Basin Groundwater Hydrology
The Groundwater Hydrology of the
Okavango Basin
FAO (2010) Internal Report prepared by MJ Jones
(Consultant) for FAO
April 2010
1
Basin Groundwater Hydrology
TOC
Contents
1
INTRODUCTION .......................................................................................................................... 5
2
GEOLOGICAL SETTING ............................................................................................................. 6
2.1
Tectonic Background .............................................................................................................. 6
2.2
The Karoo System ................................................................................................................... 7
2.3
Post Karoo Geological Events .............................................................................................. 14
2.4
The Post-Cretaceous development of the Okavango Basin .................................................. 18
2.5
The Kalahari Group .............................................................................................................. 20
2.6
The Lower Kalahari Group Formations ................................................................................ 20
2.7
The Kalahari Sands ............................................................................................................... 24
2.8
The Tectonic Development of the Okavango Graben ........................................................... 25
2.9 Impact of the Palaeo-geomorphological Legacy in the Okavango Basin ................................... 27
3
THE GROUNDWATER OCCURRENCES AND QUALITY IN THE OKAVANGO BASIN . 33
3.1
Potential Aquifer Recharge ................................................................................................... 33
3.2
Depth to Groundwater ........................................................................................................... 37
3.3
Groundwater Quality............................................................................................................. 37
4
HYDROGEOLOGICAL PROVINCES OF THE OKAVANGO BASIN .................................... 39
4.1
The Cubango-Cuito Basin ..................................................................................................... 39
4.2
The Omatako and Eiseb Basins ............................................................................................ 49
4.3
Ghanzi Block ........................................................................................................................ 51
4.4
The Okavango Delta ............................................................................................................. 53
4.5
Makgadikgadi Block ............................................................................................................. 56
4.6
Central Kalahari Block .......................................................................................................... 59
5
CONCLUSIONS ........................................................................................................................... 69
6
REFERRENCES ........................................................................................................................... 70
2
Basin Groundwater Hydrology
Table of Figures
Figure 1: .................................................................................................................................................. 6
Figure 2 ................................................................................................................................................... 7
Figure 3 ................................................................................................................................................... 7
Figure 4 ................................................................................................................................................... 8
Figure 5 ................................................................................................................................................... 9
Figure 6 ................................................................................................................................................... 9
Figure 7 ................................................................................................................................................. 10
Figure 8 ................................................................................................................................................. 11
Figure 9 ................................................................................................................................................. 11
Figure 10 ............................................................................................................................................... 12
Figure 11 ............................................................................................................................................... 12
Figure 12 ............................................................................................................................................... 13
Figure 13 ............................................................................................................................................... 13
Figure 14 ............................................................................................................................................... 15
Figure 15 ............................................................................................................................................... 16
Figure 16 ............................................................................................................................................... 17
Figure 17 ............................................................................................................................................... 18
Figure 18 ............................................................................................................................................... 19
Figure 19 A - C ..................................................................................................................................... 20
Figure 20 ............................................................................................................................................... 22
Figure 21 ............................................................................................................................................... 23
Figure 22 ............................................................................................................................................... 23
Figure 23 ............................................................................................................................................... 25
Figure 24 ............................................................................................................................................... 25
Figure 25 ............................................................................................................................................... 26
Figure 26 ............................................................................................................................................... 26
Figure 27 ............................................................................................................................................... 27
Figure 28 ............................................................................................................................................... 28
Figure 29 ............................................................................................................................................... 29
Figure 30 ............................................................................................................................................... 29
Figure 31 ............................................................................................................................................... 30
Figure 32 ............................................................................................................................................... 32
Figure 33 ............................................................................................................................................... 34
Figure 34 ............................................................................................................................................... 37
Figure 35 ............................................................................................................................................... 39
Figure 36 ............................................................................................................................................... 40
Figure 37 ............................................................................................................................................... 40
Figure 38 ............................................................................................................................................... 42
Figure 39 ............................................................................................................................................... 43
Figure 40 ............................................................................................................................................... 46
Figure 41 ............................................................................................................................................... 49
Figure 42 ............................................................................................................................................... 51
Figure 43 ............................................................................................................................................... 52
Figure 44 ............................................................................................................................................... 52
Figure 45 ............................................................................................................................................... 54
Figure 46 ............................................................................................................................................... 54
Figure 47 ............................................................................................................................................... 55
Figure 48 ............................................................................................................................................... 56
Figure 49 ............................................................................................................................................... 57
Figure 50 ............................................................................................................................................... 57
Figure 51 ............................................................................................................................................... 58
Figure 52 ............................................................................................................................................... 59
Figure 53 ............................................................................................................................................... 59
Figure 54 ............................................................................................................................................... 60
3
Basin Groundwater Hydrology
Figure 55 ............................................................................................................................................... 60
Figure 56 ............................................................................................................................................... 61
Figure 57 ............................................................................................................................................... 62
Figure 58 ............................................................................................................................................... 65
Figure 59 ............................................................................................................................................... 66
Figure 60 ............................................................................................................................................... 67
Figure 61 ............................................................................................................................................... 67
Figure 62 ............................................................................................................................................... 67
Figure 63 ............................................................................................................................................... 68
Figure 64 ............................................................................................................................................... 68
4
Basin Groundwater Hydrology
1 INTRODUCTION
Faced with the problems of scale and disparities in the quality and quantity of available
databases, a strategic groundwater assessment of large river basins must go back to basic
principles where the key objectives are to establish the resources in terms of a) their
occurrence b) their chemical quality and c) the periodic changes in groundwater storage.
In the Okavango Basin, circumstances have resulted in a very unequal spatial distribution of
sound geological and other essential environmental data. While the available topographic,
climatic, soil, vegetation and hydrological information is sufficient to achieve a broad
appreciation of the groundwater processes within the Okavango Basin, quantifying the
resources is constrained by the lack of geological and hydrogeological data in the Cubango
and Cuito catchments.
The overview of the resources and definition of the groundwater circulation are partially
simplified by the fact that large areas of the basin receive little, if any, modern recharge and
can, therefore, be considered as inactive. Very limited geological and hydrogeological data is
available for the Cubango and Cuito catchments where active groundwater recharge
supports the essential baseflow to the Okavango River runoff. The hydrological and climate
data needed to establish aquifer recharge is available but information on the nature of the
groundwater occurrences, the depth to water and the yield of wells usually available from the
records of previous well drilling and testing programmes has not been located. Without this
supporting field verification, geophysical survey, aerial photography and remote sensing
interpretations have a limited application. This poses a major constraint to developing the
necessary understanding of these two significant and potentially under-utilised basins.
Against this background, judgement has been used to select and cite what appears to be the
most realistic and representational of the available data.
This review broadly follows a hydrogeological terrain concept that combines classification of
groundwater occurrences and their tectonic and geomorphological setting with specific
climatic zones as this approach conveys an idea of what type of groundwater system or
occurrence to expect. Thus the term "large sedimentary basin and arid" conveys the
prospect of extensive stratiform sandstone aquifers largely containing "fossil water" and that
only receives modern recharge along the mountainous rim of the basin. Similarly "Basement
complex and arid zone" conveys a mountainous terrain with widespread exposures of
crystalline metamorphic and batholithic rocks that are cut by numerous faults, fissures and
stress joints. Preferentially erosion along these lines of weakness results in a heavily incised
surface water drainage pattern with the fissure and fault zones below the valley floors are
ideally located to receive indirect recharge as are the large tracts of outwash deposits
(colluvial deposits) along the mountain front piedmont.
5
Basin Groundwater Hydrology
2 GEOLOGICAL
SETTING
2.1 Tectonic
Background
The main groundwater occurrences in the Okavango Basin are found in the Basement rocks,
the Karoo System formations and the Kalahari Group formations and minor, but locally
important occurrences, exist in the lightly metamorphosed Late Pre-Cambrian Palaeozoic
sediments. Given the importance of the Karoo System and Kalahari Group aquifers it is
necessary to develop a clear understanding of the geological setting of the Basin.
An almost ubiquitous cover of Kalahari Group formations hides the complex underlying
geological structure of the Okavango Basin that is enclosed by ancient tectonic cratons
(Figure 1) that apatite fission track dating shows to have been land since 300 to 450Ma (M.
Raab et al. 2005).
The cratons are coherent relics of previous continental breakup and drift episodes and
comprise batholith cores and intensely metamorphosed crystalline rocks. The cratons were
conjoined by later Precambrian and Early Palaeozoic metamorphic sedimentary sequences
of the Limpopo, Namaqua and Demara Belts and the Lufilian Arc to form the southern
extremity of the Pangaea supercontinent by Mid to Late-Carboniferous (ca 320-330Ma). The
southern margin of Pangaea was an active convergent, low-angle, continental and oceanic
plate subduction zone. The resulting compressional forces created an extended coastal
mountainous range. The South African Cape Fold Belt is a remnant of these mountains.
Figure 1: Provisional simplified
schematic of the Basement
structure of the Okavango Basin.
Dated between 3.6 and 2.5Ga, the
Zimbabwe and Kaapvaal Cratons
became attached by the Limpopo
Mobile Belt around 2.0Ga. The
collision of these cratons with 1.8-
1.6Ga Congo Craton between 500
and 550Ma saw the deformation of
the island arc sediments of the
Demara and Lufilian belts (adapted
from Geological Survey of Namibia,
Figure 1:
2005).
Extensional forces and hot spots (Figure 2) associated with superplume activity initiated the
breakup of the Pangaea super-continent into first, the Gondwana and Laurasia
supercontinents in the Permian Period (230 Ma) and then, the breakup of Gondwana in to
Africa, South America, Madagascar, India, Australia and Antarctica in the Late Jurassic and
Cretaceous (130 and 100Ma).
6
Basin Groundwater Hydrology
Figure 2: Pangaea prior to the ca
230Ma separation of Laurasia and
Gondwana due to mantle plume
initiated rifting. The hot spots shown
now lie along or near to the mid-
Atlantic ridge.
Shows location of South Pole ca
300Ma and north limit of recognised
lower Karoo glacial deposits
(Adapted from Earle, S., 2001).
Figure 2
Each tectonic development left regional structural trends in the fabric of the underlying
Basement. Figure 3 shows the deep geological structure under the Kalahari Group cover in
northern Botswana. This has been established from detailed geophysical surveys and from
various mineral exploration programmes coupled with the geological mapping of the
outcrops of the Demara folded quartzites and intercalated limestones in the bed of the
Okavango River at the Popa Falls on the Angola-Namibia border and of the Pre-Cambrian
felsic granitoid gneiss outcrops in the Tsoilo Hills to the west of the Okavango Delta. The
degree of Basement complexity under the Namibia and Angolan areas of the Okavango
Basin remains to be established but will have impacted on the deposition of later Karoo and
Kalahari sediments as will the impact of renewed tectonic activity as typified by the current
movements along the Okavango graben faults.
Figure 3
Figure 3: Northern Botswana, Basement geology and tectonic elements (redrawn from S.
Yawsangratt, 2002)
2.2
The Karoo System
7
Basin Groundwater Hydrology
The start of the deposition of the Karoo System sediments in the Late Carboniferous and
Permian (300Ma) coincided with an intra-cratonic sag (S. Lawrence and I. Hutchinson, 2009)
or the flexural subsidence of the back-bulge to the Cape Fold Belt (O. Catuneanu et al.,
2005) prior to the breakup of Pangaea. Figure 4 shows the regional tectonic setting during
the deposition of the Ecca Sandstone in the Central Botswana and the Great Karoo.
Compression forces in the western half of southern Africa saw uplift and erosion of the
Karoo Basin margins with a north-eastward focused drainage pattern. To the east and north,
extensional forces and rifting controlled the tectonic development of linear Karoo basins
typified by the Luangwa and central Zambezi Valleys.
Figure 4: Inferred Permian
tectonic setting during the
deposition the Karoo in
southern Africa (adapted from
S. Lawrence and I. Hutchinson,
2009).
Figure 4
A NE-SW trending shear zone approximating to the northern margin of the Caprivi Strip
delineates boundary between the Central Kalahari Karoo Basin and the Angolan Karoo
sequence to the North as shown on Figure 5.
During the deposition of the Early Karoo formations around 300Ma, southern Africa lay close
to the southern pole and under the influence of glaciation that extended northwards to the
Gulf of Guinea and central Sudan (Figures 2 and 7). Subsequently as plate movement
drifted the African continent to the north and slowly rotated it counter-clockwise, as the
deposition of the Karoo sequence progressed the prevailing palaeo-climate changed of from
cold and semi-arid to hot and humid although there were considerable temporal and spatial
fluctuations.
8
Basin Groundwater Hydrology
Figure 5: The Okavango
Basin - distribution of main
Karoo occurrences
(extracted from O.
Catuneanu et al., 2005)
Light grey shading indicates
Karoo subcrop under
Kalahari Group formations.
Coastal escarpment marking
regional water divide
indicated in red.
OGT approximate site of
cored BH shown on Figure
11.
Figure 5
The established Karoo System stratigraphy in the Kalahari Basin and the more provisional
Angolan sequence are shown on Figure 6. The dating and regional correlation of the
formations is being established but significant gaps still exist (B. N. Modie and A. Le Herisse,
2009)
Figure 6
Figure 6: Okavango Basin Karoo System stratigraphy with indicative thicknesses, adapted
from O. Catuneanu et al. (2005).
In depth studies of the main South African Karoo Basin and the Kalahari Basin have
established the prevailing palaeo-geographical and palaeo-climatic conditions during the
deposition of the Karoo formations as shown on Figures 7 and 8. Most of the clastic
sediments have an arkosic composition that reflects rapid denudation, transportation and
deposition.
9
Basin Groundwater Hydrology
Figure 7: Central Kalahari and main South
African Karoo Basins, palaeogeography
during the deposition of the Ecca Group
sediments ca. 275Ma (abstracted from K.
Scheffler, D. Buehmann and L.Schwarkm,
2006). Inset shows the configuration of
Pangaea prior to breakup.
Figure 7
The Karoo formations of the Okavango Basin have all been subject to deep weathering both
during and after deposition as shown on Figure 9. This weathering profile, determined by
detailed examination, including analysis of 34 samples taken from a 340m cored borehole
drilled near Orapa (see approximate site on Figure 5): It is interpreted as establishing the
changing sedimentary and climatic environment during the deposition of the Karoo
formations (K. Scheffler, D. Buehmann and L.Schwark, 2006). The profile reflects the key
factors that impacted the formations during deposition, the changing climate, the marine
retreat and changes in the sediment sources. These sources were firstly from the southern
Cargonian highlands and then during the deposition of the largely lacustrine Tihabala
Formation from the northern Windhoek highlands before finally reverting to the southern
sources again during the deposition of the continental arkoses of Ntane Formation.
The distribution of 1:1 kaolin clays and 1:2 smectite clays is attributed to changing redox
conditions in the profile shown on Figure 9. From the groundwater resources perspective
the clay distribution coincides with the view of open and closed weathering profiles as
applied to the formation of weathered Basement complex aquifers. Following this view
suggests that some of the weathering of the profile could have developed during the
subsequent denudation processes associated with the formation of the African erosion
surface.
10
Basin Groundwater Hydrology
Figure 8: Southern Africa, palaeoclimate
changes during the deposition of the
Dwyka, Ecca and Beaufort Groups
(abstracted from K. Scheffler, D.
Buehmann and L.Schwark, 2006).
Figure 8
The maximum thickness of the Karoo formations in the South African Karoo Basin is around
1500m and, in the Kalahari Basin 800m. This points to a low energy environment and
relatively stable long-term tectonic conditions compared to the accelerated denudation and
sedimentation conditions of the eastern rifted basins in Zambia and Tanzania (the Luangwa
and Ruhuhu basins) where 3 to 5km of the truncated Karoo System formations remain.
Figure 9: Karoo formation weathering profile
from borehole OGT (Figure5) showing
influence of climate and mineralogical
changes notably in the K feldspar
(Orthoclase) content in the arkoses derived
from a southern sources in Cargonian
highlands. K. Scheffler, D. Buehmann and
L.Schwark, 2006
Figure 9
The deltaic environment for the deposition of the Ecca formations in the central Kalahari
Basin comprises an upward-coarsening, lower delta-front member marking the final marine
retreat that is followed by a series of upward-fining deltaic and fluvitile members (T.
Segwabe1 and E. Bordy, 2009) as shown on the fence diagram (Figures 10). The withdrawal
of the Early Ecca sea to the west largely separated the central Kalahari Basin from the
Aranos Basin in Namibia. Figure 10 shows the configuration of the pre-Karoo landsurface,
the Dwyka glacial deposits, the deltatic-fluvitule Ecca formations and the Beaufort floodplain
sediments. It is apparent that the regional palaeo-water divides approximately followed the
modern geography (Figure 5).
11
Basin Groundwater Hydrology
Figure 10
Figure 10: Distribution of Karoo formations across the central Kalahari Basin along the cross
sections marked on Figure 10 (adapted from (T. Segwabe1 and E. Bordy, 2009). The depths
are centred on top of the Ecca Formation and not relative to ground level.
The distribution and thickness of the Karoo Dwyka, Ecca and Beaufort Groups across the
Kalahari Basin (Figure 11) reflects the active control imparted by the deep tectonic structure
with the cratonic core of the Ghanzi Ridge and Magondi Belt forming a mark northern
boundary to the central Kalahari Karoo Basin. The southern margin of the Ghanzi Ridge is
labelled the Makgadikgadi Line (L. V. Ramokate, 1997 and T. B. Rahube, 2003) and
incorporates the Tsau normal fault zone. The Kunyere Thamalakane Fault line defines the
northern margin of the Ghanzi Ridge and the southern limit to the Okavango Delta graben. It
is not certain that the basin configuration shown on Figure 11 is the result of the palaeo-
geomorphology or, as is more likely, due to intra-depositional sag possibly related to a pre-
Karoo rift branch that is on nearly the same alignment as, and followed by, the Okavango
dyke swarm.
Figure 11: Thickness
of the Ecca Group in
the Central Kalahari
Basin (adapted from T.
Segwabe and E.
Bordy, 2009)
Figure 11
North of the Ghanzi Ridge, the Dwyka, Ecca and Beaufort Group formations are less well
defined and where present, they appear less well developed as shown on Figure 6:
Information for the Angolan and Namibian areas of the Okavango Basin is limited. The
imprint of the deep geological structure, however, is expected to be repeated but remains to
be established. A break in main intra-continental water divide in Namibia saw the deposition
of shallow and deep marine shales in the Price Albert Formation of the Ovambo Basin.
12
Basin Groundwater Hydrology
Figure 12: Okavango Basin -
regional superficial geological
map (extracted from E. G.
Purday, 1989).
Figure 12
The persistent unconformity between the Beaufort and Stormberg Groups point to
widespread Triassic tectonic events that culminated with the extrusion of the Stormberg
(Drakensberg) trap basalts associated with the relatively short-lived Karoo large igneous
province (LIP) that was active between 180 and 190Ma.
The sandstones and arkoses of the Ntane formation and Stormberg Basalt lavas are the
main potential aquifers and in the Okavango Basin. They are well exposed in eastern and
northern Botswana and at places in Namibia as shown on the superficial geological map,
Figure 12 and the geological map of Botswana showing the solid geology below the Kalahari
Group sediments, Figure 13.
Figure13: Geological map of Botswana
showing the Karoo Group solid geology
below the Kalahari Group sediments
(adapted from J. L. Farr et al. 1981)
Figure 13
The intrusion of the Okavango mafic dyke swarm ca 179Ma in the Jurassic marked the end
of the Karoo System deposition. Extending on a N110°E trend for some 1500km and with a
width of some 100km, this dyke swarm is considered to be aligned on a branch of the pre-
13
Basin Groundwater Hydrology
Cambrian triple rift junction that was weakly active during the deposition of the Karoo System
(B. Le Gall, et al., 2005 and F. Jourdana, G. Férauda and H. Bertrand, 2006).
2.3
Post Karoo Geological Events
Two significant global changes and two major continental scale changes impacted on the
present Okavango Basin.
The first global change is the northern drift of the African continent and anticlockwise rotation
as shown on Figure 14. This transit has resulted in regional climate shifts that saw the
Okavango Basin enter the southern latitudinal arid belt during the Eocene-Tertiary (ca.
50Ma).
Deep geophysical investigations have shown the geographical stability of the African plate
for at least 300Ma to be due to tectonic inertia caused by a large low shear velocity province
in the mantle under much of the African continent (E. J. Garnero, T. Lay, and A. McNamara,
2007). The presence of this mantle structure and two phases of superplume activity along
the margins of this structure (T. H. Torsvik, et al. 2006, 2008 and K. Burke and Y. Gunnell,
2008) are indirectly responsible for the African Plateaus. The first superplume activity started
in the Jurassic (ca. 225Ma) with the breakup of the Pangaea super-continent into Gondwana
and Laurasia to form the North Atlantic and culminated in the Cretaceous (ca. 130Ma) when
the Tristan superplume triggered the separation of the African and South American plates to
form the South Atlantic. By this time the separation of the African and Indo-Antarctic-
Australian tectonic plates had already occurred. This phase of continental separation
resulted in passive margins to the southern African plate.
The second phase is associated with the Afar superplume that saw the separation of the
African and Arabian plates. Incidental with these events were the crustal extensions
responsible for the formation of the East African Rift systems.
The second global change that impacted on the palaeo-climate was the global decline in sea
levels and the creation of the modern oceanic circulation pattern that followed the formation
of the Antarctic Ice Sheet around 34Ma. This occurred after the breaching of the land links
between Antarctica and Australia and South America (K. Burke and Y. Gunnell, 2008) that
opened the Southern Ocean. The subsequent influence of the cold Benguela Current
compounded the hot semi-arid desert conditions along the western half of southern Africa.
14Ma Miocene
50Ma Eocene
65Ma end of Cretaceous
14
Basin Groundwater Hydrology
Figure 14: Palaeo-
geographic maps showing
rotational and northern drift
of the African Continent.
95Ma Cretaceous
150Ma Jurassic
Figure 14
The continental changes covered the interplay between tectonic events and denudation the
moulded the African landscape.
The first continental change was the formation of the African erosion surface. This surface
was
formed in the Mid-Cretaceous following the breakup of the South American and African
continents around 130 Ma. Closely following this separation, was the elevation of the
Kalahari High Plateau (M. de Wit, 2007) that triggered two denudation cycles that lowered
much of the African continent to a low-relief, peneplaned landsurface (P. van der Beek, et
al., 2002 and A. Moore, T. Blenkinsop and F. Cotterill, 2009, M. D. Rowberry, T. S. McCarthy
& S. Tooth, 2009). Apatite fission track analysis dates the two phases of shoulder uplift that
occurred between 138 and 160Ma and between 90 and 115Ma (F. Guillocheau, et al.,
2009). While the tectonic mechanism responsible for the uplift of the Kalahari Plateau has
yet to be identified, the dates correspond with the two phases of kimberlite emplacement.
Offshore lithological and stratigraphical evidence from oil exploration studies confirms the
denudation of the Kalahari plateau in the Late Cretaceous as the main source of sediments
(F. Guillocheau, et al., 2009).
The denudation process started at the coastline between 95 and 115Ma and culminated in
a final pulse around 80Ma across most of the interior. By this stage many of the main water
divides found in modern southern Africa were established. At least 5km of rocks are
estimated to have been eroded during this phase of accelerated denudation that averaged
40m/Ma. After 80Ma, the offshore deposition record shows a sharp decline in sediment
transportation from the main basins of the pre-Limpopo and pre-Orange Rivers as
denudation dropped to 5m/Ma (M. Séranne and Z. Zanka, 2005). This leads to conclusion
that much of the southern African topography was of low relief and lay close to the prevailing
sea level after 80Ma (see Figure 15).
The second regional change was the uplift of these low-lying peneplains from 30Ma onwards
to form the characteristic, modern high African plateau landscape.
15
Basin Groundwater Hydrology
Figure 15: Palaeo-topography
of Africa in the Mid to Late
Cretaceous ca.100-90Ma
(adapted from M. de Wit, 2007)
Figure 15
These African plateaus were first described and attributed to erosion by E. J. Wayland
(1934) , B. Willis (1936), F. Dixey (1946, 1956) and L. C. King (1962).They postulated
structural unloading due to denudation and periodic isostatic uplift as the trigger for a
succession of peneplanation cycles. A major objection to this concept of periodic isostatic
uplift is that the unloading rebound is seen as a continuous process: Subsequently,
superplume activity was proposed as the driving mechanism for regional periodic uplift (G.
C. Bond 1979). This mechanism, however, is not supported by deep geophysical
investigations that identify the large low shear velocity province in the mantle underlying
much of the African continent as the source of the tectonic buoyancy (E. J. Garnero, T. Lay,
and A. McNamara, 2007).
The noted geographical stability of the African plate for at least 300Ma due to this mantle
structure and two phases of superplume activity are indirectly responsible for the African
Plateaus ((T. H. Torsvik, et al. 2006, 2008 and K. Burke and Y. Gunnell, 2008).
While still contested by other workers (A. Moore, T. Blenkinsop and F. Cotterill, 2009), K.
Burke and Y. Gunnell (2008) present a coherent interpretation of the geological processes
behind the African erosion surfaces. Of relevance to the Okavango Basin, their main findings
are:
· the African continental plate has remained close to its present geographical position
for at least 200 million years with only latitudinal and minor rotational drifting taking
place (Figure 14);
· two deep seated tectonic events have shaped the landsurface, these were the
eruption of the Karoo (225 -130Ma) and Afar (30Ma) large igneous provinces (LIPs)
that arrested or pinned the slow movement of the African plate;
· the plate-pinning event caused by the Afar superplume coupled with shallow mantle
convection resulted in the formation of the characteristic swells and basins found
across the Continent: These regional uplifts had previously been ascribed to isostatic
movements and then large mantle plumes;
· the African erosion surface developed during the 100Ma interval between the end of
130 Ma Karoo and the 30Ma Afar events when tectonic activity was largely
quiescent;
· between 80Ma and 30Ma much of Africa had a low relief and lay close to the
prevailing sea level;
16
Basin Groundwater Hydrology
· the present large-scale geomorphological features of the African landscape have
taken shape since 30Ma and local tectonic movements, however, make the African
surface appear polycyclic.
K. Burke and Y. Gunnell (2008) reassign many of the plateaux in southern and central Africa
previously considered to be post-Gondwana or Gondwana as part of a polycyclic African
surface that has been differentially uplifted by movements associated with regional rifting
and shallow mantle convection driven swells. They find there are unlikely to be any
continuously exposed African land surfaces much older than 40Ma. They cite the P. van der
Beek, et al. (1998) maximum apatite fission track (AFT) ages of 40Ma for the landsurface on
Kipengere and Livingstone Mountains in south west Tanzania and for the topographically
lower Lupa and Ufipa plateau each side of Lake Rukwa. Landsurfaces older than 40Ma,
however, are preserved where buried under younger sedimentary formations and
subsequently exhumed as described in Namibia by M. J. Raab, et al. (2002, 2005)
The distribution of the post 30Ma basin and swell structures that moulded the landscape of
southern Africa is shown on Figure 16. The scale of the regional uplifts is in the order of 2 to
5km as determined for the Biè Swell in Angola shown on Figure 17. Consideration of
accelerated post-uplift denudation and isostatic balancing, however, suggests that actual
land levels did not vary greatly from their current elevations.
Figure 16: Distribution of the post 30Ma
basin and swell structures (adapted from K.
Burke and Y. Gunnell, 2008)
Key:
Swells (blue)
1- Biè (Angola);
2- Namibia;
3- North Zambia;
4 Zimbabwe;
5- South Africa;
6- Mayombe;
7- East African;
Basins (yellow)
1- Kalahari
2- Congo
Figure 16
The Kalahari Basin shares a similar inter cratonic location to the Congo Basin and there is
no clear tectonic mechanism for their formation. They may occupy a region where the
shallow mantle convection rotated and sunk rather than a tectonic downthrown rifted zone as
typified by the Sudd Basin. For the Congo Basin, S. Buiter and B. Steinberger (2009),
however, suggest a deep seated gravity anomaly as exerting a downward pull under the
basin.
Irrespective of the tectonic mechanism involved, the formation of the Kalahari Basin
disrupted the regional drainage basins and impacted of the deposition of the Kalahari Group
formations.
17
Basin Groundwater Hydrology
Figure 17: The Neogene (ca 30Ma)
Biè Swell in Angola from M. P.
Jackson and M. Hudec, 2009
Figure 17
2.4
The Post-Cretaceous development of the Okavango Basin
As the palaeo-geomorphological development of the Okavango Basin had key control over
the distribution of the Kalahari Group formations that contain some of the locally important
aquifer horizons in the region, consideration must be given to the environmental changes
that occurred during their deposition.
Two main reconstructions of the post-Karoo drainage systems of central and southern Africa
have been proposed. The first is largely based on the interpretation of persistent tectonic
uplift along three concentric horseshoe shaped sets of flexural axes (Moore, A.E. & P.A.
Larkin, 2001, A. T. Moore, A., T. Blenkinsop and F. Cotterill, 2009 and J. Stankiewicz and M.
J. de Wit, 2006). This model proposes that the palaeo-Okavango River was part of the
Limpopo River system that also included the upper Zambezi catchment until around the end
of the Cretaceous (Figure 18A). From the end of the Cretaceous 34Ma to the Mid-
Pleistocene (2.5Ma) these combined catchments were cut-off from the Limpopo are
considered to have formed the closed Kalahari basin where the fluvitile and lacustrine
Kalahari Group formations were deposited (Figure 18B). In the Pleistocene, the Okavango
Basin was detached from Zambezi River as the influence of the East African Rift System
(EARS) spread westwards (Figure 18C).
A: The Late Cretaceous (65Ma)
B: Palaeocene to Mid Pleistocene (65-
1.5Ma)
18
Basin Groundwater Hydrology
C: Mid-Late Pleistocene (~ 1Ma)
D: Present
Figure 18A-D: The post Cretaceous development of the drainage system in Southern Africa
(modified from A. E. Moore and P. A. Larkin, 2001 and J. Stankiewicz and M. J. de Wit,
2006).
Figure 18
No stratigraphic record in the east coast delta sediments of the Limpopo and Zambezi
between 65 and 30Ma is preserved to provide ancillary evidence on the denudation and
eastward sediment transfer history during this interval.
The alternative models (K. Burke and Y. Gunnell, 2008) for the development of the palaeo-
drainage system of the southern Africa between 80 and 15Ma are shown on Figure 19A, B
and C and are supported by the stratigraphy and lithology of the off-shore sediments of the
deltas of the main rivers as shown. Subsequent to uplift of the South African swell ca. 30Ma,
the regional drainage was disrupted and Transtswana River was cut off from the Kalahari
(Orange) River. Flow in the proto-Okavango River was further impounded by the rising
Zimbabwe and North Zambian swell.
Figure 19A: The palaeo-drainage
system of southern Africa ca. 70-
30Ma (adapted from K. Burke and Y.
Gunnell, 2008). Pre 30Ma Kalahari
River and Karoo River (Orange River
Delta) peak deposition between 122
and 95Ma with pulse at 80Ma. Little
deposition since 30Ma
19
Basin Groundwater Hydrology
Figure 19B: The palaeo-drainage
system of southern Africa ca. 30Ma.
Uplift of South African swell reverses
flow of the upper course of the
Transtswana River.
Renewed accelerated denudation
triggered in the Zambezi Basin
associated with activity along the East
African Rift System.
Figure 19C: The palaeo-drainage
system of southern Africa ca. 15Ma.
Limpopo River captures headwaters
of the Orange (Vaal) River and
potentially the Proto-Okavango River.
Rifting continues to dominate the
development of the Zambezi-Shire
River basin.
Figure 19 A - C
As the outwash fan and terminal lakes in the Okavango expanded during the Tertiary, the
deposition of the Kalahari Group formations merged with the Owambo-Cuvelai-Cunene
River system fans and lakes. Intermittent tectonic movements and climate fluctuations
resulted in expansion and contraction of the lakes surrounding pans throughout the Tertiary.
2.5
The Kalahari Group
The main subdivisions of Kalahari Group are the pre-Pleistocene Lower Kalahari fluvial and
lacustrine formations and the upper Pleistocene-Holocene aeolian Kalahari Sands.
2.6
The Lower Kalahari Group Formations
The hidden begrock geology of palaeo-African Erosion Surface below the Kalahari Group
includes the topographically higher areas underlain by Pre-Cambrian and Early Palaeozoic
crystalline and the topogarphically lower sedimentary basins occupied by various Karoo
Group formations.
Figure 20 shows the thickness and distribution of the Kalahari Group formations. The basal
members of the Lower Kalahari Group must incorporate and preserve the imprint of the
palaeo-African Erosion Surface drainage pattern and the associated alluvial and colluvial
deposits.
20
Basin Groundwater Hydrology
Figure 20: Distribution
and thickness of the
Kalahari Group
formations in southern
Africa (adapted from M.
H. T. Hipondoka, 2005
and based on data
from D. S. G. Thomas,
1988, D. S. G. Thomas
and P. A. Shaw 1991
and I. G. Haddon,
2000).
Some further certainty to dating the base of the Lower Kalahari Group formations in the
Okavango Basin is provided by the Kimberlite pipe intrusion phases in the Mid to Late
Cretaceous (120 - 80Ma). After intrusion, these were subject to denudation and laterisation
before being covered by the Kalahari Group sediments. The presence of laterites points to
humid conditions continuing sometime after 80Ma (F. Guillocheau, et al., 2009). As regional
denudation became subdued at around 4m/Ma between 80 and 30Ma and the hydrological
regime became strongly seasonal, it is reasonable to envisage that large parts of the palaeo-
drainage system became ephemeral and that possibly in places fragmented into a series of
closed (endorheic) basins (Figures 19A - 19C).
T. C. Partridge and R. R. Maud (1987) concluded that the widespread fluvitile and lacustrine
Kalahari sediments were deposited between the Mid-Palaeocene (ca. 60Ma) and the
Pliocene (ca. 2.5). They consider that the extensive calcrete and silcrete deposits formed
from the Mid-Pliocene onwards as the result of increasing aridity. Adding detail, I. G. Haddon
and T. S. McCarthy (2005) assign a Late Cretaceous date to the river gravels and outwash
deposits found at the base of the Kalahari Group. They envisage Early Palaeocene tectonic
block faulting, tilting and down-warping of the Kalahari Basin as leading to the merging of
colluvial tracts and the spread of shallow lakes that were infilled by lacustrine clays and
marls. The denudation of the Basement rocks and Karoo Group formations over surrounding
uplifted swells was the source of the sediments but there was considerable re-working of the
sediments. There is agreement that the formation of extensive calcrete and less extensive
silcrete beds reflects more arid periods but generally, the palaeo-climate comprised clearly
defined wet and dry seasons in-line with the modern climate.
Two approaches are taken to defining the geology of the Lower Kalahari Group Formations.
The first categorises the formations by lithology and the second attempts to apply a more
rigorous time base stratigraphical classification. The lithological approach is dictated by a
lack of exposed sections and a consistently datable fossil or mineralogical record.
D.S.G. Thomas and P. P. Shaw (1991) adopt the lithological approach and recognise four
main units in the Lower Kalahari Group succession:
- conglomerate and gravel units that sporadically occur at the base of the Lower
Kalahari Group and occasionally within the other units;
21
Basin Groundwater Hydrology
- pink to red, fine-grained, homogenous marls;
- varicoloured, sandstones;
- calcretes, silcretes and other duricrusts.
Given much of the Kalahari Group occurrence is less than 100m thick in the Okavango
Basin (Figure 19), this lithological classification has been widely applied to hydrogeological
investigations in central and southern Botswana where the focus is largely on recharge
studies. It is also appropriately applied to the thick, rapidly-infill stratigraphy of the Okavango
Delta proper. Here the Lower and Upper Kalahari Group reach a maximum thickness of over
400m (Figure 21).
Figure 21: Thickness of Kalahari
Group formations under the
Okavango Delta (from W.
Kinzelbach 2006).
Figure 20
In the Ovambo Basin in Namibia, the Kalahari Group occurrence is thicker than 100m and a
formal lithostratigraphic succession has been established as shown in Figure 22. This
succession recognises sub-divisions that are largely based on palaeo-environmental factors
revolving around low-energy deltas and shallow, ephemeral lakes under arid and semi-arid
climatic conditions.
How this lithostratigraphy applies to the Namibian and Angolan parts of the Okavango Basin
remains to be confirmed but I. G. Haddon and T. S. McCarthy (2005) propose a provisional
cross correlation for the entire Kalahari Group occurrence. As the Kalahari Group
occurrence in Namibia and Angola are relevant to the groundwater resources of the
Okavango Basin, the Ombalantu Formation and part of the Belseb Formation in the Ovambo
Basin are correlated with the clayey Tsumkwe Formation in Eiseb Graben adjacent to the
Botswana border (Figure 23).
22
Basin Groundwater Hydrology
Figure 21
Figure 22: The Kalahari Group stratigraphy of the Ovambo Basin, Namibia (redrawn based
on M. H. T. Hipondoka, 2005).
Although more remote from the East African Rift System from where these tectonic features
are widely assumed to be propagated, the Eiseb Graben (Figure 3) is identified as a south-
western extension of the Okavango Graben (H. Wanke, 2005) and the currently unconfirmed
stratigraphy of the Lower Kalahari Group under the Okavango Delta may share a similar
correlation to that of the Eiseb Graben.
Figure 23: Log for
Borehole WW41024
drilled in the Eiseb
Graben (adapted from
C. Stadtler et al. 2005).
Figure 22
The map of Kalahari Group thickness (Figure 20) shows the lower Cubango and Cuito
Rivers in southern Angola and along the Angola-Namibia border is clearly cross the NE
extension of the Ovambo Basin were at least the upper part of the Lower Kalahari Group will
closely correlate with the succession shown on Figure 22. The same assumption, however,
may not apply to the virtually unreported Kalahari Group occurrence in the upper catchment
of the Cuito River where over 300m of the Group sediments underlie the water divide
between Cuito and Kasai River Basins. The surface water drainage pattern is very linear and
sub-parallel with strongly meandering consequent tributaries running SSE off the shoulder of
the Biè swell. The tributary lateral valleys are wide with poorly defined drainage channels
and the hydrogeological indications are that aquifer horizons of the Kalahari Group are
effectively absorbing the majority of the annual precipitation. In view of the geomorphological
23
Basin Groundwater Hydrology
setting it is likely that the Kalahari Group in this area contains considerably more coarse
clastic material in the succession than found downstream. Future investigations will establish
if there are significant calcretes and silcretes in the northern Kalahari Group formations.
While the more humid, long term, prevailing climate could be against the formation of these
secondary deposits, if they are found to be extensive the role of groundwater circulation in
their formation will require investigation.
2.7
The Kalahari Sands
Up to 50m thick, the Kalahari Sands extend over entire Okavango Basin and much of the
more extensive Kalahari Basin in Namibia, Angola, DR Congo, Zambia, Zimbabwe and
South Africa: The thickness may exceed 200m in northern Namibia. Largely derived from the
Lower Kalahari Group and Karoo System formations, the Sands show typical aeolian
landforms including dune fields. In the lower Cuito Basin and around the Okavango Delta the
dunes have a uniform E-W orientation. In detail, however, the finer individual sand grains
show less rounding and the deposits are less well-sorted than normally associated with
purely aeolian deposits (B. M. Savory, 1965).
Although the deposition of the aeolian Kalahari Sands in the Pliocene and Pleistocene are
largely linked to climate changes, continuing tectonic movements changed the geometry of
the drainage system. The climate changes were associated with northern hemisphere glacial
events as shown by the analysis of recent scientific core drilling of sediments at the bottom
of Lake Malawi. These have provided a detailed climatic record dating back to around 1.0Ma
(R. P. Lyons et al., 2009, C. A. Scholz, et al., in press). This record shows the region to have
experienced to two relatively recent mega-droughts between 70,000 and 135,000 Ka
(thousand years before present) when Lake Malawi dropped to more than 550m below
current levels. The 135 Ka drought lasted 20,000 to 30,000 years and the 70 Ka drought ca
5,000 years. These mega-droughts were more severe than those associated with the last
glacial maximum. Since 70Ka, the Lake Malawi has stabilised around the current level but
significant precipitation fluctuations have occurred in parallel with Holocene climate changes
in the northern hemisphere.
Possibly as early as the Pliocene (ca. 5Ma) tectonic shifts formed the Palaeo-Makgadikgadi
Lake Basin (D. R. C. Grey and H. J. Cooke 1977 and T. C. Partridge and L. Scott, 2000).
Evidence of the Pliocene and Pleistocene geomorphology and extent of the Palaeo-
Makgadikgadi Lake during the pluvial periods has not been established: And it may not exist
due to subsequent re-working and re-deposition of the Pleistocene alluvium during the last
high lake level stands as shown on Figure 24.
24
Basin Groundwater Hydrology
Image 1: Kalahari
Sands, slump and slip
features in the Upper
Cuito Catchment
(Google Earth Image
centred on 130 27' 52" S,
180 54' 46" E, ground
elevation 1436m).
Figure 23
The ferrasols developed on the Kalahari Sands have very high infiltration capacity and
effective porosity: When saturated the soils and Kalahari Sand profiles have a low structural
strength giving rise to frequent rotational and slump slips on even moderate slopes (Image
1).
2.8
The Tectonic Development of the Okavango Graben
Figure 24 shows the proven and inferred distribution of the main faults associated with the
Okavango Graben.
Figure 24: The proven and
inferred faults associated with
the Okavango Delta and
distribution of the Pleistocene
and Recent alluvial tracts
(adapted from I. G. Haddon
and T. S. McCarthy, 2005).
Figure 24
Despite unresolved views as to the exact nature of the Okavango Graben as shown on
Figure 25, the Basement bedrock geology and structural features shown on Figure 3 clearly
have had a long term control over tectonic development of the modern grabens and fault
blocks as highlighted by M.P. Modisi, et al. (2000), E. A. Atekwana, et al. (2005) and H.
Wanke (2005). The currently dominant NE-SW structural trends aligned with the East African
Rift System are superimposed on older rifts termed the NW Botswana rift system that was
25
Basin Groundwater Hydrology
initiated around 1100Ma: That is prior to the formation of the Pangaea supercontinent (R.
Key and R. Mapeo, 1999). This rift system was active again during the break-up of Pangaea
and Gondwana and the deposition of the Karoo Group. The separate phase of pre-Karoo
rifting on a NW-SE trend is less well defined but is reflected in the interpreted bedrock
structure as shown on the block diagram Figure 26, and as previously noted, is followed by
the Okavango mafic dyke swarm.
Figure 25: NW-SE schematic section
through the Okavanga Delta area
showing possible interpretations of
the graben structure (from J. T. Meier,
T. Himmelsbach and J, Böttcher,
2008).
Figure 25
Gravity, airborne and surface magnetic geophysical surveys (C.V. Reeves and D. G.
Hutchins, 1976, M. P. Modisi, et al., 2000 and S. Yawsangratt, 2002) have supplemented the
limited information available from deep boreholes drilled in the Delta area shown on Figure
27. M.P. Modisi, et al. (2000) considered the structure to be a half graben with late Tertiary
to Recent down throws ranging from 200 to 300m. E. A. Atekwana, et al. (2005), however,
consider it to be a full graben (Figure 25) with downthrow of 400 to 700 m. The intra-Karoo
and post-Karoo downthrows are unknown.
Figure 26: Simplified block
diagram showing the
graben faulted structure
underlying the Okavango
Delta based on
geophysical data and
borehole information as
indicated on Figure 26
(based on W. Kinzelbach,
et al., 2006 and C. Milzow,
Figure 26
et al., 2009)
H. Wanke (2005) traces the Okavango rift faults into the Eiseb Graben and confirms the rift
system as actively controlling the deposition of the Karoo Group as well as the Kalahari
Group. He also notes very recent displacement (post 38Ka) of the Pleistocene dunes by the
movements along the faults.
26
Basin Groundwater Hydrology
Figure 27: The Okavango
Delta, bedrocks underlying the
Kalahari Group formations,
depths based on borehole data
(from UNESCO, 2007).
Figure 27
2.9 Impact of the Palaeo-geomorphological Legacy in the Okavango Basin
Given the key factors in soil formation are parent material, climate, topography, soil biota
(vegetation and organisms) and time, the most obvious palaeo-geomorphological legacy is
the prevalence of predominantly loose arenaceous soils that cover around 90% of the Basin:
Figure 28 shows the distribution of the main soil groups in the catchment and their
characteristics. The classification system used is largely based on how the soil was formed
(pedogenesis).
Since the start of the Karoo Group deposition around 310-300Ma, all the soil forming
materials have been derived from within the Okavango Basin by the mechanical and
chemical weathering of Basement rocks apart from the Stormberg Basalts, the mafic dyke
swarm, the dacite sills and the two phases of Kimberlite pipe intrusion. The Karoo Group
sediments can be considered as a mechnaical and chemical transition stage (as can the
Lower Kalahari Group) as the continental weathering, transportation and deposition
processes continued to mould the landscape and create the soils of the Basin.
27
Basin Groundwater Hydrology
Figure 28: Soils map - Okavango
Basin, (extracted from HWSD,2009 -
Harmonized World Soil Database
Viewer, FAO/IIASA/ISRIC/JRC by L.
Verelst, IIASA).
Main soil characteristics:
Arenosols (AR): Sandy soils featuring
very weak
or no soil development
Calcisols (CL): Soils with accumulation
of
secondary calcium carbonates
Cambisols (CM): Weakly to moderately
developed soils
Ferralsols (FR): Deep, strongly
weathered soils
with a chemically poor, but physically
stable subsoil
Fluvisols (FL): Young soils in alluvial
deposits
Figure 28
Leptosols (LP): Very shallow soils over
hard rock
or over unconsolidated very gravelly
material
Luvisols (LV)/Solonetz (SN): Soils with
subsurface accumulation of high activit
clays and high base saturation
Regosols (RG): Soils with very limited
soil development
Solonchaks (SC): Strongly saline
soils
The relationship between the modern soils and the geology of the bedrock outcrops shown
on Figure 12 is clear.
The predominance of ferrasols in the headwater catchment of the Cubango River conforms
to the expected chemical and mechanical weathering products derived from felsic crystalline
rocks under a warm humid to sub-humid climate (mean annual precipitation ca 600-
1200mm) and with freely draining (open) and closed dual groundwater flow systems.
28
Basin Groundwater Hydrology
Figure 29
Figure 29: Approximations of the progressive open weathering profile and main secondary
mineral products for a generic felsic granite.
Based on numerous published studies, the accepted weathering profile over a freely drained
felsic Basement rock mainly comprises unaltered quartz grains, gibbsite (Al2Si2O5(OH)4 or
more rarely halloysite (Al2Si2O5(OH)4.2H2O) derived from the plagioclase feldspars, kaolinite
(Al4Si4O10(OH)2) and minor amounts of smectite: Figure 29 illustrates an empirical
distribution of the mineral fractions down such a profile. The weathered zone has a high
micro-porosity and retains sufficient permeability for to allow recharge and drainage of
weathering products.
Figure 30
Figure 30: Calculated stripped ion content ratio derived form typical silicate minerals. The
compositions are expressed as mole ratios relative to HCO3- (after R. Garrels, 1967).
With the objective of predicting water quality, R. Garrels (1967) calculated the proportions of
ions in waters resulting from the weathering of the indicated silicate minerals to kaolinite as
shown on Figure 30. The compositions employed for biotite, pyroxene and hornblende are
typical those of igneous rocks. Thus, the major ion hydrochemistry of groundwater flowing
through the weathering front is largely predictable with high concentration of calcium,
magnesium and sodium as the main cations and a low concentration of iron and aluminium.
Bicarbonate is the dominant anion: The draining groundwater will have a pH in the range of
6.5 to 8.5 with a higher pH capable of supporting higher dissolved amorphous silica levels
(M. L. L. Formoso, 2006).
29
Basin Groundwater Hydrology
Image 1: Dambo landforms in the headwaters of the Cubango River located ca 8km South
of Chinhama, Angola (Google Earth Image centred on 130 09' 33" S, 160 27' 40" E, elevation
1624m).
Characteristic dambo landforms on the African Erosion Surface underlain by granitic
Basement rocks are shown on Image 1. The term dambo is restrictively used to describe
open, treeless, grass-floored, seasonally flooded valleys that lack of a clearly defined flow
channel or gully. Hydrogeologically, dambos are associated with confined or closed
weathering front aquifer and deep groundwater flow as shown on Figure 31.
The chemical weathering processes associated with the formation of dambos is focused on
the removal of the most mobile cations, calcium, magnesium and sodium from the bottom of
the saprolite profile by the deep groundwater flow.
Figure 31
Figure 31: Schematic section of the integrated dambo model showing the three main
morphological components, the recharge window, the confined saprolite aquifer zone under the
interfluve and the dambo with the underlying gibbsite rich flow artery. Also shown are the
geometry of the collapse zone and the springs discharging from the superficial aquifer. Grey
saprolite zone indicative of high smectite content and very restricted water movement. The deep
groundwater flow under the dambo floor discharges at a terminal dambo spring.
30
Basin Groundwater Hydrology
The soils overlying the water divides and interfluves of weathered crystalline Basement are
dominantly free-draining ferrasols with an effective porosity of around 40%, a field capacity
of 250 to 330mm/m and a wilting point of 165 to 210mm/m. They have a high infiltration
capacity, often over 75mm/hr (NCSR 1971) and resist erosion other than the etchiplanation
processes as described by E. J. Wayland (1933), B. Willis (1936) and J. Büdel (1982). This
process allows for slow slope wash of sands down the interfluve towards the dambo floor
where they form a wedge of residual colluvium along the break of slope.
Geomorphologically, therefore, denudation from dambos catchments is less than 4m/Ma.
The distribution of the leptosols is limited to the crystalline felsic Basement outcrop in the
Tsoilo Hills west of the Delta and over the Demara meta-sediment outcrops of the Ghanzi
Ridge. This distribution reflects the more arid prevailing climate conditions. The mean annual
precipitation is less than 600mm. The Ghanzi Ridge is a topographic high standing around
300m above the surrounding plains. The leptosols are the product of mechanical weathering
processes with only superficial chemical weathering of the feldspars and ferro-magnesium
silicate accessory mineral surfaces. Transported under arid and semi-arid conditions, these
mechanically derived soils provide the source of future arkosic sandstones.
Apart from minor intercalated carbonate horizons, the moderately metamorphosed arkoses
and shales of the Demara Group shared a similar depositional environment to the Karoo
Group and under humid or sub-humid conditions could be expected to be subject similar
chemical weathering to that shown for the sedimentary formations of the Karoo Group
shown on Figure 9.
Active recharge and groundwater flow promoted by free drainage of the geological
formations are essential for chemical weathering to be effective over extended periods of
time. This implies that the weathering of the Karoo Group formations shown on Figure 9
must have occurred during intervals when these conditions were met. Despite the Ecca and
the Beaufort formations being laid down under deltaic, fluvial and lacustrine conditions, the
survival of the minerals susceptible to chemical alteration (Figure 30) points to limited
chemical weathering during and immediately after the deposition of these formations. The
Ntane formation comprises largely continental arkosic sandstone and is buried under the
sub-aerially extruded Stormberg basalt flows. These two formations have been substantially,
but not completely chemically weathered.
The weathering of the Karoo formations must have taken place during the last accelerated
denudation phase in the late Cretaceous that led to the formation of the African Erosion
Surface and prior to the deposition of the first Kalahari Group formations. The groundwaters
circulating in the Ecca, Beaufort and Ntane formations would have been essentially calcium,
magnesium and sodium cation and bicarbonate anion rich, and could provide the main
source of the calcrete forming minerals. Likewise secondary weathering of the smectites
means circulating groundwaters exposed to the Stormberg Basalts would have been silica
rich and could provide the source of the silcrete forming minerals.
Remote from the modern Basement outcrops, the re-working and deposition of material from
the older formations as the Kalahari Group, has eliminated the bulk of the minerals susceptible
to chemical weathering. The resulting weakly developed quartz rich, base-ion and nutrient poor
sandy arenosols are, therefore, the dominant soils in the Okavango Basin.
Surveys of natural vegetation cover in Central and Southern Africa show a close correlation
between the occurrence of the arenosols, ferralsols and leptosols soils and the distribution of
characteristic "miombo" woodland (Figure 32). The typical grasses and trees of the miombo
woodland are totally adapted to the base-poor, residual sandy soils developed by the in-situ
weathering over the crystalline Basement or derived from clastic sediments sourced from the
crystalline Basement and the certain Karoo sedimentary formations.
31
Basin Groundwater Hydrology
That the deeply rooted miombo woodlands and the sandy soils of the Okavango Basin
evolved together over a significant period of geological time when climatic conditions have
fluctuated within the broad range is a major factor in their current distribution. The long-term
climate has, and does, range from semi-arid to humid with precipitation varying between
550mm and 1,500mm and an annual temperature fluctuating between 15oC and 25o C. More
significantly has been the long-term division of the annual climate into clear cut wet and dry
seasons. Variations in the density and prevalence of the deeply rooted miombo woodlands
under present climatic conditions are subject to investigation under the International
Geosphere-Biosphere Programme (IGBP) Kalahari Transect Project (K. K. Caylor and I.
Rodriguez-Iturbe, 2004 and R. J. Scholes, P. G. H. Frost and Y. Uhong Tian, 2004). Below
an elevation of around 1000m, the miombo woodlands give way to "mopane" woodlands
across the sandy Karoo and Kalahari soils where groundwater levels are closer to the
surface.
Figure 32: Distribution of Miombo
Woodlands in Central and
Southern Africa, from F White
(1983) quoted in A. Malmer and
G. Nyberg (2008)
Figure 32
The remaining soil classes have much more restricted occurrences closely tied to their
geological or geomorphological setting as follows:
· Calcisols are associated with the extensive calcrete occurrences along the margins of
the pans or "dry or fossil" river valleys;
· Cambisols are thin residual restricted to the Demara Group outcrops on the arid
Botswana-Namibia border;
· Fluvisols occur along the course of the perennial surface water courses in the middle
Cubango and Cuito Rivers and in the Okavango Delta;
· Luvisols are associated with the chemical weathering of the surface outcrops of
Stormberg Basalts and are rich in 2:1 clays.
· Regosols are poorly developed and occur over the colluvial material found along the
courses of the ephemeral rivers draining the high ground on the Botswana-Namibia
border.
· Solonchaks are the strongly saline soils associated with the Makgadikgadi Pan.
32
Basin Groundwater Hydrology
3
THE GROUNDWATER OCCURRENCES AND QUALITY IN THE
OKAVANGO BASIN
The tectonic framework, the distribution of the broad lithological divisions set out in Box 1
and the description of the late Tertiary and Quaternary evolution provide most of the
necessary geological background required for the definition of the main hydrogeological
provinces found within the Okavango Basin.
The main aquifers are clastic consolidated and unconsolidated formations with high primary,
inter-granular porosity in the Karoo System and Kalahari Group formations. In addition, there
are locally important aquifers in the Demara metasediments typified by the karst dolomites
around Grootfontein, the Tosoilo and Aha Hills and in the colluvial outwash aprons and
valleys fronting the crystalline Basement highlands.
In line with the surface water division of the Okavango Basin in to active and inactive
hydrological systems based on the whether the sub-basins support perennial or non-
perennial river flows, the first order division of the groundwater occurrences is based on the
link between recharge potential and depth to groundwater plus consideration of the
groundwater quality.
3.1 Potential
Aquifer
Recharge
Aquifer recharge is the net infiltrating water that moves from the land surface or the
unsaturated zone to the saturated zone. Recharge can be expressed as a millimetre
equivalent of precipitation or as cubic metres of flow. There are three main natural aquifer
recharge mechanisms "Direct" or diffuse recharge, "Indirect" or concentrated recharge and
"Underflow".
The four environmental factors controlling aquifer recharge are:
· Climate that includes the intensity, duration and volume of the precipitation;
· The hydrogeological influences that include the geomorphology, geology and
pedology of the land surface;
· The surface water run-off regime and ;
· The vegetation cover/land use.
Direct or diffuse recharge describes the process where precipitation passes vertically from
the ground surface through the unsaturated zone to the water table. Direct annual recharge
mirrors the rainfall pattern and is very variable. Based on mean annual values, in the tropics
the cut-off value for meaningful direct recharge is 600mm. Below this mean value, direct
recharge is erratic and unpredictable. Above 600mm recharge can be expected to rise
exponentially, become predictable and above around 1200mm, the available aquifer storage
can become a factor as to whether recharge is accepted or rejected.
Indirect aquifer recharge occurs wherever runoff takes place. This runoff can be generated
locally or come from a remote upstream source. Indirect infiltration of runoff is the dominant
recharge mechanisms in semi-arid and arid regions where rainfall is limited to three to five
storms a year with individual storms lasting only few hours. In addition, it has been
33
Basin Groundwater Hydrology
recognised that 5 per cent of the storms cause over 50 per cent of the stream flow and that
approximately 15 per cent of the floods produce 90 per cent of the total stream flow.
Underflow recharge is used to describe large-scale lateral groundwater transfers between
regional aquifers.
The scope for the aquifer recharge in the Okavango Basin, therefore, revolves around the
distribution and amount of precipitation, the infiltration capacity of the soils and the
geological setting of the aquifers. The annual rainfall is generally limited to a few months and
the year is divided into clearly defined wet and dry seasons. Under the prevailing arid and
semi-arid climate across much of the Okavango Basin, the main differences in the rainy
season are its duration and the number of rain days. As the climate becomes drier, rainfall in
the wet seasons tends to become more variable in both quantity and distribution. Many of
the rainfall events are of relatively short duration and highly localised making it inherently
difficult to sensibly quantifying any hydrologically response particularly when the analysis is
based on mean daily, monthly or annual data. This particularly applies when trying to
establish aquifer recharge.
As the need to monitor the hydrochemistry of precipitation has emerged to help elucidate
infiltration and aquifer recharge, differentiation between the cyclonic or convection origin of
the rainfall becomes particularly relevant in the semi-arid zones that receive storms spilling
over from neighbouring humid zones. In these zones, frequently less than 25 percent of the
rainfall may come directly from the oceans. The remaining 80 percent of the precipitation
originates as recycled evapo-transpiration.
Figure 33 shows a preliminary distribution of aquifer recharge mechanisms and potential for
recharge events to occur based on a subjective appreciation of numerous worldwide
recharge assessments: The exceptional recharge to the karstic dolomite aquifers in the
Otavi Mountains around Grootfontein is has not been incorporated in this Figure.
Figure 33: Preliminary
distribution of potential aquifer
recharge in the Okavango
Basin based on subjective
consideration of mean annual
precipitation. No allowance is
made for influent seepage from
the main rivers (based on
Department of Environmental
Affairs (Botswana), 2006).
Perennial rivers in mid blue,
seasonal and ephemeral in
light blue.
Figure 33
34
Basin Groundwater Hydrology
Box 1: Okavango Basin - simplified hydrogeological classification of with basic properties and characteristics
Geological Formation
Water
Aquifer
Geomorphological setting Dominant
Large (L), medium (M)
bearing
Characteristi
recharge
and (S) small (localised)
properties
cs
mechanism
scale examples
Unconsolidated
Colluvial outwash deposits , mountain
Poor -
Mountain front
Indirect >>
(S)
front scree, talus.
excellent
direct
Primary,
Alluvial sand, deltaic sand
Fair
Deltas, valley floor
Indirect
(L) Okavango Delta (M)
inter-
moderate
floodplains
(~influent)
Cubango and Cuito River
granular
Valleys
porosity
Interbedded alluvium, sand, silt, clay
Poor fair
Valley floor floodplains
Indirect
(M) Cubango and Cuito
and
(~influent)
River Valleys
permeabilit
Aeolian sand, loess
y
Fair good
Dunes, blanket cover
Direct >
(M) Northern basin areas
indirect
Aquicludes, clay, clayey sands & silts,
Very poor
River backwater, flood
None
marls, calcareous mud.
poor
plain, lagoon
Consolidated
Conglomerate, arkose
Poor fair
Direct indirect (M) Ntane and Ecca
Group
Sandstone Primary
Sedimentary basin
Direct indirect (M) Ntane and Ecca
secondary
Group
Interbedded sandstone, siltstone and
Secondary
Sedimentary basin, rift
Direct indirect (M) Ecca, Beaufort Group
mudstones
valley floor (E and S
Africa)
Siltstone
Secondary Poor fair
(M) Ecca, Beaufort Group
Limestone and Dolomite (marble)
Secondary
Sedimentary
basins,
Direct - indirect (S) Calcretes, Transvaal
, Karst
elevated massifs
Group
(L) Otavi Mts.,
Grootfontein
Crystalline Rocks
Extrusive basalt, lava flows
Primary
Fair to
Incised plateau,
Direct
(L) Stormberg Basalts
35
Basin Groundwater Hydrology
secondary excellent
indirect
Granites, Basement Complex (fissured),
Secondary Poor
Mountain, erosion scrap
Indirect
Localised across
low and medium grade metamorphic
moderate
direct
Basement outcrop
Palaeozoic rocks
Basement Complex Weathering Front
Secondary Fair -
Continental peneplain
Indirect
(S) Upper Cubango Basin
moderate
direct
Aquicludes
Unweathered, massive Basement
Secondary Very poor
Throughout Basement
outcrop
Mafic dyke swarms
Secondary Very poor
Okavango Dyke Swarm
36
Basin Groundwater Hydrology
3.2
Depth to Groundwater
Figure 34 shows the depth below ground of the groundwater rest water level for the main
aquifer horizons in the Okavango Basin. The broad configuration of the deep groundwater
levels mirrors the potential recharge and suggests a first order hydrogeological division of the
Basin based on a line following the southern margin of the Ghanzi Ridge that extends
eastwards to the south of the Makgadikgadi Pan as shown on Figure 35.
Figure 34: Depth below ground
level to the groundwater rest water
level for the main aquifer horizons
in the Okavango Basin (adapted
from Department of Environmental
Affairs (Botswana), 2006 and
Mendelsohn, J.M. & el Obeid, S.
2004 with additional extrapolated
data for Namibia from H. Klock
and P. Udluft, 2002).
Figure 34
Apart from under the Eiseb Graben, to the north of this line the depth to groundwater is less
than 40m and there is a potential for aquifer recharge that points to active groundwater
circulation. South of this line, the depth to groundwater is predominantly over 60m and, under
current climatic conditions the aquifers receive little or no recharge.
3.3 Groundwater
Quality
As a virtually closed drainage system and with high evaporation losses, the ground and
surface waters of the Okavango Basin will be steadily accumulating dissolved solids as they
move downstream.
The natural sources of dissolved solids in the ground and surface waters are entirely from the
weathering of the exposed and buried bedrock formations and from the mineral content of the
rainfall. There are no hydrothermal springs recorded within the Basin that could supply deep,
mineralised, juvenile groundwater. (There are hot springs outside the Basin near Huambo
and at Kasane.)
Given an expected annual chemical weathering denudation rate of the open groundwater flow
system defined in Figure 28 of around 0.02 to 0.04mm for the Pre-Cambrian crystalline
Basement occurrences in upper Cubango headwaters the resulting groundwater baseflow total
dissolved solid (TDS) content will be in the order of 300 to 400mgl. The lower calculated
denudation rate is not entirely outside the ranges modelled for silicate chemical weathering by
C. S. Riebe, et al. (2001). The higher rate conforms with the 0.04 mm/year measured in Puerto
Rico by B. F. Turner, R. F. Stallard and S. L. Brantley, (2003), R. C. Fletcher, H. L. Buss and S.
L. Brantley (2006), S Brantley, et al., (2006), M.I. Lebedeva, R.C. Fletcher, V.N. Balashov and
S.L. Brantley, (2007) H. L. Buss, et al., (2008) and S. Brantley (2008). The pH of the
groundwater increases from around a pH5 where the recharge enters the aquifer to a pH over
8.5 as the weathering processes reach a chemical equilibrium.
37
Basin Groundwater Hydrology
Provisional modelling of the closed dambo landforms shown on Figure 30 point to a lower annual
denudation rate of 0.017 to 0.034mm from the shallow groundwater flow system that discharges
onto the dambo floor from the interfluves: This points to a shallow interfluves groundwater TDS
content of around 100 to 200mgl. The deep weathering front groundwater flow will have a similar
TDS content to the open system. These chemically weathering denudation-rates align with the
0.01 0.5 mm/year as calculated from the silica fluxes in the world's ten largest rivers by G. E.
Hilley and S. Porder (2008).
As indicated, the Karoo and Kalahari sedimentary formations are largely derived from denuded
Pre-Cambrian Basement rocks and the metasediments of the Demara Group that were
chemically weathered to a greater or lesser extent depending on the prevailing climatic
conditions during transportation and deposition. The under humid conditions, the chemical
weathering processes steadily released the more active metallic elements and anions as
indicated in Figure 30. Rapid mechanical and fluvial transportation associated with the humid to
and semi-arid conditions prevailing during the deposition of the Karoo Group formations, notably
the Ecca and the Ntane Formations, allowed for the survival of the more chemically resistant
sodium and potassium feldspars that characterise arkosic sandstones. These arkosic
sandstones have been deeply weathered since deposition as shown on Figure 9. Across much
of Kwenang in the Ecca formations contain unconfined groundwater with TDS in the order of 400
to 600mgl (J. L. Farr and S. S. D. Foster, 1978, B. Th. Verhagen,1995 and A. A. Aganga, C. M.
Tsopito and K. More, 1997). Although this is slightly higher than that expected from weathered
crystalline Basement rocks, the Ecca groundwaters receive little if any modern recharge and are
considered to date back to ca 12,000 years BP (J. J. De Vries, J. J. and M. Von Hoyer, 1988).
Experience elsewhere of modern Karoo groundwater quality under humid conditions suggest
lower groundwater TDS with values often well below 100mgl (CCKK, 1982).
The weathering and depositional history of the Kalahari Group stripped of the most mobile ions
and left the formations composed of essentially inert residue quartz, gibbsite and goethite. The
very low active mineral content results in groundwater TDS values of less than 100mgl. At
Mongu in Zambia, the TDS levels recorded from the Kalahari Sands of the Zambezi floodplain
are less than 50mgl.
The igneous Stormberg Basalts, the mafic dyke swarm and the kimberlite pipes, however, are
highly susceptible to chemical weathering. This is reflected by the distribution of the base rich
luvisols and solonetz soils associated with the Stormberg Basalt occurrences in eastern and
central Botswana and the strongly saline solonchaks in and around the Makgadikgadi Pan that is
terminal drainage point for the runoff from these occurrences. Chemical weathering of the
Stormberg Basalts strips the ultramafic biotites, pyroxenes and hornblendes of the mobile ions
that are transported by ground and surface water flow to the Pan leaving the residual of active
smectite clays to accumulate in the soils. As no well- developed groundwater occurrences are
associated with these igneous rocks, the bulk of the chemical weathering products accumulate
at the surface and are removed by surface runoff and interflow.
38
Basin Groundwater Hydrology
4 HYDROGEOLOGICAL
PROVINCES OF THE OKAVANGO
BASIN
Figure 35 shows a provisional distribution of main hydrogeological provinces identified using
the proceeding criteria. The Cubango-Cuito Basin, the Omatako and Eiseb Basins and the
Ghanzi Block are upstream of the Okavango Delta and the Makgadikgadi and Central
Kalahari Blocks lie downstream of the Delta.
Figure 35: The Okavango Basin
hydrogeological provinces.
Perennial rivers in mid blue,
seasonal and ephemeral in light
blue.
Figure 35
4.1
The Cubango-Cuito Basin1
The lack of most forms of groundwater data means the resource assessment of these
catchments has to be subjectively based on experience from similar hydrogeological
provinces. Figure 36 shows the superficial geology of the Basins with the crystalline
Basement outcrops underlying the almost the entire headwaters and upper basin of the River
Cubango as far downstream as Caiundo and forming a clearly defined hydrogeological
terrain. The lower Cubango and the the Cuito Basins are dominated by Kalahari Group
formations. While ample examples of Basement groundwater occurrences are available from
Zambia, Zimbabwe, Malawi and Tanzania, the only comparable Kalahari basins with a similar
climate are in Western Zambia, northern Angola and DR Congo but these again lack
groundwater data.
1 Unless otherwise noted basin areas and mean runoff values for this section are taken for the FAO Okavango
River Basin Transboundary Diagnostic Analysis (TD) Final Draft Report prepared in 1999.
39
Basin Groundwater Hydrology
Figure 36: The superficial geology
of the Cubango and Cuito Basin.
Figure 36
The Groundwater Resources of the Basement in the Upper Cubango Basin
Despite the lack of factual groundwater data, the surface water flow record for the River
Cubango at Caiundo (Figure 37) coupled with satellite imagery enables a number of pertinent
observations to be made regarding the groundwater occurrences in the crystalline Basement:
· With a catchment area of 38,486km2, the mean 8 year (1963-1970) annual runoff of
5.716km3 or 148mm is equivalent to 13.4% of the mean annual catchment rainfall of
1106mm (OKACOM, 2009). The basic graphical quantification (Appendix 1) of
baseflow contribution from the 11 year record shown on Figure 37 varies from a low of
0.75 km3 in 1972 to a high of 3.09 km3 in 1969. The mean 11 year baseflow
represents around 37.5% of the total mean annual catchment runoff and 5% of the
OKACOM (2009) mean annual rainfall.
Figure 37
Figure 37: Hydrograph for the River Cubango at Caiundo with baseflow separation shown in
red and dry season anomaly highlighted in yellow (adapted from D.A. Hughes, et al., 2004).
· Direct translation of the baseflow to recharge gives the following annual values:
Year 196
196
196
196
196
196
196
197
197
197
197
3
4
5
6
7
8
9
0
1
2
3
Baseflow
km3/a
3.08 2.56 2.55 2.28 0.98 2.76 3.09 2.88 1.52 0.75 1.15
Baseflow/
80 67 66 59 25 72 80 75 39 19 30
recharge mm
· Although these recharge values are low compared to the regional estimates with
similar annual rainfall as shown on Table 1, this comparison is of limited validity as
40
Basin Groundwater Hydrology
most of the values cited in this Table are for very small experimental catchments of
less than 20km2. However, there is scope for similar empirical analysis of the flow
records from upstream hydrographic stations. These analyses would also benefit from
acceptable estimates of mean precipitation over the associated catchment areas.
· The groundwater hydrographs for 1966-1971 from observation wells at Kabwe,
Central Zambia (Figure 38) indicate how recharge is more dependent on the rainfall
distribution and antecedent soil moisture conditions than the absolute seasonal total.
M. Owor, et al. (2009) demonstrate in more detail this correlation between rainfall
intensity and Basement aquifer recharge in Uganda.
Location Rainfall
Estimat
% of
Method Reference
mean
ed
mean
mm
recharg
rainfall
e mm
Aroca
1400
200
14.2
Soil moisture
R. G. Taylor and K.
Catchment,
balance
W. F. Howard,
Uganda
(1996)
Livulezi, Malawi
~ 1140
145
12.7
Baseflow
E. P. Wright (1992)
114
10
separation
234
20
Chloride balance
Chloride balance
Bua, Malawi
~ 940
14
1.5
Baseflow
E. P. Wright (1992)
188
20
separation
Chloride balance
Diamphe, Malawi ~ 850
75
8
Baseflow
E. P. Wright (1992)
97
11.4
separation
152
17.9
Chloride balance
Chloride balance
D28, Zimbabwe
~ 890
80
9
Baseflow
E. P. Wright (1992)
80
9
separation
115
14
Chloride balance
Chloride balance
Kabwe, Zambia
Miombo
924
80
8.7
Soil moisture
J. F. T. Huston
woodland
281
30.0
balance
(1982)
Cropped farm
Soil moisture
land
balance
Nyatsime
~ 799
131
16.4
GW level data
B. Mudzingwa*
catchment,
~ 795
130
16.3
Chloride balance
(1999)
Zimbabwe
~ 796
74
9.3
Reservoir method
~ 900
162
18.0
Flux analysis
Marondera
~ 900
136
15.1
Chloride balance
M. P. McCartney*
Grassland
(1998)
Research
~ 900
185
20.5
Chloride balance
M. Jarawaza
Catchment,
~ 863
190
22.0
GW level data
(1999)
Zimbabwe
Chiweshe,
~ 900
71.6
4.1-7.9
Baseflow
R. Mjanja* (2000)
Mazowe,
separation
Zimbabwe
Table 1: Published recharged estimates for weathered Basement aquifers in southern and
central Africa (* Source K. M. Sankwe, 2001).
· The baseflow separation shown on Figure 37 and the hydrograph for observation well
`T' on Figure 38 shows river flow and groundwater levels rising (highlighted in yellow)
in response to reduced evapotranspiration losses in the cooler dry season.
41
Basin Groundwater Hydrology
Figure 38
Figure 38: Groundwater levels recorded in Observation Borehole A blue (Water Affairs
Department, 47K-1) and Well `T' red. Weathered Schist and monthly rainfall between 1966
and 1971 at Kabwe, Zambia (re-drawn from M. J. Jones and K. D. Töpfer 1972).
The early vegetation flush of the miombo woodland is considered to reduce the soil moisture
levels close to wilting point towards the end of the dry season (September-October).
Depending on the temporal and intensity distribution of the precipitation, there can be a six to
twelve week delay after the wet season starts before recharge is registered across the
weathered crystalline Basement aquifers.
Image 2: Crystalline Basement terrain in the Upper Cubango showing the juxtaposition of
dambo and normal headwater drainage basins across the African Erosion Surface west of
Chinhama (Google Earth Image centred on 120 45' 35" S, 160 38' 22" E, elevation 1721m).
Image 2 typifies the crystalline Basement terrain in the upper Cubango Basin and points to a
fairly uniform distribution of the weathered profile aquifers. Groundwater should be reliably
available within the normal constraints associated with these aquifers. The productive
weathered profiles should be less than 70m thick and the optimum well depth should be less
than 100m. Rest water levels should be within 20m of the surface and well yields will be in
the 2-10m3/hour range. The groundwater quality will be generally potable with an EC of less
than 750 µSm although the local geology may result in limited occurrences of unacceptable
iron, manganese and fluoride levels.
42
Basin Groundwater Hydrology
The humid climate of the Upper Cubango Basin should be taken into account when seeking
comparisons from similar weathered zone Basement aquifers: This makes northern Zambian
and Malawi data more appropriate for comparison than examples from Zimbabwe. In
addition, when assessing climate change impacts, it should be noted that the weathered
profile aquifers remain robust and productive under the sub-humid and semi-arid climate of
southern Zambia and Zimbabwe.
Hydrologically, the surface water runoff regime will be largely predictable from the weathered
Basement terrain. The high infiltration capacity (>75mm/hr) of the soils over the recharge
windows and interfluves means very little, if any, runoff is generated over these area during
most storm events. The main sources of surface runoff from normal storm events will be
generated over rock outcrops and the clayey dambo and regular valley floors.
The Groundwater Resources of the Kalahari and Karoo Groups in the Lower Cubango and
the Cuito Basins above the Mukwe Hydrographic Station
The six year surface water flow record (1966-71) for the River Cuito at Cuito Cuanavale
(Figure 39) shows the high baseflow contribution to the total flow. The River Cuito is clearly
strongly effluent and is support by continuous groundwater inflow above and downstream of
Cuanavale. Although a number of abstraction boreholes exist within the catchment, no
hydrogeological information is available to assist with an assessment of the groundwater
resources. However, the following observations can be made:
Figure 39: Hydrograph for
the River Cuito at Cuito
Cuanavale with baseflow
separation shown in red and
dry season anomaly
highlighted in yellow
(adapted from D.A. Hughes,
et al., 2004).
Figure 39
· With a catchment area of 15,857km2 (TWINBAS, 2007), the 5 year (1966-1970) mean
annual runoff of 3.162km3 or 199mm is equivalent to 18.5% of the mean annual
catchment rainfall of 1073mm (OKACOM, 2009). The basic graphical quantification of
the baseflow contribution for the 5 year record shown on Figure 39 varies from a low
of 2.23km3 in 1967 to a high of 3.395 km3 in 1970. The mean 5 year baseflow
represents around 73% of the total mean annual catchment runoff and 17.4% of the
OKACOM (2009) mean annual catchment rainfall.
· The subdued wet season surface flood flows reflect low topographic gradients and
high retention of rainfall in the soil, subsoil and the underlying geological formations.
· Direct translation of the baseflow to recharge gives the following annual values:
Year
1966 1967 1968 1969 1970
43
Basin Groundwater Hydrology
Baseflow km3/a 2.305 2.23 3.415 3.475 3.395
Baseflow/
145 141 215 219 214
recharge mm
· The impact of the lowering of the cooler dry season evapotranspiration losses in the
baseflow recession is clearer than that seen in the hydrograph for the upper Cubango
at Caiundo. This indicates increased dry season groundwater flow from the interfluves
to the rivers and illustrates vigorous groundwater circulation. The predominance of
miombo woodland cover on the interfluves indicates that the top few metres of the soil
and subsoil profiles will be at, or close to, wilting point and will be receptive to
infiltration at the beginning of the wet season.
· The rapid throughput of groundwater under the interfluves can clearly seen on Image
3. This shows large areas of apparently natural open grassland straddling or close to
the main surface water divides in the upper catchments of the Cuito Basin and the
spring fed ponding of groundwater in the valley floor: Box 2 sets describes these
geomorphological features and it distribution in some detail.
The main unknowns in this hydrogeological province hinge on the limited geological and
hydrogeological data that is available for the Lower Cubango and Cuito catchments where
active groundwater recharge supports the essential baseflow to the Okavango River runoff.
Information on the nature of the groundwater occurrences, the depth to water and the yield of
wells from the records of previous well drilling and testing programmes has not been located.
Karoo System formations outcrop to the north and northeast of the Upper Cuito-Kasai-
Kwando water divides but the stratigraphy and lithology are largely undefined (Figure 6). The
assumption, however, must be that aquifer horizons similar to those found in the other Karoo
Basins will be present: Even if this is prove the case, the quality of the deeper groundwaters
may be unacceptable.
Image 2: Open grassed areas straddling the Cuito-Kasai Basin water divide with
groundwater fed lake at the head of a Cuito tributary valley Google Earth Image from
August 2007 centred on 120 39' 55"S, 180 21' 28"E, ground elevation 1493m asl.
44
Basin Groundwater Hydrology
The Kalahari Group sediments are reportedly over 300m thick but again stratigraphic and
lithological details are poorly reported and again it must be assumed that the groundwater
bearing properties of the formations must be similar to those reported from the Ovambo
Basin in Namibia.
As a potential aggregate groundwater resource, the Karoo and Kalahari Group formations are
expected to be reasonable rather than prolific. Groundwater should be widely available but
individual well yields are expected to be in the range of 2 to 10 l/sec.
The strongly effluent rivers that drain the Kalahari Group and Karoo System in the Lower
Cubango and the Cuito Basins indicate the groundwater storage of the aquifer formations will
be fully saturated for most of the water year. The rainfall-infiltration-recharge balance
suggests that given heavy groundwater abstraction, the hydrologic system would replenish
the aquifers. Equally, downstream where the effluent groundwater discharge to the rivers
reverses and the river becomes influent, groundwater abstraction close to the rivers will
induce artificial recharge from the river to the wells.
A further outstanding question concerns the thickness of the active groundwater flow zones:
It is possible that it extends less than 100m below the ground surface under the river beds. It
could extend down to more than 250m if there are well developed Karoo System sandstone
aquifers running under the basins. If this is the case, there is a slight potential for artesian
groundwater to occur in the lower reaches of both basins. The trellis drainage pattern will
capture groundwater under the interfluves that flow laterally parallel to the main river valley. A
comprehensive regression analysis of the baseflow hydrograph recorded at Mukwe would
provide approximate estimation of the volume of groundwater held in storage in the Cubango
and Cuito Basins.
The longitudinal profiles for the Cubango and Cuito Rivers (Figure 40) provide
geomorphological evidence of the past tectonic history of the basins. The three breaks in the
Cubango profile above Caiundo point to base level changes since the uplift of the Biè swell
around 30Ma. While the convex profiles of the intervening reaches are similar to dambo
interfluve profiles, the convex nature could be a function of the drainage pattern and
accelerated erosion as tributary rivers join the main valley. Further research should enable
correlation between the dating of the triggering of the base level changes and depositional
changes in the Kalahari Group sedimentation.
The upper Cuito profile mirrors the Cubango but the nick point some 40km upstream of the
confluence of the two rivers suggests Late Pliocene Pleistocene capture of the Cuito by the
Cubango.
The suggestion that the trellis drainage pattern is a sign of a youthful Cuito drainage system
(page 37 - J.M. Mendelsohn and S. el Obeid, 2004) breaks down when viewed against trellis
drainage pattern seen around Katako Kombi in the DR Congo (Box 2, Image B2-1) and nick
points along the Cuito profile. The trellis pattern could be a residual following the E-W dune
lines seen around Vila Nova da Amada or function of the low structural strength of the soil
and Kalahari Sand profiles (Image 1).
45
Basin Groundwater Hydrology
Figure 40
Figure 40: Longitudinal profiles of the Cubango and Cuito Rivers (based on J.M.
Mendelsohn and S. el Obeid, 2004).
While some groundwater data is available for the Lower Cubango Basin below the Cubango,
equivalent data from the Lower Cuito Basin remains to be established but there is every
reason for assuming close similarities between these two areas. The merged flow from the
two Basins forms the Okavango River above the Dealta and is measured at several sites.
The 1960-1974 record of flow Mukwe is shown on Figure 41.
46
Basin Groundwater Hydrology
Box 2: Hydrological-land cover-soil configurations associated with Basement
Complex rocks in the Congo Pedicle (DRC) to the east of Ndola (Zambia)
The following are extreme examples in the debate whether groundwater recharge is greater
under forested or open grassland areas. Both examples have a significant spatial distribution
across Central and Southern Africa. Geology, geomorphology and, to a lesser extent climate
have a key role in their distribution.
The extensive open grasslands cover of
the Pleistocene superficial alluvial deposits
along the water divides near Katako Kombi
(Photo B2-1) in Kasai-Oriental Region
(DRC) have a very high infiltration capacity
capable of adsorbing daily rainfalls in
excess of 70mm without water-logging. In
common with many land-cover units, these
open grasslands are readily mapped using
satellite imagery (Image 3-1). The
superficial deposits are several tens of
metres thick and are freely draining. The
water table is deep despite mean monthly
rainfall totals between 45mm and 200mm.
The sandy soils are sufficiently fine to
support a good perennial grass cover. As
Photo B2-1, Landscape near Katako Kombi,
can be seen in Photo 3-1, tree cover on the
Kasai Oriental, DR Congo, 1983
water divides is limited to a few scatter
clumps of acacia. In contrast the sharply
incised valleys are densely forested. The
alluvial deposits are underlain by less
permeable Karoo and Cretaceous
formations and the contact between the
formations is marked by a well defined line
of perennial springs. Further south and into
Angola, the Kalahari Beds replace the
superficial alluvial cover along the water
divides but the extensive geographical
distribution of nearly identical open
grasslands and heavily forested valleys
persists.
In contrast (Image B2-2) shows a reversal in the hydrological-land cover-soil configuration
associated with the Basement Complex rocks in the Congo Pedicle (DRC) to the east of
Ndola (Zambia). The water divides are moderately forested and the valley floors are
characteristically wide open dambos. The heavy black expansive clay vertisols of the dambo
floors are completely treeless, and despite relatively steep longitudinal gradients, they have
no, or only a poorly defined water course for much of their length. Readily water logged
during the rainy season, sheet flow is the dominant runoff mechanism. The valley slopes
have characteristic heavily-leached, sandveld soil profiles with a shallow, perched water table
that discharges by slow seepage along the dambo margins. The deeper groundwater flow
through the weathered Basement Complex aquifer zone that underlies both the dambo and
the valley slopes, discharges at lower end of the dambo and is marked by the emergence of
a clearly defined stream channel. The geographical distribution of dambos is exclusively
associated with crystalline Basement Complex rocks, the well established peneplain erosion
surfaces and a humid tropical climate with a mean annual rainfall in excess of about 450mm.
The term dambo, however, is frequent misapplied to waterlogged topographic lows
associated with limestone and dolomite outcrops like the Itawa Dambo (near Ndola) and the
47
Basin Groundwater Hydrology
Chambishi examples in Zambia (P. Hadwen and M. J. Jones 1971, C. van der Heyden and
New, M. G., 2003). In Tanzania the term mbuga is used to describe not only true dambos but
almost all occurrences of black expansive clay vertisols and low-lying, swampy valleys.
Zambia
DCR
Image B2-1, Satellite Image (Google Earth)
Image B2-2, Dambos in Congo Pedicle,
West of Katako Kombe, DR Congo
DCR, due East of Ndola, Zambia, (Google
Earth).
48
Basin Groundwater Hydrology
Figure 41
Figure 41: Hydrograph for the Okavango River at Mukwe with baseflow separation shown in
red (adapted from D.A. Hughes, et al., 2004).
The following observations can be made regarding the Mukwe hydrographic record:
· With a catchment area of 226,236km2 (TWINBAS, 2007), the 13 year (1960-1973)
mean annual runoff is 9.5843km3 or 42mm rainfall equivalent (TWINBAS, 2007). The
basic graphical quantification of the baseflow contribution for the 13 year record
shown on Figure 41 varies from a low of 4.58km3 in 1960 and to a high of 8.73km3 in
1963.
· Direct translation of the baseflow to recharge gives the following annual values:
Year 196
196
196
196
196
196
196
196
196
196
197
197
197
0
1
2
3
4
5
6
7
8
9
0
1
2
Baseflo
4.5
5.4
8.7
7.2
6.4
6.6
5.9
7.7
6.8
6.0
4.9
6.5
7.9
w km3
8
3
3
2
2
5
3
4
9
5
5
Baseflo
w
20 24 29 39 32 28 29 26 35 34 30 27 22
recharg
e mm
· The strong baseflow component of the Okavango River as recorded at Mukwe below
the Cubango-Cuito confluence is shown on Figure 41. Comparison of the combined
annual baseflows in the River Cubango at Caiundo and the River at Cuito Cuanavale
with the River Okavango baseflow at Mukwe suggests that the groundwater
throughput across the lower reaches of the Cubango and Cuito is largely in balance.
The continual reworking of the geological formations in the basin accounts for the low mineral
content of the active groundwater flows in the Cubango-Cuito Basin sediments. The
geological history of the clastic sediments of multiphase re-working has stripped the mobile
metallic ions from the mineral assemblage and this has virtually removed the possibility of
active 2:1 clays from the unconsolidated alluvium, colluvium and soils. The major
environmental and development issues with the groundwater occurrences are their high
susceptibility to anthropogenic contamination and their high contribution to the surface water
flows in to the Delta.
4.2
The Omatako and Eiseb Basins
Although topographically separate river basins with different discharge points, for the
purposes of this hydrogeological report the Omatako and Eiseb Basins are considered
together. Basement felsic cratonic rocks and metasediments belonging to the Demara Group
49
Basin Groundwater Hydrology
outcrop along the northern regional water divide. East-west trending outcrops of the Karoo
System occupy the Waterberg and Eiseb Basins that disrupt the Basement and Demara
outcrops along the western and southern water regional divides. Eastwards, the Basement,
Demara and Karoo rock outcrops pass under thickening sediments of the Kalahari Group
except where isolated Basement and Demara outliers form the Tsoilo and Aha Hills.
The crystalline Basement and Demara metasediments do not have a thick weathered mantle
cover and the groundwater occurrences are limited to the mountain front and valley-fill,
colluvial, outwash sediments and the fissure and fracture zones in the underlying bedrocks.
The depth to groundwater in these areas is generally less than 40m below ground. Where
well-developed, the limestone and dolomite formations in the Demara metasediments form
important local groundwater occurrences around Grootfontein and in the Tsoilo and Aha Hills
areas.
The nature of groundwater occurrences in the Karoo System in the Waterberg Basin are
large unrecorded but the depth to groundwater near the regional water divide is deep (>40m)
but becomes shallower (<40m) eastwards. The hydrogeological mapping of the area
(Hydrogeological Map of Namibia, 2001) indicates the Karoo formations along the northern
side of the Waterberg Basin have a high inter-granular aquifer potential. Due to the thick
Kalahari Group cover (<up to 300m) the Karoo System in the Eiseb Basin has not been
investigated.
Apart from the Tsoilo and Aha bedrock outliers, the entire central and eastern areas of the
Omatako and Eiseb Basins are underlain by a continuous blanket of Kalahari Group
sediments that are from 50m to over 300m thick. The groundwater potential over most of the
Eiseb Basin is mapped as very limited and away from the ephemeral water courses, the
groundwaters are essentially saline (C. Stadtler, et al., 2005). The prevailing depth to
groundwater reflects the topography. The groundwater levels are deeper under the high
ground in the western half of the Basins and shallower approaching the Okavango River and
the Delta. Similar hydrogeological conditions are considered to extend to the east of the
Namibia-Botswana border towards the Delta.
The groundwater under the interfluves of the Omatako River and tributaries is reported as
saline.
Although the Basement rock outcrops and the storm intensities and frequency in the upper
catchment of the Omatako River are sufficient to support runoff events to support urban water
supplies from Omatako Dam, there is no record of any surface water flows from either basin
reaching the main Okavango River or the Delta. Similarly there are no indications of
significant groundwater underflow from the Basins to the Okavango River or Delta.
The distribution of potable groundwater reflected by the distribution of rural and urban water
supply boreholes in the Namibian catchment areas (Figure 42), however, points to the
importance of indirect recharge from storm flood events in the surface water courses in the
middle and lower catchments. H. Wanke (2005) reports on the use of the chloride balance
method to investigate diffuse recharge across the Omatako and Eiseb Basins as tabulated on
Figure 42. The validity of the low diffuse recharge rates where groundwater levels are more
than 10 to 15m below ground is examined in the section on the Central Kalahari Block.
Recharge to the limestones and dolomites around Grootfontein is reported on by G. Schmidt
& D. Plöthner (2000) and H. Klock and P. Udluft (2002) used remote sensing to augment
recharge evaluations to provisionally quantify the regional aquifer recharge and the
groundwater balance for the Omatako and Eiseb Basins. They estimate an annual
abstraction of 50 to 100Mm3 from the approximate 1230 water supply boreholes shown on
Figure 42 and calculate the annual regional recharge to be in the order of 140 to 730Mm³.
This is equivalent to an annual recharge of 0.9 to 4.5 mm assuming uniform distribution.
50
Basin Groundwater Hydrology
As the main focus of future water resource developments will be concentrated along the
Okavango River between Katwitwi on the Angola-Namibia Border and Mohembo at the head
of the Delta, the groundwater occurrences in part of the Okavango Basin should be closely
monitored and investigated due to the vulnerability of the Kalahari Group aquifers to over-
exploitation and susceptibility to contamination.
Figure 42: Distribution of water supply
boreholes in the Okavango Basin
within Namibia (from L. Veríssimo,
2009). Shows borehole sites following
river courses in the middle and lower
catchments of main water courses.
Chloride mass balance diffuse
recharge determinations (from H.
Wanke, 2005). Sites shown in red on
map.
Location Mean
Diffuse
rainfall recharge
mm
mm
North
420 0.42
Omatako
Omatako Vlei
440
3.70
Figure 42
Omatako
440 0.86
Sand
Etemba
410
9 23
The extension of the Eiseb Basin towards the Delta is largely dominated by saline
groundwaters that make any pockets of fresh groundwater equally vulnerable and requiring
careful exploitation.
4.3 Ghanzi
Block
Running some 350km, the ~50 km wide SWS ENE axis of the Ghanzi Ridge rises some
300m above the surrounding Kalahari Basin. Formed of strongly folded clastic and minor
carbonate metasediments of the Demara Group, apart from the thin limestones, these
formations are unlikely to contain significant deep groundwater. The exploitable groundwater
storage will be restricted to the colluvial outwash aquifers deposited in local depressions in
the Kalahari superficial cover or along the valley floors (Figure 43 and 44) that are likely to
be underlain by fracture or fissure zones that have been accentuated by structural unloading
during the denudation of the land surface. The colluvial deposits were derived by the
mechanical erosion and transport of the metasediments: These will be recharged by during
flood flows associated with local or more extensive, semi-annual, storm events (G. Tredoux
and S. Talma, 2007).
51
Basin Groundwater Hydrology
Figure 43: A cattle-post water point sited
in a loosely closed topographic depression
with the corralled cattle waste clearly
viable clustered around the water point.
Located 25 km NE of Ghanzi, Botswana
(Google Earth Image centred on 210 34'
22" S, 210 50' 29", elevation 1140m).
Figure 43
The extent of the flood recharge was highlighted by the death of some 200 head of cattle due
to acute nitrate poisoning traced to the groundwater supply boreholes after an exceptional
rainfall event in 2000. C. Colvin, et al. (2008) report nitrate levels of 14 to 508mgl in samples
collected in October 2000 from 13 boreholes along the Ghanzi Ridge. They identify the
source of the build-up of nitrates to the concentrated cattle ranching activity in the areas
around the supply boreholes. They also report the decline in the nitrate levels from 509mg in
October 2000 to 45mgl in November 2004. This supports a very localised pollution plume
model associated with a restricted contamination source located close to the contaminated
water point.
Figure 44: East Hanahai, Botswana, cattle watering points located in a well defined
ephemeral river valley forming part of the Okwa drainage system. Clear surface accumulation
of cattle waste visible, Google Earth Images, ground elevation of valley floor, 1077m .
Figure 44
A similar build up of nitrate levels in groundwater was noted at Lubbock, Texas where the
Lubbock Land Application Sites (LLAS) was first used in 1925 to dispose of around 3,800
m3/day of partially treated waste water over 80 ha using furrow and boarder irrigation.
Lubbock is a major meat processing and packing centre. The problematical build up of
nitrates in the unsaturated zone under the LLAS was first documented in 1968. The
background level in groundwaters under the two sites averages 16mgl against the maximum
permissible level of 10mgl. After the LLASs were formally cited by the State environmental
52
Basin Groundwater Hydrology
agency in 1989, clean up measures involving reduced irrigation applications and changes in
cropping from grains to fodder marginally improved the nitrate levels but groundwater in wells
to the south of the eastern LLAS was found contaminated. Direct grazing of the fodder by
cattle was also found to have no or little effect on the soil nitrate levels.
C. B. Fedler, et al. (2003) report on effectiveness of the irrigation over the LLAS in the de-
nitrification of the wastewater. They conclude that the de-nitrification processes are only
partly understood, but consider it is highly dependent on soil moisture, temperature, soil type
and carbon content. Removal rates ranged from zero to 80% of the nitrates in the applied
water: The soil carbon content acts as an electron donor and significantly enhances the de-
nitrification process.
The nitrate problem along the Ghanzi Ridge is focused by the valley and depression floor
setting of the groundwater occurrences and is further reinforced by the low natural de-
nitrification potential of the thin base ion and carbon poor soils and subsoils.
Hydrogeologically the nature of the thin, poorly-developed soils and the granular nature of the
colluvial aquifer makes tight sanitary sealing of the borehole annulus uncertain and it is
possible that the borehole - formation interface may provide a major pathway for infiltrating
surface water runoff.
C. Colvin, et al. (2008) show that the water supply borehole nitrate problem is not confined to
the Ghanzi Ridge but is widespread across southern Africa where it has been investigated
around Serowe (S. Stadler, 2005 and J. Meier, T. Himmelsbach and J, Böttcher, 2008). Here
the source for the nitrate build up in the soil profile is concluded to be the result of natural
biomass processes: An anthropogenic origin was dismissed.
Despite the topographic prominence of the Ghanzi Ridge its annual contribution to the
surface and groundwater resources of the wider Okavango Basin under the current climatic
regime is marginal.
4.4
The Okavango Delta
With the comprehensive records of borehole drilled in the Delta (Figure 45) and the
substantial volume of surface and groundwater information, reports and publications, this
report considers two main points:
· The definition of the usable aquifers and their resource potential.
· The groundwater transfer between the Okavango River input at the head of the Delta
and the discharge zones to the south east along the Thamalakane Fault, towards the
River Linyati and Savute Channel in the north east via the Seinda Spillway and
possibly to the south west towards Lake Ngami.
Groundwater development for the Maun municipal water supply and for the towns and
villages surrounding the Delta coupled with formal registration of water supply boreholes
shows the fresh aquifer horizons to be unconfined or semi-confined, fine to medium grained
sands lying within the top 120m of the Kalahari Group filling the Okavango Graben. The
variability of the alluvial sediments can be seen on the borehole composite logs shown on
Figure 46.
Depending on the topographic location, the groundwater levels lie within 3 to 20m below the
ground surface. A persistent shallow sand unit lying within 18m of the surface is identified
and contains good quality groundwater. Below 18m, rapid lateral and vertical changes in the
alluvial fill result in a complex multilayered groundwater occurrence as shown on Figure 47.
The water quality can also change rapidly from fresh to brackish to saline down to around
120m below which no fresh groundwater lenses are considered to occur.
53
Basin Groundwater Hydrology
Extensive pumping tests show borehole yields to be equally variable with a maximum of
around 10l/sec and the mean to be 2 to 3l/sec: Interpreted transmissivity values range from 4
to 30m2/day. The shallow groundwater occurrences of the Kalahari Group are reliably
recharged by effluent seepage from the Okavango flows depending on the scale of the
seasonal floods.
Figure 45: Okavango Delta -
borehole location map of boreholes
recorded by the Botswana Water
Affairs Department (adapted from
TWIBAS, 2007)
Figure 45
As seen in the Cubango and Cuito Basins, definition of the active groundwater flow zones
under the Delta lies at the core to understanding of the groundwater circulation and
resources. The geometry of the tectonic structure under the Delta dictates that all
groundwater flow must take place through the Kalahari Group sediments but no well
developed continuous sand horizons have been located within the multilayered formations.
The ubiquitous salinity of the groundwaters below 120m points the elimination of deep
groundwater circulation under the Delta.
Figure 46
Figure 46: Typical composite borehole logs prepared for the Maun Water Supply study
54
Basin Groundwater Hydrology
(adapted from Mangisi, N., 2004).
The relationship between the surface water low flows in to the Delta and the groundwater
regime is considered to be in broad balance. The limited aquifer storage leads to the rejection
of excessive flood flows and the surface water hydrological response is an increase spill into
the Boteti River with exceptional floods spreading to Lake Ngami in the west and the Mababe
Basin to the east (Figure 25). The high surface water seasonal groundwater recharge is
largely lost to evapotranspiration during the low flow months.
If there is large-scale transfer of groundwater towards from the head of the Delta and
downstream
towards the River Linyati and Savute Channel in the north east via the Seinda Spillway it is
not obvious.
Figure 47
Figure 47: Schematic cross-section of the Kalahari Group sediments in the Okavango
Graben showing the multilayered aquifer horizons ( based on C. Milzow, et al., 2009 using
data collected during the Department of Water Affairs investigations for the Maun Water
Supply)
Equally groundwater contour mapping around the Delta has not been located. Such mapping
would establish if the Delta lies at the centre of a groundwater mound with potential radial
flows to the surrounding Kalahari formations and the older alluvium as suggested by T.S.
McCarthy (2006) and C. Milzow, et al. (2009), or if the Delta is the focus of groundwater flow
from the Eiseb Basin to the west. The latter mechanism is substantially ruled out by the depth
to groundwater and the hydrochemical zoning shown on Figure 48.
55
Basin Groundwater Hydrology
Figure 48: Groundwater levels and
hydrochemistry around the
Okavango Delta. The prevalence of
sulphate rich groundwaters and the
steep gradient hydraulic west of the
Delta may be directly linked to the
underlying bedrock surface and
geology (from T.S. McCarthy,
2006).
Figure 48
4.5 Makgadikgadi
Block
Lying to the southeast of the Thamalakane- Kunyere Faults, the centre of the Makgadikgadi
Block occupies a topographic depression that holds the Ntwetwe and Sowa Pans that are the
terminal drainage points for all the surface and potential groundwater flows in the Okavango
Basin.
The Ntwetwe Pan is directly linked to the Okavango Delta surface water overflows via the
Boteti River. The Sowa Pan is the focus of annual transient surface water flows from the
Ntata River that drains the eastern water divides between the Okavango, Limpopo and
Zambezi Basins. The ephemeral Letlhakane River connects the Ntwetwe and Sowa Pans.
Under current climatic conditions no distant surface water flows are expected to reach the
Makgadikgadi Block from the Central Kalahari or Ghanzi Blocks. Under past more humid
conditions the terminal pans merged and expanded to cover much of the older alluvial
occurrences shown on Figure 24. Although the regional groundwater gradients are towards
the Pans, the actual groundwater transfer appears extremely limited.
Extensive surface and airborne geophysical and geological exploration surveys have
established the tectonic structure of the Makgadikgadi Block. Narrow outcrops of crystalline
Basement and Karoo System formations form the regional water divide between the
Okavango and Limpopo Basins and the centre of Makgadikgadi Basin is underlain by up to
300m of Kalahari Group sediments. The Basement and Karoo formations are cut by the
closely spaced 179Ma Jurassic Okavango dyke swarm that follows pre-existing rift structure
trends. Subsequent movements associated with the formation of the 30Ma swells and basins
and the East African Rift System have broken the underlying geology into a series of faulted
blocks that follow the same structural weaknesses as shown on Figure 49. The Karoo surface
was clearly heavily eroded prior to the deposition of the Kalahari Group formations and the
modern groundwater chemistry has been closely linked to the underlying Karoo formation (S.
Stadler, 2005).
56
Basin Groundwater Hydrology
Figure 49
Figure 49: Schematic E-W geological section in the Orapa Area (adapted from I.F. Blecher
and R. A. Bush,1993 quoted in B. Th. Verhagen, 2003).
Away from the regional interfluves, the top of the saturated groundwater horizons lie within
the Kalahari Group formations. Along the Boteti River and around the Ntwetwe and Sowa
Pans where groundwater occurrences are within 20m of the surface, evapotranspiration
losses can be expected to be high.
To varying degrees, the deeper groundwater south of the Thamalakane- Kunyere Faults is
saline except where seasonal recharge events associated with the Delta overflows takes
place along the Boteti River as illustrated on Figure 50. Further airborne EM surveys along
the Boteti River (D. Sattel and L. Kgotlhang, 2004) indentify low conductivity signature zones
between Rakops and Lake Zau as shallow sand lenses containing fresh groundwater within
the Kalahari Group formations.
Figure 50: Airborne electromagnetic
(AEM) map superimposed on Landsat
view of the Okavango Delta. Red and
yellow tones indicate high salinity
groundwater and blue and green tones
represent fresh groundwater (From C.
Milzow, et al., 2009 and data from
Campbell et al., 2006)
Figure 50
Similar localised fresh groundwater lenses occur along the main ephemeral water courses
that drain to the Ntwetwe and Sowa Pans. The fresh groundwater lenses frequently occur as
perched aquifers within the Kalahari Group formations (S. Stadler, 2005) and are exploited in
the dry valleys by hand-dug wells. The main groundwater developments, however, are
centred on the Orapa and Letlhakane Diamond Mines and on scattered cattle watering
points.
Wellfield abstraction for the Orapa and Letlhakane water supplies and the dewatering
operations at the mines began in 1960 and have modified the regional SE-NW groundwater
57
Basin Groundwater Hydrology
flow as shown on Figure 51. The 2003 wellfield abstraction was 8.9Mm3 and the dewatering
4.1 Mm3. The wellfields large draw groundwater from the Ntane Sandstone that is considered
locally to be in hydraulic continuity with the overlying Stormberg Basalts and the Kalahari
Group formations (B. Th. Verhagen, 2003).
Figure 51: Groundwater
contours (m asl) of the Orapa
and Letlhakane are for 2002
(from S. Stadler, 2005).
Figure 51
Although the groundwater contours shown on Figure 51 do not reflect the complex block
faulting of the Karoo Group formations, S. Stadler (2005) characterizes the groundwater
chemistry on the basis of the bedrock geology with sulphate rich groundwaters associated
with the chemical weathering of the Stormberg Basalts and withy the siltstones and clays of
the Mosoltsane formation. This implies a degree of differential groundwater flow and
compartmental groundwater occurrences. The sulphate rich waters associated with the
Stormberg Basalts also provide the source of the sodium carbonate deposits mined from the
Sowa Pan.
The hydrogeological investigations undertaken to support Orapa and Letlhakane water
supplies include evaluations of recharge and chemical quality (B. Th. Verhagen, 2003 and S.
Stadler, 2005). Little consensus exists over the role of direct diffuse recharge to the main
Ntane Sandstone aquifer zones but the potential for annual indirect recharge from ephemeral
stream flow is high given a mean annual rainfall of between 400 and 600mm. The fractured
and fissured nature of the Stormberg Basalts is generally considered to provide preferential
flow paths for infiltration and isotope and water balance studies indicate the global recharge
reaching the regional water table at around 2% of the annual rainfall.
The impact of this volume of recharge may be enough to sustain regional groundwater flow
as indicated on Figure 52. The on-going groundwater level monitoring may well show the
validity of this possibility. However, for current groundwater resource planning purposes,
even the highest recharge estimates are considerably below the gross groundwater
abstraction.
58
Basin Groundwater Hydrology
Figure 52
Figure 52: Schematic hydrogeological section from Serowe to Orapa showing potential for
groundwater flow and relative recharge (from S. Stadler, 2005).
Groundwater developments in the Ntane Basin and eastwards from the Sowa Pan are
currently limited to cattle posts and rural communities and the priority for systematic study of
the groundwater resources is low compared to the priorities driven by the development
demands elsewhere in the Makgadikgadi Block.
4.6
Central Kalahari Block
In contrast to the other groundwater provinces within the Okavango Basin, the semi-arid
climate and relatively deep groundwater levels means the Kalahari Group formations lie
above the regional water table across most of the Central Kalahari Block. The main
groundwater occurrences are found in the Ecca Group Kweneng Sandstone, the Ntane
Sandstone and the Stormberg Basalt belonging to the Karoo System: These formations
provide the sole source of water for the economically important cattle ranching industry.
Figure 53: The distribution of the
main Karoo System fromations in
Central Southern Baotswana
(adapted from J. L. Farr et al.
1981).
Figure 53
In line with the depth to the water table, there are no visible groundwater discharges at the
surface from these aquifer formations across this groundwater province. The groundwater
quality in these formations varies from good close to the past recharge areas to saline where
the aquifers are totally confined. Figure 53 shows the Karoo System subcrop under the
Central Kalahari Block.
59
Basin Groundwater Hydrology
Figure 54
Figure 54: Botswana Geological Survey GS10 study blocks. Jwaneng wellfield section shown
on Figure 58 (adapted from J. H. Whitelaw, J. L. Farr and S. S. D. Foster, 1977).
Between 1976 and 1981, the UK funded Geological Survey of Botswana GS10 project
undertook an assessment of the groundwater resources that largely concentrated on the
Karoo System aquifers within the Central Kalahari Block. The main area covered for detailed
study was a 11,300km2 Block 2 to the west south of Letlhakeng were the geological results
from a 70 hole coal exploration drilling programme were augmented by the GS10 well drilling
and testing (Figure54). The Geological Survey held records of some 220 water supply wells
drilled in the area: At the time of the GS10 survey, only 120 wells were located on the ground
and were in use for watering cattle. The GS10 project proved the Ecca Group Kweneng
Sandstone to be the main aquifer in Block 2.
Since 1980, significant urban and mining groundwater supplies have been developed within
Block 2. The Molepolole urban water supply is taken from a wellfield located around
Matlagatse to the northwest on the Letlhakeng Road. The geological cross-section (Figure
55) shows the favourable geometry of the Kweneng Formation subcrop to maximise storage
and recharge under more humid conditions with the less permeable shales and siltstones of
the Beaufort Kwetla formation probably in position to force excess groundwater flow into the
surface water drainage system.
Figure 55
Figure 55: Geological cross section along Molopole-Letlhakeng Road (adapted from A.
Gieske, E. T. Selaolo and H. E. Beekman, 1995 with additional data from R. T. Chaoka, et al,
2006).
60
Basin Groundwater Hydrology
Under present climatic conditions, however, the potential and processes of aquifer recharge
are limited. Attempts to quantify diffuse recharge using rigorous hydrochemical and isotope
investigations have produced conflicting results as can be seen from the studies in the
Matlagatse and Letlhakeng areas by the GS10 project (S. S. D. Foster, 1978) and the follow-
up Groundwater Resources Monitoring and Recharge Study (GRES) reported on by A.
Gieske, E. T. Selaolo and H. E. Beekman (1995) and E. T. Selaolo, et al., (2003).
Figure 56 shows seasonal changes in soil moisture determine by neutron probe logging by
the GRES project in the Letlhakeng area. The GS10 conclusion was that diffuse recharge
was undetectable while the GRES project identified diffuse recharge rates of a few
millimetres. Allowing for the adaption of the natural Kalahari vegetation to the semi-arid
climate and the high percentage of fine silty sand in the soil profile, the wilting point saturation
level will be towards or even below the low end of the accepted normal range of 5 to 16% by
volume (see Box 3).
Figure 56
Figure 56: GRES temporal soil moisture changes at the Maipatlelo site (redrawn from E. T.
Selaolo, et al., 2003).
The results from multiple parameter analysis of samples from a 41.7m deep, augered hole at
Maipatlelo near Letlhakeng are shown on Figure 57. Assuming a mean density of 1.9 to 2.2
for the soil column, the soil moisture values by weight can be adjusted to volumetric values in
the range of 2 to 25%: The saturation levels shown on Figure 56, therefore, are at, or below
wilting point from 5m downwards. The parameter profiles, however, do show anomalies that
indicate past recharge events. Although these anomalies can be tied to lithological variations
in the geological profile it is felt that more consideration should be given to the periodic floods
that occur in the "fossil" valleys.
At Letlhakeng in 1978 a maximum flood level during the 1967 flood down the Meratswe and
Gaotlhobogwe Valleys (Figure 55) was marked some 40cm up the Post Office Wall: This was
several metres above the nearby valley floors. A similar flood effecting over 120 households
occurred in June 2009 after storm event with over 100m of rain falling within 24 hours. D. J.
Nash (1996) records various qualitative descriptions of historic floods that can be considered
potential aquifer recharge events. The fate of the flood waters is seldom reported, but they
are dispersed by infiltration in to the unsaturated soils and subsoil zones with some runoff
ultimately reaching the local unconfined water table, if it exists, and the rest being returned to
the atmosphere by evapotranspiration. Excess flood waters may reach the terminal
Deception Pan that forms the focus of the Central Kalahari drainage system but most is
expected to be temporarily ponded and lost by open-water evaporation along the course of
the dry valleys.
61
Basin Groundwater Hydrology
Figure 57: Composite multiple
parameter log for the 41.7m
deep, augered hole at
Maipatlelo near Letlhakeng
(redrawn from E. T. Selaolo, et
al., 2003).
Figure 57
The complex dynamics of the three phase (gas, fluid and solid) nature of the unsaturated
zone suggest a more active role for preferential flow paths than diffuse mechanisms for the
downward transmission of recharge through the profile. On this basis the limited recent
recharge identified by B. T. Verhagen (1995) in the Jwaneng Wellfield area SW of Letlhakeng
is possibly feasible.
The absence of groundwater discharge coupled with the limited recent recharge means the
evaporation and transpiration losses from the top of the saturated formations as the driving
mechanism accounting for the depth of the groundwater levels. Determined using lithium
chloride as a tracer, tree rooting depths are reported to reach below 53m by M. Keeletsang
(2004) and 70m by O T Obakeng 2007. As is seems unlikely that the roots would extend
downwards through an unsaturated formation with only hydroscopic water available, its
seems probable they follow preferential recharge pathways that given the reported solution
effects (J. L. Farr and S. S. D. Foster, 1978) could be could be enhanced by precipitation
stem flow. If the net annual evapotranspiration losses are in the order of 2 to 4mm, they
would be sufficient to account for a 30 to 50m decline in groundwater levels over a period in
excess 10,000 years. As these losses will be fairly uniform over the entire Kalahari Karoo
subcrop, the pre-existing groundwater gradients could be preserved although possibly in a
modulated form.
62
Basin Groundwater Hydrology
BOX 3: Soil Moisture, Infiltration and
ranges from 2% to over 70%. In general the
Recharge under Arid and Semi-arid
coarser grained and better sorted the soil, the
Conditions
lower the field capacity. Soils with field
capacities over 55% generally have a high
Within a porous soil profile four theoretic
humus content. Between field capacity and
steady-state conditions can be reached. First
full saturation, only the largest pores are
total dryness (inherently requires zero
likely to contain soil gas and these may be
atmospheric humidity), second hydroscopic
discontinuous, therefore the movement of
saturation (water trapped by molecular
water vapour is very much curtailed and
attraction to soil particles requires soil gas
gravity controls the main movement of water
to be at 100 percent humidity), thirdly field
as a liquid. Water, however, may be removed
capacity and fourthly fully saturated (all pore
from the profile by evaporation, transpiration
space saturated). The capillary fringe above
and gravity drainage. Below the water table,
a water table occupies a relatively stable
gravity dominates all movement although
status between the third and fourth steady
density differences cause by thermal
state conditions. Between the total dryness
gradients can distort the flow pattern. Water
and hydroscopic saturation all water
is removed by evaporation, transpiration, and
movement in the soil profile must be as
lateral outflow.
vapour. The water can only be removed by
evaporation. The percentage volume of
The movement and volume of both water
hydroscopic moisture is very small and
vapour and liquid in unsaturated profile is
relates to the soil grain size distribution and
subject to physical size and shape of pore
the moisture film thickness is < 0.5 x 10-3 mm
openings and the presence of preferential
(Fig 1).
flow paths. The same physical properties
control the water bearing properties below
the water table. Experimental work in
southern France by Chaptal (1932) quoted by
Gottman (1942) and Hills (1966) was
modelled on the traditional Middle Eastern air
well system for atmospheric water harvesting.
This system and the "solar-still" survival
technique rely on temperature differentials
and relative humidity to achieve the
condensation of water. The basic maximum
water availability can be calculated from the
simplified values given below:
Ambient Air
Saturated Humidity (*RH
Temperature Relative Humidity =
o C
100%)
Figure 1: Relationship between soil water film
Partial Pressure
thickness and moisture tension. (Source:
(mb)
g/m3
PhysicalGeography.net)
Between the hydroscopic and field capacity
10 11.97
9.6
states, the water may move through the soil
20 22.89 18.4
profile as vapour or as a liquid with capillary
30 41.58 33.4
tension acting as the main driving potential
and gravity only playing a minor role. The
Thus cooling one cubic metre of saturated air
capillary forces may be exerted in any
from 30o C to 20o C will yield 15 gram of water
direction and water may be removed by both
and from 20o C to 10o C will yield 8.8 gram.
evaporation and transpiration. There is,
Extracting water from a soil profile will
however, a bottom limit (wilting point) to the
obviously have a lower yield as only the void
ability of plants to suck water from the soil;
spaces will contain air and the air is not
nominally this corresponds to a moisture
necessary completely saturated if the profile
content of between 5% and 16% by volume.
is below the hydroscopic saturation point. Soil
The moisture content of soils at field capacity
microclimate measurements under desert
63
Basin Groundwater Hydrology
conditions, however, indicate that the top of
dew point* 11o C) and soil gas humidity a few
the mid-day fully saturated soil atmosphere
centimetres below the surface (ca 90% at
(and hence the hydroscopic saturation point)
25oC dew point*
is usually found at < 2.0m below ground.
18o C).
Much field, laboratory and mathematical
*RH = 100%
research has been undertaken to quantify all
aspects of the proceeding observations as
part of investigations in to infiltration and
aquifer recharge. For example, Kitchen et al.
(1980) report on a project in Cyprus that
successfully employed physical soil moisture
determinations, lysimeters, chloride balance
and isotope profiling investigations to
determine recharge to coastal aquifers. The
recommendation concerning the
appropriateness of the chloride balance
profiling technique for investigations in
remote semi-arid regions with deep
watertables and thick arenaceous cover, has
lead to its widespread application across the
Kalahari in Southern Africa: Foster, et al.
(1982), Gieske (1992), Beekman, et al.
(1994, 1996, 1997 and 1999), Selaolo (1998)
and Selaolo, et al. (1994 and 1995) report on
various recharge research projects
undertaken in Botswana while similar projects
elsewhere in Africa, Australia, SE and E Asia,
and the US have been reviewed to develop a
global synthesis of groundwater recharge in
arid and semi-arid regions (Scanlon, et al.
2005). The African recharge estimates are
from 1mm to 20mm. with the lowest rates
recorded in areas with deep watertables
(>25m.).
The validity of recharge estimates below
5mm and possible as high as 20mm is
speculative as no account appears to be
given to the upward movement of water
vapour through the profile to replace the
evaporation losses from the top 2m of the
profile. The mobility of soil gases associated
with hydrocarbon pollution plumes has been
extensive studied (Scanlon, 2002) and the
seasonal desiccation to depths of more than
6m of stiff bentonite clays have been
recorded in Texas (Simpson, 1934 quoted in
Meinzer, 1942). Without considerably more
data to prove the contrary, it is felt that there
is a significant movement of water vapour up
the unsaturated profile to replace the
evaporation water losses at the landsurface
even over a deep watertable. The potential
for the evaporation loss is borne out by the
effectiveness of the solar still and comparison
of the air relative humidity (ca. 30% at 30oC
64
Basin Groundwater Hydrology
Reported absolute isotope values and ratios largely support the proceeding observations.
Figure 58
Figure 58: Jwaneng Wellfield geological section (adapted from D. Buckley, 1984 and J. H.
Whitelaw, 1978)
The aquifer properties determined for the Ecca Group formations in the Jwaneng wellfield area
are shown on Table 2 and imply that the groundwater occurrence ranges from unconfined at
site W6 on Figure 58 to totally confined by the overlying Beaufort Group sediments at site W17.
Elsewhere in study block 2 the Ecca Sandstone is largely semi-confined: This again restricts the
opportunity for recharge to reach the aquifer horizons in the Ecca Group subcrop.
Site Rest
Water
Test
Transmissivity Storage Permeability Effective
water
inflows
yield
(T) m2day
(S)
K mday-1
porosity
level m
(Q)
%
bgl
l/sec
W6 65
75-90
7.5
490
2x10-3
0.59 n.a.
9.1
620
1x10-2
W13 60 95-115
10.5
680
3x10-4
0.09 13.3
W14 68 95-115
5.1 320
3x10-4
W16 59 80-100
10.5
190
6x10-5
W17 51 80-115
19.0
1100+
3x10-4
0.14 18.6
7.0
620
7x10-4
W18 50 135-160
12.8
450
1x10-4
W20 56 90-110
10.5
130
7.2
100
1x10-3
Permeability and effective porosity values - laboratory determinations 0.02 12.4
for core samples.
Table 2: Jwaneng Wellfield well testing and aquifer properties (adapted from J. L. Farr and S.
S. D. Foster, 1978).
Annual abstraction from the Jwaneng wellfield in 2008 was reported around 5.5Mm3, dewatering
at the mine extracts 3.7Mm3 and drainage from the slimes dams yields a further 3.6Mm3 (M.C.
Brook, 2009). Maximum drawdowns of 5m across the wellfield after 14 year abstarction were
considerably less than the 30m predicted by the initial aquifer model (M.C. Brook, 1990 and B.
T. Verhagen, 1995). Subsequent modelling calibration incorporated increased leakage and
recharge rather than increasing storage on the basis of isotope data that was interpreted to
indicate significant modern recharge. The hydrochemical analyses used, however, were from
stratigraphically mixed pumped samples from abstraction wells and this may affect the integrity
of the isotope findings.
Investigation of the Ecca Group aquifers in the GS10 study blocks 1 and 4 remains limited but in
Block 1 the water quality is known to deteriorate as the aquifer horizons become increasingly
65
Basin Groundwater Hydrology
confined and groundwater flows stagnate under the younger formations (C. Cheney, et al.,
2006). Below the Kalahari Group cover, much of Block 4 is underlain by the Stormberg Basalts
that forms provides unpredictable groundwater yields (J. H. Whitelaw, J. L. Farr and S. S. D.
Foster, 1977).
The demand for increased raw sources for the Serowe urban water supply and for the Morupule
Power Station has driven hydrogeological investigation and development of the Ntane
Sandstone Formation in the GS10 study Block 3.
The Block straddles the water divide between the Okavango and Limpopo Basins and the
geometry of the Karoo and Kalahari formations is shown on Figures 59, 60, 61 and 62. At
Serowe, the surface and groundwater divides approximately coincide with a marked
groundwater mound. This points to local diffuse recharge occurring at least across the
Stormberg Basalts outcrops (Figures 60 and 61). Infrared satellite images north and east of the
Paje wellfield indicate widespread active seasonal groundwater seepage along the foot of the
Kalahari Group escarpment (Ecosurv 2009). The Ecoserv configuration of the groundwater
contours point to the area around the Serwe Pan as receiving contemporary recharge: The
groundwater quality provides additional confirmation and the detailed studies around Serowe
reported by J. Meier, T. Himmelsbach and J, Böttcher (2008) date the nitrate rich groundwaters
in the recharge areas as younger than 1Ka. To the west, down gradient confined waters under
the Stormberg Basalts are dated to +15Ka.
Figure 59
Figure 59: The geology, wells and wellfields in the Serowe area.. Hydrogeological sections on
Figures 60 and 61 are shown in mid blue (redrawn from B. T. Verhagen, 1995 and Ecosurv,
2009).
The role of the regional fault pattern and the mafic dykes on the groundwater flow appears
localised but is sufficient to break the aquifer up into a series of poorly interconnected
compartments as shown on Figures 60, 61 and 62.
66
Basin Groundwater Hydrology
Figure 60
Figure 60: Hydrogeological section B-B' (shown on Figure 59) through the Morupule Power
Station Wellfield area (redrawn from Ecosurv, 2009).
The aquifer modelling of the Morupule Power Station groundwater supply boreholes reproduces
these aquifer compartments to assess impact of various abstraction scenarios on other local
groundwater users. Future monitoring should validate the extent to which the faults and dykes
dominate the geometry of the regional drawdowns but Ecoserv (2009) note a 40m head
differential associated with the fault dyke labelled D10 on Figure 59.
Figure 61
Figure 61: Hydrogeological section A-A' (shown on Figure 57) through the Serowe Water
Supply Wellfields (adapted from Swedish Geological Company (SGC), 1988)
While the two cross-sectional interpretations of groundwater levels in the Serowe wellfields area
shown on Figures 61 and 62 highlight the inferred impact of ongoing and increasing abstraction
in amplifying the head differentials across the main fault and dyke groundwater barriers.
Figure 62
Figure 62: Hydrogeological section through the Serowe Water Supply Wellfield after a
decade of abstraction at around 2.5Mm3/year (adapted from J. Meier, T. Himmelsbach and J,
Böttcher, 2008).
67
Basin Groundwater Hydrology
All groundwater resource studies around Block 3 agree that current abstraction greatly exceeds
current recharge. At Serowe, S. Stadler (2005) illustrates the pre-development 1988 and the
2002 groundwater contours (Figure 63) and Figure 64 shows the observed shift in the
groundwater divide in response to the Serowe and Paje wellfield abstraction.
Figure 63
Figure 63: Serowe Block, groundwater contour maps for 1988 (left) and 2002 (right) showing
impact of Serowe Water Supply abstraction (adapted from S. Stadler, 2005).
As the groundwater occurrences are substantially defined, current and future monitoring will
enable a time-scale to be assigned to the sustainability of large-scale abstraction. The thickness
of the saturated aquifer zones will be a critical to in the assessment of long term resources and
demand pressures will have to be met by expansion of wellfields to new aquifer development
areas.
Figure 64
Figure 64: Serowe Block, 1988-2002 shift of the groundwater divide (from S. Stadler, 2005).
These new areas will undoubtedly extend in to be Central Kalahari Block where current
abstraction is large limited to cattle posts and rural settlements.
68
Basin Groundwater Hydrology
5 CONCLUSIONS
The main unresolved questions in the Okavango Basin concern the geology and groundwater
occurrences of the Cuito and lower Cubango Basins.
A limited, closely monitored investigation programme aimed at establishing the stratigraphy,
lithology, water bearing characteristics and hydrochemistry of the Karoo System and Kalahari
Group formations in these Basins should determine whether the groundwater flow system is
superficial as found under the Okavango Delta; of intermediate depth as found in the Serowe
and Makgadikgadi Blocks; or deep as found in Kwenang in the Central Kalahari Block. The
almost complete lack of hydrogeological data for the Cuito and lower Cubango Basins requires
addressing.
It is felt that the prevailing hydrology and hydrogeology of the crystalline Basement occurrences
of Upper Cubango Basin is sufficiently well understood for planning purposes.
The hydrogeology and groundwater resources of the Omatako and Eiseb Basins are poorly
documented. In view of likely further surface and groundwater developments close to the
Okavango River between Katwitwi on the Angola-Namibia Border and Mohembo at the head of
the Delta and west of the Delta in Botswana, attention should be given to expanding the
groundwater database for future planning purposes.
The limited groundwater resources of the Ghanzi Block will naturally restrict future
developments and the current situation is sufficiently well documented for planning purposes.
Future monitoring and investigation of the Delta should aim at completing the contouring of
groundwater levels around the Delta margins.
Driven by mineral exploration and developments, the hydrogeology of the Karoo System in large
areas of the Makgadikgadi and Central Kalahari Blocks has been scientifically investigated and
is reasonably well understood. For future planning purposes, however, when new areas will
need to be opened up for development, steps should be taken to complete groundwater
inventories and to install long term monitoring points.
Finally, more hydrogeological research should be directed to evaluating the role of storm flood
flows in the ephemeral rivers as the source of indirect recharge. The considerable effort that has
been centred on direct, diffuse recharge has failed to distinguish indirect recharge as probably
the main mechanism across the Okavango Basin.
69
Basin Groundwater Hydrology
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Appendix 1
Upper Cubango River at Caiundo, Monthly Baseflow Component
Year/Month 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973
Jan
0.12 0.16 0.03 0.06 0.05 0.03 0.10 0.14 0.05 0.04 0.03
Feb
0.13 0.22 0.06 0.15 0.04 0.20 0.23 0.24 0.05 0.07 0.05
Mar
0.22 0.40 0.18 0.37 0.03 0.44 0.39 0.37 0.20 0.06 0.13
Apr
0.36 0.42 0.45 0.40 0.06 0.48 0.49 0.46 0.30 0.05 0.18
May
0.46 0.42 0.50 0.36 0.17 0.42 0.46 0.41 0.26 0.09 0.20
Jun
0.43 0.32 0.42 0.25 0.18 0.38 0.38 0.37 0.21 0.11 0.17
Jul
0.38 0.21 0.30 0.20 0.14 0.28 0.30 0.29 0.15 0.10 0.14
Aug
0.30 0.17 0.20 0.15 0.11 0.21 0.23 0.23 0.12 0.08 0.10
Sep
0.22 0.12 0.16 0.12 0.08 0.14 0.18 0.15 0.08 0.06 0.06
Oct
0.17 0.08 0.12 0.09 0.05 0.08 0.12 0.10 0.05 0.04 0.04
Nov
0.15 0.02 0.07 0.07 0.04 0.05 0.11 0.07 0.03 0.03 0.03
Dec
0.14 0.02 0.06 0.06 0.03 0.05 0.10 0.05 0.02 0.02 0.02
Total km3 3.08 2.56 2.55 2.28 0.98 2.76 3.09 2.88 1.52 0.75 1.15
mm
80 67 66 59 25 72 80 75 39 19 30
mean 2.15
km3
56mm
River Cuito at Cuito Cuanavale, Monthly Baseflow Component
Year/Month 1966 1967 1968 1969 1970
Jan 0.16
0.17
0.22
0.265 0.28
Feb 0.18
0.17
0.27
0.305 0.33
Mar 0.21
0.18
0.35
0.34
0.35
Apr 0.24
0.20
0.34
0.32
0.34
May 0.24
0.22
0.32
0.31
0.32
Jun 0.23
0.21
0.31
0.3
0.28
Jul 0.19
0.21
0.29
0.30
0.28
Aug 0.18
0.20
0.27
0.28
0.26
Sep 0.18
0.175
0.26
0.27
0.25
Oct 0.18
0.17
0.26
0.26
0.24
Nov 0.18
0.17
0.30
0.26
0.24
Dec 0.18
0.18
0.25
0.27
0.24
Total km3 2.31
2.23 3.42 3.48 3.40
mm 145
141
215
219
214
Mean 2.96
km3 187mm
78
Basin Groundwater Hydrology
Okavango River at Mukwe, Monthly Baseflow Component
Year/Month
1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972
Jan
0.28 0.26 0.34 0.41 0.47 0.34 0.35 0.35 0.34 0.30 0.40 0.34 0.32
Feb
0.26 0.28 0.34 0.45 0.50 0.33 0.38 0.34 0.39 0.28 0.37 0.36 0.30
Mar
0.28 0.33 0.39 0.55 0.55 0.39 0.50 0.37 0.61 0.47 0.41 0.49 0.35
Apr
0.34 0.41 0.50 0.76 0.68 0.50 0.72 0.49 0.97 0.78 0.65 0.65 0.40
May
0.40 0.47 0.66 0.96 0.84 0.70 0.83 0.64 1.10 1.08 0.87 0.74 0.47
Jun
0.46 0.57 0.78 1.10 0.93 0.81 0.80 0.76 1.05 1.07 0.85 0.68 0.57
Jul
0.57 0.63 0.79 1.08 0.83 0.78 0.70 0.74 0.90 0.90 0.76 0.61 0.56
Aug
0.56 0.67 0.72 0.97 0.70 0.70 0.62 0.67 0.75 0.80 0.68 0.56 0.50
Sep
0.46 0.58 0.64 0.82 0.55 0.61 0.53 0.50 0.63 0.66 0.58 0.49 0.45
Oct
0.38 0.46 0.51 0.62 0.43 0.50 0.45 0.40 0.47 0.52 0.50 0.43 0.38
Nov
0.32 0.40 0.42 0.53 0.38 0.40 0.40 0.35 0.37 0.46 0.46 0.37 0.34
Dec
0.27 0.37 0.41 0.48 0.36 0.36 0.37 0.32 0.32 0.42 0.36 0.33 0.31
Total km3
4.58 5.43 6.5 8.73 7.22 6.42 6.65 5.93 7.9 7.74 6.89 6.05 4.95
mm
20 24 29 39 32 28 29 26 35 34 30 27 22
mean 6.54km3
29mm
79
Basin Groundwater Hydrology
The Okavango River Basin Transboundary Diagnostic Analysis Technical Reports
In 1994, the three riparian countries of the Okavango
Diagnostic Analysis to establish a base of available
River Basin Angola, Botswana and Namibia
scientific evidence to guide future decision making.
agreed to plan for collaborative management of the
The study, created from inputs from multi-disciplinary
natural resources of the Okavango, forming the
teams in each country, with specialists in hydrology,
Permanent Okavango River Basin Water Commission
hydraulics, channel form, water quality, vegetation,
(OKACOM). In 2003, with funding from the Global
aquatic invertebrates, fish, birds, river-dependent
Environment Facility, OKACOM launched the
terrestrial wildlife, resource economics and socio-
Environmental Protection and Sustainable
cultural issues, was coordinated and managed by a
Management of the Okavango River Basin (EPSMO)
group of specialists from the southern African region
Project to coordinate development and to anticipate
in 2008 and 2009.
and address threats to the river and the associated
communities and environment. Implemented by the
The following specialist technical reports were
United Nations Development Program and executed
produced as part of this process and form substantive
by the United Nations Food and Agriculture
background content for the Okavango River Basin
Organization, the project produced the Transboundary
Transboundary Diagnostic Analysis.
Final Study
Reports integrating findings from all country and background reports, and covering the entire
Reports
basin.
Aylward, B.
Economic Valuation of Basin Resources: Final Report to
EPSMO Project of the UN Food & Agriculture Organization as
an Input to the Okavango River Basin Transboundary
Diagnostic Analysis
Barnes, J. et al.
Okavango River Basin Transboundary Diagnostic Analysis:
Socio-Economic Assessment Final Report
King, J.M. and Brown,
Okavango River Basin Environmental Flow Assessment Project
C.A.
Initiation Report (Report No: 01/2009)
King, J.M. and Brown,
Okavango River Basin Environmental Flow Assessment EFA
C.A.
Process Report (Report No: 02/2009)
King, J.M. and Brown,
Okavango River Basin Environmental Flow Assessment
C.A.
Guidelines for Data Collection, Analysis and Scenario Creation
(Report No: 03/2009)
Bethune,
S.
Mazvimavi,
Okavango River Basin Environmental Flow Assessment
D. and Quintino, M.
Delineation Report (Report No: 04/2009)
Beuster, H.
Okavango River Basin Environmental Flow Assessment
Hydrology Report: Data And Models(Report No: 05/2009)
Beuster,
H. Okavango River Basin Environmental Flow Assessment
Scenario Report : Hydrology (Report No: 06/2009)
Jones, M.J.
The Groundwater Hydrology of The Okavango Basin (FAO
Internal Report, April 2010)
King, J.M. and Brown,
Okavango River Basin Environmental Flow Assessment
C.A.
Scenario Report: Ecological and Social Predictions (Volume 1
of 4)(Report No. 07/2009)
King, J.M. and Brown,
Okavango River Basin Environmental Flow Assessment
C.A.
Scenario Report: Ecological and Social Predictions (Volume 2
of 4: Indicator results) (Report No. 07/2009)
King, J.M. and Brown,
Okavango River Basin Environmental Flow Assessment
C.A.
Scenario Report: Ecological and Social Predictions: Climate
Change Scenarios (Volume 3 of 4) (Report No. 07/2009)
King, J., Brown, C.A.,
Okavango River Basin Environmental Flow Assessment
Joubert, A.R. and
Scenario Report: Biophysical Predictions (Volume 4 of 4:
Barnes, J.
Climate Change Indicator Results) (Report No: 07/2009)
King, J., Brown, C.A.
Okavango River Basin Environmental Flow Assessment Project
and Barnes, J.
Final Report (Report No: 08/2009)
Malzbender, D.
Environmental Protection And Sustainable Management Of The
Okavango River Basin (EPSMO): Governance Review
Vanderpost, C. and
Database and GIS design for an expanded Okavango Basin
Dhliwayo, M.
Information System (OBIS)
Veríssimo, Luis
GIS Database for the Environment Protection and Sustainable
Management of the Okavango River Basin Project
Wolski,
P.
Assessment of hydrological effects of climate change in the
Okavango Basin
Country Reports
Angola
Andrade e Sousa,
Análise Diagnóstica Transfronteiriça da Bacia do Rio
Biophysical Series
Helder André de
Okavango: Módulo do Caudal Ambiental: Relatório do
Especialista: País: Angola: Disciplina: Sedimentologia &
Geomorfologia
80
Basin Groundwater Hydrology
Gomes, Amândio
Análise Diagnóstica Transfronteiriça da Bacia do Rio
Okavango: Módulo do Caudal Ambiental: Relatório do
Especialista: País: Angola: Disciplina: Vegetação
Gomes,
Amândio
Análise Técnica, Biofísica e Socio-Económica do Lado
Angolano da Bacia Hidrográfica do Rio Cubango: Relatório
Final:Vegetação da Parte Angolana da Bacia Hidrográfica Do
Rio Cubango
Livramento, Filomena
Análise Diagnóstica Transfronteiriça da Bacia do Rio
Okavango: Módulo do Caudal Ambiental: Relatório do
Especialista: País: Angola: Disciplina:Macroinvertebrados
Miguel, Gabriel Luís
Análise Técnica, Biofísica E Sócio-Económica do Lado
Angolano da Bacia Hidrográfica do Rio Cubango:
Subsídio Para o Conhecimento Hidrogeológico
Relatório de Hidrogeologia
Morais, Miguel
Análise Diagnóstica Transfronteiriça da Bacia do Análise Rio
Cubango (Okavango): Módulo da Avaliação do Caudal
Ambiental: Relatório do Especialista País: Angola Disciplina:
Ictiofauna
Morais,
Miguel
Análise Técnica, Biófisica e Sócio-Económica do Lado
Angolano da Bacia Hidrográfica do Rio Cubango: Relatório
Final: Peixes e Pesca Fluvial da Bacia do Okavango em Angola
Pereira, Maria João
Qualidade da Água, no Lado Angolano da Bacia Hidrográfica
do Rio Cubango
Santos,
Carmen
Ivelize
Análise Diagnóstica Transfronteiriça da Bacia do Rio
Van-Dúnem S. N.
Okavango: Módulo do Caudal Ambiental: Relatório de
Especialidade: Angola: Vida Selvagem
Santos, Carmen Ivelize
Análise Diagnóstica Transfronteiriça da Bacia do Rio
Van-Dúnem S.N.
Okavango:Módulo Avaliação do Caudal Ambiental: Relatório de
Especialidade: Angola: Aves
Botswana Bonyongo, M.C.
Okavango River Basin Technical Diagnostic Analysis:
Environmental Flow Module: Specialist Report: Country:
Botswana: Discipline: Wildlife
Hancock, P.
Okavango River Basin Technical Diagnostic Analysis:
Environmental Flow Module : Specialist Report: Country:
Botswana: Discipline: Birds
Mosepele,
K. Okavango River Basin Technical Diagnostic Analysis:
Environmental Flow Module: Specialist Report: Country:
Botswana: Discipline: Fish
Mosepele, B. and
Okavango River Basin Technical Diagnostic Analysis:
Dallas, Helen
Environmental Flow Module: Specialist Report: Country:
Botswana: Discipline: Aquatic Macro Invertebrates
Namibia
Collin Christian &
Okavango River Basin: Transboundary Diagnostic Analysis
Associates CC
Project: Environmental Flow Assessment Module:
Geomorphology
Curtis, B.A.
Okavango River Basin Technical Diagnostic Analysis:
Environmental Flow Module: Specialist Report Country:
Namibia Discipline: Vegetation
Bethune, S.
Environmental Protection and Sustainable Management of the
Okavango River Basin (EPSMO): Transboundary Diagnostic
Analysis: Basin Ecosystems Report
Nakanwe, S.N.
Okavango River Basin Technical Diagnostic Analysis:
Environmental Flow Module: Specialist Report: Country:
Namibia: Discipline: Aquatic Macro Invertebrates
Paxton,
M. Okavango River Basin Transboundary Diagnostic Analysis:
Environmental Flow Module: Specialist
Report:Country:Namibia: Discipline: Birds (Avifauna)
Roberts, K.
Okavango River Basin Technical Diagnostic Analysis:
Environmental Flow Module: Specialist Report: Country:
Namibia: Discipline: Wildlife
Waal,
B.V. Okavango River Basin Technical Diagnostic Analysis:
Environmental Flow Module: Specialist Report: Country:
Namibia:Discipline: Fish Life
Country Reports
Angola
Gomes, Joaquim
Análise Técnica dos Aspectos Relacionados com o Potencial
Socioeconomic
Duarte
de Irrigação no Lado Angolano da Bacia Hidrográfica do Rio
Series
Cubango: Relatório Final
Mendelsohn,
.J.
Land use in Kavango: Past, Present and Future
Pereira, Maria João
Análise Diagnóstica Transfronteiriça da Bacia do Rio
Okavango: Módulo do Caudal Ambiental: Relatório do
Especialista: País: Angola: Disciplina: Qualidade da Água
Saraiva, Rute et al.
Diagnóstico Transfronteiriço Bacia do Okavango: Análise
Socioeconómica Angola
Botswana Chimbari, M. and
Okavango River Basin Trans-Boundary Diagnostic Assessment
Magole, Lapologang
(TDA): Botswana Component: Partial Report: Key Public Health
Issues in the Okavango Basin, Botswana
81
Basin Groundwater Hydrology
Magole,
Lapologang
Transboundary Diagnostic Analysis of the Botswana Portion of
the Okavango River Basin: Land Use Planning
Magole, Lapologang
Transboundary Diagnostic Analysis (TDA) of the Botswana p
Portion of the Okavango River Basin: Stakeholder Involvement
in the ODMP and its Relevance to the TDA Process
Masamba,
W.R.
Transboundary Diagnostic Analysis of the Botswana Portion of
the Okavango River Basin: Output 4: Water Supply and
Sanitation
Masamba,W.R.
Transboundary Diagnostic Analysis of the Botswana Portion of
the Okavango River Basin: Irrigation Development
Mbaiwa.J.E. Transboundary Diagnostic Analysis of the Okavango River
Basin: the Status of Tourism Development in the Okavango
Delta: Botswana
Mbaiwa.J.E. &
Assessing the Impact of Climate Change on Tourism Activities
Mmopelwa, G.
and their Economic Benefits in the Okavango Delta
Mmopelwa,
G.
Okavango River Basin Trans-boundary Diagnostic Assessment:
Botswana Component: Output 5: Socio-Economic Profile
Ngwenya, B.N.
Final Report: A Socio-Economic Profile of River Resources and
HIV and AIDS in the Okavango Basin: Botswana
Vanderpost,
C.
Assessment of Existing Social Services and Projected Growth
in the Context of the Transboundary Diagnostic Analysis of the
Botswana Portion of the Okavango River Basin
Namibia
Barnes, J and
Okavango River Basin Technical Diagnostic Analysis:
Wamunyima, D
Environmental Flow Module: Specialist Report:
Country: Namibia: Discipline: Socio-economics
Collin Christian &
Technical Report on Hydro-electric Power Development in the
Associates CC
Namibian Section of the Okavango River Basin
Liebenberg, J.P.
Technical Report on Irrigation Development in the Namibia
Section of the Okavango River Basin
Ortmann, Cynthia L.
Okavango River Basin Technical Diagnostic Analysis:
Environmental Flow Module : Specialist Report Country:
Namibia: discipline: Water Quality
Nashipili,
Okavango River Basin Technical Diagnostic Analysis: Specialist
Ndinomwaameni
Report: Country: Namibia: Discipline: Water Supply and
Sanitation
Paxton,
C.
Transboundary Diagnostic Analysis: Specialist Report:
Discipline: Water Quality Requirements For Human Health in
the Okavango River Basin: Country: Namibia
82
Basin Groundwater Hydrology
83