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Study Name:
Orange River Integrated Water Resources Management Plan
Report Title:
Review of Surface Hydrology in the Orange River Catchment
Submitted By: WRP Consulting Engineers, Jeffares and Green, Sechaba Consulting, WCE Pty Ltd,
Water Surveys Botswana (Pty) Ltd
Authors:
H Mare
Date of Issue: August 2007
Distribution:
Botswana: DWA: 2 copies (Katai, Setloboko)
Lesotho: Commissioner of Water: 2 copies (Ramosoeu, Nthathakane)
Namibia: MAWRD: 2 copies (Amakali)
South Africa: DWAF: 2 copies (Pyke, van Niekerk)
GTZ: 2 copies (Vogel, Mpho)
Reports:
Review of Existing Infrastructure in the Orange River Catchment
Review of Surface Hydrology in the Orange River Catchment
Flood Management Evaluation of the Orange River
Review of Groundwater Resources in the Orange River Catchment
Environmental Considerations Pertaining to the Orange River
Summary of Water Requirements from the Orange River
Water Quality in the Orange River
Demographic and Economic Activity in the four Orange Basin States
Current Analytical Methods and Technical Capacity of the four Orange Basin States
Institutional Structures in the four Orange Basin States
Legislation and Legal Issues Surrounding the Orange River Catchment
Summary Report

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TABLE OF CONTENTS
1
INTRODUCTION ..................................................................................................................... 1
1.1
General ......................................................................................................................... 1
1.2
Objective of the study ................................................................................................... 8
1.3
Purpose and Structure of this Report......................................................................... 10
2
DATA BASE INVENTORY .................................................................................................... 11
2.1
General ....................................................................................................................... 11
2.2
Importance Index and data confidence ranking system ............................................ 11
2.3
Description of the database inventory........................................................................ 15
3
SITUATION ASSESSMENT OF HYDROMETROLOGICAL CONDITIONS IN ORANGE
RIVER BASIN ................................................................................................................................ 21
3.1
General ....................................................................................................................... 21
3.2
Rainfall Data ............................................................................................................... 23
3.3
Evaporation................................................................................................................. 25
3.4
Land use impacting on runoff ..................................................................................... 27
3.5
Flow data .................................................................................................................... 27
4
CONCLUSIONS AND RECOMMENDATIONS .................................................................... 35
4.1
Conclusions ................................................................................................................ 35
4.2
Recommendations...................................................................................................... 36
5
REFERENCES ...................................................................................................................... 37
LIST OF FIGURES AND TABLES
Figure 1-1: Orange River................................................................................................................. 1
Figure 1-2: Approximate Water Balance for Natural Runoff in the Orange River Basin................ 3
Figure 1-3: Major Water Demands along the Lower Orange River................................................ 5
Table 1-1: Orange River Water Balance at 2005 Development Level ........................................... 5
Figure 1-4: Major Water Transfer Schemes from Gariep and Vanderkloof dams. ........................ 7
Figure 1-5: Phase 1 of the Lesotho Highlands Water Project. ....................................................... 8

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Table 2-1: Data importance index ................................................................................................. 11
Table 2-2: Data confidence ranking system ................................................................................. 13
Table 2-3: Main sub-catchments in the Vaal River catchment ..................................................... 15
Table 2-4: Main sub-catchments in the Orange River catchment ................................................ 16
Table 2-5: Sub-catchments within each main sub-catchment...................................................... 17
Table 2-6: Data elements included in the inventory ..................................................................... 19
Table 3-1 : Rating of rainfall data .................................................................................................. 24
Table 3-2: Rating of evaporation data........................................................................................... 25
Table 3-3: Rating of observed flow data ....................................................................................... 28
Table 3-4: Summary of natural flow data ...................................................................................... 29
Table 3-5: Natural flow statistics ................................................................................................... 31
Table 3-6: Rating of available natural flow data............................................................................ 32

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1
INTRODUCTION
1.1
General
The Orange River originates in the Lesotho Highlands and flows in a westerly direction
2 200 km to the west coast where the river discharges into the Atlantic Ocean (see Figure
1-1). The Orange River basin is one of the largest river basins south of the Zambezi with a
catchment area of approximately 0.9 million km2.
Figure 1-1: Orange River
It has been estimated that the natural runoff of the Orange River basin is in the order of
11 600 million m3/a of which approximately 4 000 million m3/a originates in the Lesotho
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Highlands
and
approximately
900 million m3/a
from
the
contributing
catchment
downstream of the Orange/Vaal confluence which includes part of Namibia and a small
portion in Botswana feeding the Nossob and Molopo rivers. Whether or not these two
rivers directly contribute to the Orange River is an outstanding issue which will be
addressed during the study. The remaining 6 700 million m3/a originates from the areas
contributing to the Vaal, Caledon, Kraai and Middle Orange rivers. (see Figure
1-2).
Figure 1-2: Approximate Water Balance for Natural Runoff in the Orange River Basin
Error! Reference source not found.It should be noted that much of the runoff originating
from the Orange River downstream of the Orange Vaal confluence is highly erratic
(coefficient of variability greater than 2) and cannot be relied upon to support the various
downstream demands unless further storage is provided.
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Figure 1-2: Approximate Water Balance for Natural Runoff in the Orange River Basin
The water flowing into the Orange River from the Fish River in Namibia (near the river
mouth) could theoretically be used to support some of the downstream demands,
particularly the environmental demands at the river mouth.
To date, however, the
contributions from the Fish River (in Namibia) cannot be utilised to support any
downstream demands since these demands are currently supplied with water from
Vanderkloof Dam which must be released well in advance since the water takes 2 to 6
weeks to reach the mouth (some 1 400 km away). Any water flowing into the Orange
River from the Namibian Fish River will therefore add to the water already released from
Vanderkloof Dam since it is currently not possible to stop or store the additional water
once it has been released.
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The figures indicated in Figure 1-2 refer to the natural runoff which would have occurred
had there been no developments in the catchment. The actual runoff reaching the river
mouth (estimated to be in the order of 5 500 million m3/annum) is considerably less than
the natural value (over 11 000 million m3/annum). The difference is due mainly to the
extensive water utilisation in the Vaal River basin, most of which is for domestic and
industrial purposes.
Large volumes of water are also used to support the extensive
irrigation (estimated to be in the order of 1 800 million m3/annum) and some mining
demands (approximately 40 million m3/annum) occurring along the Orange River
downstream of the Orange/Vaal confluence (see Figure 1-3) as well as some irrigation in
the Lower Vaal catchment and Eastern Cape area supplied through the Orange/Fish
Canal (see Figure 1-4Error! Reference source not found.) (Eastern Cape Fish River). In
addition to the water demands mentioned above, evaporation losses from the Orange
River and the associated riparian vegetation account for between 500 million m3/a and
1 000 million m3/a depending upon the flow of water (and consequently the surface area)
in the river (Mckenzie et al, 1993, 1994 and 1995). An approximate water balance for the
Orange River is provided in Table 1-1 to provide perspective on the various demands
supported from the river.
Several new developments have already been commissioned or have been identified as
possible future demand centres for water along the Lower Orange River. In Namibia such
developments include the Haib copper mine, Skorpion lead and zinc mine (already
developed), the Kudu gas fired power station at Oranjemund and several irrigation projects
for communal and commercial irrigation along the northern riverbank. Similar potential
also exists on the South African side of the river with particular need to develop irrigation
for previously disadvantaged farmers.
In Lesotho there is considerable development
planned for the Lesotho Lowlands area and also the potential for further transfers from the
Lesotho Highlands Water Project. In Botswana, the developments that may influence the
Orange River are restricted mainly to groundwater abstraction.
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Figure 1-3: Major Water Demands along the Lower Orange River.
Table 1-1: Orange River Water Balance at 2005 Development Level
Water Balance Component
Volume (million m³/a)
Environmental Requirement
(1)
900
Namibia
(2)
120
Lesotho & Transfers to RSA
(3)
820
RSA Orange River Demand
(4)
2 560
RSA Vaal River Demand
(5)
1 560
Evaporation & losses
(6)
1 750
Spillage
(7)
3 780
Total
11 490
Spillage under natural conditions
10 900
Notes
(1) - Includes natural evaporation losses from Orange River.
(2) - Includes water use from Orange & Fish rivers.
(3) ­ With Full Phase 1 of LHWP active.
(4) ­ Includes transfers to the Eastern Cape.
(5) ­ Vaal Demand supplied from locally generated runoff.
(6) ­ Excludes evaporation losses from the as it is already included in component 1.
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(7) ­ Average spillage at 2005 development level
In Lesotho, the first phase of the Lesotho Highlands Water Project was recently completed
and represents one of the largest water transfer schemes in the world. Some details of the
scheme are shown in Figure 1-5. It should be noted that the water transfers shown in the
figure are approximate values only and are likely to change due to revision of
environmental requirements etc.
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Figure 1-4: Major Water Transfer Schemes from Gariep and Vanderkloof dams.
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Figure 1-5: Phase 1 of the Lesotho Highlands Water Project.
1.2
Objective of the study
In view of the existing and possible future developments which will influence the availability
of water in the Orange River, a project has been initiated by ORASECOM and
commissioned and funded by GTZ involving all four basin states (Botswana, Lesotho,
Namibia and South Africa. The main objective of the project is to facilitate the development
of an Integrated Water Resources Management Plan for the Orange River Basin. The plan
will in turn facilitate the following specific objectives:
· Maximise benefits to be gained from Orange River water;
· Harmonise developments and operating rules;
· Foster peace in the region and prevention of conflict;
· Encourage proper and effective disaster management;
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· Ensure that developments are sustainable and encourage the maintenance of
bio-diversity in the basin, and
· Management of potential negative impacts of current and possible future
developments.
In order to achieve the above objective it is envisaged that the resulting Water Resources
Development Plan will be founded on the following four basic principles:
· Reasonable utilisation of available water resources;
· Equitable accrual of benefits to basin states;
· Sustainable utilisation of water resources, and
· Minimisation of harm to the environment.
The strategy to be adopted by the project team to meet the objectives should involve the
following:
· Sharing of information on existing and proposed future developments;
· Facilitation of a common understanding of key issues based on comparable
technical and institutional capacity;
· Development of comparable legislation and institutional structures;
· Adoption of comparable standards and management approaches;
· The development of a Water Resource Management Plan for the future
development and management of the water resources of the Orange River.
It is anticipated that the development of the Water Resource Management Plan will be
undertaken in phases and the remainder of this document refers to the work involved with
Phase 1 of the project. Phase 1 will involve the following:
· A desktop study to establish the status quo within the basin and to create an
agreed base from which the subsequent phases of the project can be developed;
· To facilitate capacity building where possible in order to strengthen expertise
throughout the four basin states;
· To identify and highlight deficiencies in the knowledge base which must be
addressed before the Water Resource Management Plan can be finalised. Some
fieldwork may be required in subsequent phases of the project;
· To develop a preliminary Water Resource Management Plan which can be used
as the basis from which the final plan can ultimately be developed;
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· To develop a draft scope of work for subsequent phases of the project from which
a Terms of Reference can be developed by the Client.
An inaugural meeting to discuss the project and in particular the expected content for the
Inception Report was held in Botswana on 8 February 2004.
1.3
Purpose and Structure of this Report
This report will be used to summarise the findings from the two main components of the
hydrology review task. The first component is the data base inventory. A description of
the data base inventory along with summaries on pertinent data and data gaps are given
in Section 2 of the report. The second component includes a description of the situation
assessment of the prevailing hydro-meteorological conditions, in the Orange River Basin,
given in Section 3. Conclusions and recommendations are given in Section 4.
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2
DATA BASE INVENTORY
2.1
General
This component of the hydrology review task involved the compiling of the latest available
hydrological and related data from reports of relevant studies. As part of this component
an importance index and data confidence ranking system was developed to be able to
classify the available hydrological information. Gaps in the hydrological information were
identified in the process and indicated on a geographical coverage. An initial database
was developed in Excel which will later in the study be incorporated in the Access
database which is developed as part of Task 2 of this study. The preliminary Excel data
base was populated with metadata such as MAR, MAP, record periods, data importance
and data confidence, evaporation etc. as obtained from existing reports. Details of the
importance index and data confidence ranking system as well as for the database
inventory will be given in the sections to follow.
2.2
Importance Index and data confidence ranking system
The data importance index was developed and used to provide an indication of the
importance of a specific hydrological related data element. The data importance index that
was developed for this purpose and also used in the population of the data inventory is
given in Table 2-1.
Table 2-1: Data importance index
Rating (0 to 5)
Description
0
Not required
1
Useful background information
2
Nice to have ­ will add very little value to the hydrology but will assist in the broader
understanding of the hydrology
3
Valuable ­ will add substantial value to the hydrology
4
Important ­ required to obtain reliable hydrology
5
Essential ­ Cannot do without
The data confidence ranking system in turn was developed to provide and indication of the
confidence one has in the available hydrological data as given in the latest study reports.
Separate data confidence ranking systems were developed for different types of data as
the characteristics of the different data types required this.
The rating system for the
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observed flow data includes two parameters. The first parameter on a scale A to D gives
an indication of the quality of the observed flow and relates to the effort made to check,
verify and patch the observed record. The second parameter gives an indication of the
accuracy of the data on a scale 0 to 5 where 5 is excellent and 0 so poor that it is not
recommended for use in hydrological analyses. This means that one can have a record of
high quality (A) which means that this observed record was properly checked, verified and
patched but the accuracy is only reasonable. The rating for this record will then be given
as an A3 (see Table 2-2 for more detail). The rating systems for the other data types are
relatively simple and are indicated by a single parameter as shown in Table 2-2.
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Table 2-2: Data confidence ranking system
Data and rating description
Rating
Additional rating description and notes
Observed flow data
Accuracy
Typical conditions / descriptions
Rating 0 - 5
0 ­ not to be used
Accuracy poor ­ weekly recording ­ short record length - > 20% gaps unreliable months
Observed checked & verified & patched
A
Observed checked and verified
B
1 ­ poor
Accuracy suspect ­ daily recording ­ short record length - > 15% gaps unreliable months
Observed no checking/verification
C
2 ­ use with caution
Accuracy reasonable - daily recording ­ reasonable record length - > 15% gaps unreliable months
No observed data
D
3 ­ reasonable
Accuracy reasonable - daily recording ­ reasonable record length - < 15% gaps unreliable months
4 ­ Good
Accuracy good for low flows not for high flows - automatic recording ­ reasonable record length - <
10% gaps unreliable months
Natural flow data
Simulated based on good rainfall data and good
A
Also take into account the reliability of the observed flow used for calibration purposes ­ if it is only reasonable rather give the
calibration
natural flow a B rating.
Simulated based on good rainfall data and reasonable
B
calibration
Simulated using regional parameters & reasonable
C
rainfall data
Estimations based on assumptions & nearby catchments
D
with more detail data etc.
No natural flow data
E
Rainfall data (Catchment & point rainfall data)
Good to excellent
A
Derived from reliable selected rain fall data that were verified and patched.
A typical selection process for individual rainfall gauges would include:
- A reasonable record length 30 to 40 years or more
- Not more than 8% to 10% of the total observed record should consist of unreliable monthly data.
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Data and rating description
Rating
Additional rating description and notes
- A gauge should lie geographically inside the sub-catchment or nearby in an adjacent catchment
- A consistent mass plot which shows a straight line with a constant gradient for the full period of the record
Reasonable
B
Derived from reliable selected rain fall data that were verified and patched but with some shortcomings such as short records,
not sufficient number of gauges, etc..
Poor
C
Quality of rainfall data suspicious & lack of data
No data
D
No data
Evaporation data
Good to excellent
A
Long reliable record with less than approximately 10% unreliable data
Reasonable
B
Record length, and reliability reasonable
Poor
C
Quality of rainfall data suspicious & lack of data
No data
D
No data
Data general
Good to excellent
A
Reasonable
B
Poor
C
No data
D
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2.3
Description of the database inventory
As indicated in Section 2.1 the initial database was developed in Excel and will later in
the study be incorporated into the Access database which is currently in the process to
be developed as part of Task 2 of this study.
For the purpose of this database the study area was divided into different sub-
catchments mainly according to those defined in the available study reports. These
sub-catchments were sorted first according to the major river catchments the Vaal
River and Orange River catchments and then according to the main sub-catchments
within each major river catchment. The Vaal River catchment is divided into five main
sub-catchments as described in Table 2-3 and shown in Figure A-1 of Appendix A.
Table 2-3: Main sub-catchments in the Vaal River catchment
Main Sub-catchment
Description
Name
Area (km²)
Upper Vaal
38 638
Vaal River catchment from Vaal Dam and upstream
Vaal Barrage
8 651
Vaal River catchment between Vaal Dam & Vaal Barrage
Middle Vaal
60 836
Vaal River catchment between Vaal Barrage and Bloemhof Dam
Lower Vaal
53 787
Vaal River catchment downstream of Bloemhof Dam excluding the the Riet
and Modder River catchments
Riet/Modder
27 627
The combined Riet and Modder River catchments
Total
189 539
Total Vaal River Catchment
The Orange River catchment (excluding the Vaal River catchment) is divided into
seven main sub-catchments which also take into account the country wherein it is
located. The Orange River main sub-catchments is described in Table 2-4 and shown
in Figure A-1 of Appendix A.
The sub-catchments as per main sub-catchment are given in Table 2-5 for both the
Vaal and Orange River catchments and are shown in Figure A-2 of Appendix A. As
part of the Lesotho Lowlands Study, updated hydrology was prepared for smaller sub-
catchments within Lesotho, as indicated in Table 2-5 and shown in Figure A-3 of
Appendix A. Several data elements were covered in the inventory with regards to the
following data categories: i.e. catchment area, observed flow, natural flow, catchment
rainfall, point rainfall, evaporation, land use and the source of the data. For each of
the sub-catchments as defined in Table 2-5 metadata were provided for all the
selected data elements as and when it was available from existing documents. The
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importance of the data was indicated according to the data importance index as
defined in Table 2-1.
Table 2-4: Main sub-catchments in the Orange River catchment
Main Sub-catchment
Description
Name
Area (km²)
Senqu
24 752
Upper reaches and origin of the Orange River in Lesotho
Upper Orange
48 595
Orange River upstream of Vanderkloof Dam and downstream of
Welbedacht Dam and the Lesotho Border at Oranjedraai.
Caledon
15 245
Caledon River catchment from Welbedacht Dam and upstream
(includes parts of RSA and Lesotho)
Lower Orange RSA
326 173
Orange River catchment downstream of Vanderkloof Dam and the Vaal
River confluence excluding the Lower Orange Areas located in
Botswana and Namibia
Lower Orange
71 000
The Orange River catchment located in Botswana
Botswana
Lower Orange
164 166
The Orange River catchment located in Namibia excluding the Fish
Namibia
River (Namibia)
Fish River Namibia
95 680
The total Fish River catchment in Namibia
Total
745 611
Total Orange River Catchment excluding Vaal River
The confidence one has in the existing data was included in the inventory for each of
the data categories and was based on the data confidence ranking system defined in
Table 2-2.
Details of the data elements included in the inventory for each of the data categories
are given in Table 2-6. The gross and net areas are given for sub-catchments. The
gross area refers to the total area in km² of the sub-catchment and the net area to the
area that contributes to the river runoff. Information with regards to the net area was
unfortuanatelly not always available from existing reports. In some areas which are
relatively flat, part of the sub-catchment drains to local pans, so that only a portion of
the runoff will be reaching the river. Only the area that contributes to runoff draining to
the river is regarded as the effective area and is referred to as the net catchment area.
There will most probably also be non contributing or endoreic areas in Namibia and
Botswana, although no data was available in this regard.
Under the observed flow category, only the observed flows used in the final hydrology
calibration process were included and not all the available observed flow in the sub-
catchment. In the process of generating hydrology for a study area, the best available
flow records are selected for calibration purposes.
The selection of these gauges
depends on the location of key points in the catchment, the reliability of the flow data,
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the amount of missing or unreliable monthly flow data and the available record length.
It is also important to note that the observed flow represents the total flow from the
upstream catchment flowing past the point of observation. The natural flow refers to
the incremental flow from the sub-catchment and therefore excludes the flow from the
upstream sub-catchments.
Table 2-5: Sub-catchments within each main sub-catchment
Vaal River Basin
Orange River Basin (excluding Vaal)
Main sub-
Sub-catchment name &
Main sub-
Sub-catchment name & reference
catchment
reference no.
catchment
no.
Upper Vaal
R1-Delangesdrift
Senqu
L1-Katse Dam
R2-Frankfort
L2-Malatsi possible Dam
R3-Grootdraai Dam
L3-Mashai possible Dam
R4-Sterkfontein Dam
L4-Matsoku Weir
R5-Vaal Dam
L5-Mohale Dam
Vaal Barrage
R6-Vaal Barrage
L6-Ntoahe possible dam
R7-Klip River
L7-Tsoelike possible dam
R8-Suikerbosrand River
L8-Oranjedraai
R9-Allemanskraal Dam
*L9-Makhaleng 1
R10-Bloemhof Dam
*L10-Makhaleng 2
R11-Boskop Dam
Upper Orange
R36-Aliwal Noord
R12-Erfenis Dam
R37-Gariep Dummy Dam
Middle Vaal
R13-Klerkskraal Dam
R38-Vanderkloof Dam
R14-Possible Klipbank Dam
R39-Kraai River
R15-Klipdrift Dam
R40-Gariep Dam
R16-Koppies Dam
Caledon
L11-Hlotse possible dam
R17-Possible Kromdraai Dam
L12-Katjiesberg possible dam
R18-Johan Neser Dam
R41-Knellpoort Dam
R19-Possible Rietfontein Dam
R42-Waterpoort possible dam
R20-Rietspruit Dam
R43-Welbedacht Dam
R21-Lower Sand/Vet River
+L13-Hlotse possible dam 1
Lower Vaal
R22-Wentzel Dam
+L14-Hlotse possible dam 2
R23-Baberspan
$L15-Ngoajane possible dam 1
R24-Taung Dam
$L16- Ngoajane possible dam 2
$L17- Muela Dam
R25-Spitskop Dam
Lower Orange
R44-Boegoeberg Weir
RSA
R26-Lower Harts
R45-Vioolsdrift/Mouth
R27-Vaalharts Weir
Lower Orange
B1- Nossop & Molopo catchment
Botswana
R28-De Hoop Weir
R29-Douglas Weir
Lower Orange
N1-Daan Viljoen Dam
Namibia
Riet/Modder
Aucampshoop
N2-Otjivero Dam
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Vaal River Basin
Orange River Basin (excluding Vaal)
Main sub-
Sub-catchment name &
Main sub-
Sub-catchment name & reference
catchment
reference no.
catchment
no.
Kalkfontein Dam
N3-Nossop remainder
Krugersdrift Dam
N4-Nauaspoort Dam
Rustfontein Dam
N5-Oanob Dam
Tierpoort Dam
N6-Auob remainder
Tweerivier
N7-Tsamab Dam
N8-Dreihoek Dam
N9-Quaternary 442 & 481 &
remainder of 482 & 483
N10-Quaternary484
N11-Quaternary485
Fish River
N12-Hardap Dam
Namibia
N13-Konkiep
N14-Lower Fish possible dam
N15-Naute Dam
N16Seeheim
Notes : * - As part of the Lesotho Lowlands Study the Oranjedraai sub-catchment were sub-divided into these two sub-
catchments.
+ - As part of the Lesotho Lowlands Study the Hlotse sub-catchment were sub-
divided
into
these
two
sub-
catchments.
$ - As part of the Lesotho Lowlands Study the Katjiesberg sub-catchment were sub-divided into these three sub-
catchments.
The unit runoff refers to the average mm runoff from the catchment and for the
observed flows will represent the unit runoff from the total catchment upstream of the
gauge. The unit runoff given for the natural flow refers to the incremental flow and
therefore represents the unit runoff from the incremental catchment only.
Rainfall data is the primary input data used in rainfall runoff models to simulate natural
runoff.
It is therefore very important that rainfall records are thoroughly checked,
evaluated, verified and patched before it can be used to simulate catchment runoff.
Only the final selected rainfall gauges used to create a representative rainfall record
for a sub-catchment were included in the inventory. The catchment rainfall record is in
most cases a percentage rainfall file which expresses the monthly rainfall as a
percentage of the mean annual precipitation of the sub-catchment. The point rainfall
file is used in models to determine the water balance of large storage reservoirs and
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therefore represents the mm rainfall at the point of the dam. The point rainfall file thus
contains monthly rainfall data in mm for the total record period.
Table 2-6: Data elements included in the inventory
Data Category
Data elements
Catchments area *
Gross area in km²
Net area in km²
Observed flow*
Flow gauge name
Flow gauge number
Record period used
Statistics
Average (million m³/a)
Standard deviation (million m³)
Coefficient of variance
Unit runoff (mm)
Natural flow*
File name
Record period
Statistics
Average (million m³/a)
Standard deviation (million m³)
Coefficient of variance
Unit runoff (mm)
Catchment Rainfall*
Rainfall gauging stations
used
File name
Record period
Statistics
Average (mm/a)
Standard deviation (mm)
Coefficient of variance
Point Rainfall*
File name
Name of dam
Record period
Statistics
Average (mm)
Standard deviation (mm)
Coefficient of variance
Evaporation*
Symonspan evaporation
(mm/a)
A-pan evaporation
(mm/a)
Dam evaporation (mm/a)
Name of the dam
Land use*
Small dams
Total storage (million m³)
Impervious area
Area (km²)
Data source
Study name
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Orange IWRMP
Task 4: Surface Hydrology
Report name & number
Modelling tools
Used for flow data
Name of model
Used for rainfall data
Name of model
Note:* - Data confidence and data importance ranking were carried out for each of these
data categories.
Evaporation data used in the hydrological studies is in most cases only 12 monthly
values representing the average gross evaporation for each month.
Gross
evaporation refers to the evaporation from a pan or dam as if no rainfall occurred
during the month. The net evaporation is obtained when the effect of rainfall is added.
In the RSA Symons-pan evaporation is generally used in hydrological analysis as well
as to obtain lake evaporation. In Namibia and Botswana A-pan evaporation is used in
stead.
The land use data category only includes land use data that is not captured under the
Task 8 (Water requirements). Land use that is included in the inventory typically is
small or farm dams as well as impervious areas within larger urban areas. To provide
some indication of the extent of these land use activities, the total combined storage in
million m³ of all the small dams within the sub-catchment and/or the total impervious
area (km²) in the sub-catchment were captured in the inventory.
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Task 4: Surface Hydrology
3
SITUATION ASSESSMENT OF HYDROMETROLOGICAL CONDITIONS IN
ORANGE RIVER BASIN
3.1
General
The Orange River rises as two main river systems, the Orange River and its
associated tributaries, and the Vaal River and its associated tributaries. The Orange
River originates in the Lesotho Highlands at elevations of about 3 300m above sea
level. In Lesotho the Orange River is known as the Senqu River and only when it
enters the RSA is it referred to as the Orange River.
The river flows west for
approximately 2 200km to the Atlantic Ocean and for the last 600km forms the border
between the RSA and Namibia. The Vaal River catchment varies in elevation from
about 3 200m above sea level at the South Eastern boundary in the Drakensberg to
approximately 970m above sea level at its confluence with the Orange River close to
Douglas.
Downstream of Douglas the Orange River is joined by the Ongers/Brak
River and the Hartbees River from the south and the Molopo and Fish Rivers from the
North. The Molopo and its tributary the Nossob form the boundary between the RSA
and Botswana while the Fish River drains a large portion of the Orange River
catchment within Namibia.
To be able to properly develop and manage a large water resource such as the
Orange River, it is of utmost importance that sufficient and reliable hydrological data
are available to be used as the main input to any water resource system analyses.
The basic approach followed in hydrological studies to be able to obtain reliable
hydrological data sets can be summarised as follows:
· Sub-divide the whole catchment into sufficient smaller sub-catchments,
representing key points in the system such as existing and future major dams
as well as other sites where reliable observed flow data are available from
existing flow gauging stations.
· Collecting and collating of key data which includes:
observed rainfall data from gauging stations. These stations must be
well distributed throughout each sub-catchment and are selected to be
used as the main input to the rainfall runoff simulation models,
observed data from stream flow gauges, preferably for each sub-
catchment. These stream flow gauges are a combination of purpose
built gauging stations and dams,
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Orange IWRMP
Task 4: Surface Hydrology
evaporation
data
for
each
sub-catchment
(Class
A-pan
and
Symonspan),
all water use data, which typically include irrigation, urban, industrial,
mining, domestic, forestation, transmission losses, environmental
requirements, etc.
· Pre-calibration data manipulation:
the quality of the data for each rainfall station within or close to the
study area is checked by considering the record length, amount of
missing and unreliable data and by producing mass plots to check the
consistency of the record,
infilling and patching of the missing and unreliable data by using
acceptable techniques such as multiple linear regression with other
stations or models specifically developed for this purpose such as
ClassR & PatchR,
using finally selected and patched rainfall records for each sub-
catchment, a representative catchment rainfall record is created for
each sub-catchment that covers the total simulation period selected
for the specific study and includes the data from a couple of rainfall
gauging stations distributed over the sub-catchment,
evaluate the quality of the stream flow data by checking record
lengths, reliability of stage/discharge ratings, the amount of missing
and unreliable data etc.,
examine water use data and taking it back in time. This is important
as rainfall runoff modelling is based on the principle of modelling
rainfall against "naturalised runoff", which is runoff that is unaffected
by human development. This is required as it is assumed that rainfall
is not affected by human developments and it will not be possible to
model and calibrate the modelled runoff against a non-stationary time
series such as observed runoff, which is affected by human
developments. To obtain naturalised runoff, the effect of the growing
demands is removed from the observed flow record.
· Rainfall runoff Model Calibration:
Calibrate the rainfall runoff model on an observed flow sequence to
produce a simulated record that provides the best possible fit to the
observed flow, while at the same time respecting the physical realities
of the sub-catchment.
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Orange IWRMP
Task 4: Surface Hydrology
This process is carried out for all the sub-catchments, resulting in
monthly incremental natural runoff records for each sub-catchment,
covering the total simulation period to be used in the study.
For the purpose of the system analysis, reliable monthly natural flow
records of at least 60 to 70 years in length are required for each sub-
catchment.
In the system analysis the hydrology of all the sub-
catchments are included and water demands representative of a
selected development level can be imposed on the system, to
determine the yield characteristics of the system or sub-systems at the
specific development level.
3.2
Rainfall Data
The mean annual precipitation (MAP) in the Orange River Basin vary from as high as
1 200mm in the Lesotho Highlands to less than 50mm in the Richtersveld and parts of
Namibia in the Lower Orange where the Orange River forms the border between the
RSA and Namibia. (See Figure A-4 of Appendix A)
Rainfall data are possibly the most important data used in a hydrological analysis, as it
forms the basis of the runoff generated in each sub-catchment. It is therefore essential
that rainfall data are checked and infilled properly before being used in subsequent
phases of the hydrological process.
The data confidence ranking system was
therefore designed to capture the level of detail of the rainfall data checking,
verification and patching processes for each of the catchment rainfall records.
Included in the hydrological data base developed for this study are details of the final
catchment and point rainfall records that were created in previous studies for the
different sub-catchments within the whole Orange River Basin. Although there are
many more rainfall gauges within a sub-catchment, only those finally selected and
used to create the average sub-catchment rainfall record are listed in the data base.
In some areas within the Orange River basin no hydrological studies were previously
carried out, and although raw rainfall data do exist for these areas, this data were not
captured in the data base as it was not verified and checked for hydrological purposes.
The catchment rainfall files are mostly a percentage rainfall file, expressing the
monthly rainfall as a percentage of the Mean Annual Precipitation (MAP) of the specific
sub-catchment. The point rainfall files are a monthly rainfall file in mm and represent
the point rainfall on a specific dam in a sub-catchment.
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Orange IWRMP
Task 4: Surface Hydrology
Rainfall data were rated for most of the sub-catchments as good to excellent as
indicated in Table 3-1. Lower ratings were limited mainly to the low rainfall areas in
the Lower Orange within the RSA, Namibia and Botswana.
A rainfall isohyetal map was produced for the entire Orange River basin, based on the
isohyetal maps received from Namibia, Botswana and the RSA (RSA isohyetal map
already included Lesotho) (See Figure A-4 of Appendix A). The isohyets from the
Namibia and the RSA compared very well along the borders between the two
countries.
The isohyets received from Botswana and those from the RSA showed
some discrepancies along the RSA/Botswana in particular at the lower rainfall areas.
The Botswana and RSA isohyets were therefore slightly adjusted to obtain a smooth
transition from the one country to the other. The discrepancies between the isohyets
before adjustment are also shown on the map. The isohyets received from Botswana
did not show the 200mm isohyet at the most southern point of Botwana and it was
based on an extension of the RSA isohyet. The combined isohyetal map as given in
Appendix A, Figure A-4, should in general still provide a good indication of the
variance in rainfall over the entire Orange River Basin.
Table 3-1 : Rating of rainfall data
Main Sub-catchment
Rating
Upper Vaal
Good to excellent
Vaal Barrage
Good to excellent
Middle Vaal
Good to excellent
Lower Vaal
Good to excellent
Riet/Modder
Good to excellent
Senqu
Generally good to excellent, except for the rainfall records used for the
Makhaleng Dam catchments used in the Lesotho Lowlands Study which
were rated as reasonable.
Upper Orange
Good to excellent
Caledon
Generally good to excellent, except for the rainfall records used for the
Hlotse and Hololo Dam catchments used in the Lesotho Lowlands Study
which were rated as reasonable..
Lower Orange RSA
Rated as reasonable as rainfall data were obtained from the WR90
publications and is not the product of a detailed hydrological study.
Lower Orange Botswana
No data available
Lower Orange Namibia
No data available
Fish River Namibia
Rated as reasonable for some gauging stations that were used in previous
studies but no catchment rainfall files were available for any of the sub-
catchments.
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Task 4: Surface Hydrology
3.3
Evaporation
S-pan evaporation varies in the Orange River basin from in excess of 2 600mm/a in
the central parts of the Lower Orange, most of Namibia and western part of Botswana
to approximately 1 200mm/a in the Lesotho Highlands (See Figure A-5 in Appendix
A).
Evaporation is one of the primary inputs to catchment rainfall runoff models and is also
used to determine water losses from wetlands, reservoirs and aquifers as well as for
determining water requirements of crops under irrigation. Two types of evaporation
pans are commonly used to measure evaporation namely the Symons pan and the
American Class A-pan. The A-pan is a round pan which is installed on a support
above the ground and is mainly used for agricultural purposes to determine irrigation
demands of different crops. The Symons pan (S-Pan) is a square pan and is buried in
the ground. In the RSA the S-pan are widely used to determine the free surface lake
evaporation at dam sites. Evaporation from the S- and A-pans differs significantly and
the A-pan evaporation is on average 1.1 to 1.3 times higher than the S-pan values.
Data captured in the data base for this study refers mainly to S-pan evaporation, as S-
pan evaporation is used as basis in the rainfall runoff models as well as to determine
lake evaporation. Due to the lower variation in the annual evaporation at a site, only
12 monthly evaporation values are generally required in hydrological analysis which
represents the average monthly evaporation for each of the months in a year. The
point evaporation as used for the major dams in the basin is also captured in the data
base and is referred to as lake evaporation.
Most of the evaporation data as used in the previous studies for the various sub-
catchments were rated as reasonable as evaporation data in general did not go
through a thorough checking, verification and patching process as in the case of the
rainfall data.
Evaporation data were in general obtained from the "Surface Water
Resources of South Africa 1990" publication.
Table 3-2: Rating of evaporation data
Main Sub-catchment
Rating
Upper Vaal
Reasonable
Vaal Barrage
Reasonable
Middle Vaal
Reasonable
Lower Vaal
Reasonable
Riet/Modder
Reasonable
Senqu
Although data was used in the previous studies, only lake evaporation were
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Orange IWRMP
Task 4: Surface Hydrology
Main Sub-catchment
Rating
given in the reports and no reference were made to catchment evaporation
except for the areas covered by the recent Lesotho Lowlands Study
(Makhaleng Dam catchments) Given data were however rated as
Reasonable.
Upper Orange
Reasonable
Caledon
Reasonable
Lower Orange RSA
Reasonable
Lower Orange Botswana
No data available
Lower Orange Namibia
No data available
Fish River Namibia
Rated as reasonable for Hardap and Naute Dam and poor for possible
Lower Fish Dam but no catchment evaporation data were available
The mean annual evaporation (MAE) isoline coverage as given in Figure A-5 of
Appendix A, were prepared from MAE isoline coverages received from RSA and
Namibia. Lesotho was fully covered by the RSA isoline map and detail was therefore
not required from Lesotho. No evaporation isolines were available from Botswana and
only A-pan evaporation for a few stations were available as indicated on Figure A-5 of
Appendix A.
These individual values however seem to tie in with the RSA and
Namibia isolines. MAE isolines received from Namibia referred to A-pan evaporation.
A-pan evaporation is on average 1.1 to 1.3 times higher than S-pan values.
The
following equation to convert S-pan to A-pan evaporation was obtained from the
"Surface Water Resources of South Africa 1990":
MAE (S-pan) = 130 + 0.726 MAE (A-pan)
When this equation was used to convert the A-pan MAE isolines received from
Namibia to equivalent S-pan isolines, the converted S-pan isolines did not match well
with the S-pan isolines used in the RSA. To be able to obtain a reasonable match
between the two sets of MAE isolines a factor of between 0.85 and 0.91 was required
to convert Namibian A-pan to equivalent RSA S-pan values.
This means that the
Namibia A-pan values are on average 1.1 to 1.18 times higher than the RSA S-pan
values. The MAE isolines for Namibia as shown on Figure A-5 of Appendix A were
derived from the Namibian A-pan isolines using conversion factors of between 0.85 to
0.91.
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Task 4: Surface Hydrology
3.4
Land use impacting on runoff
Although there are a large variety of land use data that will impact on the available
runoff, most of these developments will be addressed in Tasks 3 and 8, review of
existing infrastructure and summary of water requirements respectively.
As part of
task 4 (Review of existing Hydrology) the focus will only be on small dams and
urbanised areas which are not covered in tasks 3 and 8.
The confidence in the available data was regarded as reasonable in all the sub-
catchments where such data were captured in the previous study reports. No data
were available for the entire Lower Orange as well as for the Senqu catchment.
The extent of these developments in each of the main sub-catchments is shown in
Figure A-6 of Appendix A. The storage in small dams accumulates to a total of 811
million m³, which is equivalent to that of a very large storage dam.
The only
impervious area that was significant enough to be taken into account in the
hydrological studies was that of Johannesburg and surrounding areas within the Vaal
River catchment which accumulated to 1.3 km².
The effect on the reduction or
increase in runoff for any of these developments, were unfortunately not given in the
existing hydrology reports.
3.5
Flow data
Reliable observed flow records of sufficient length provides extremely valuable data to
be used in any hydrological analysis as it captures the characteristics of the actual flow
generated in the catchment over time. The observed records are however seldom
used as is in system analyses, due to the fact that it also captures the effects of human
related development over time, in the catchment. One therefore needs to remove the
effects of human development from the flow record, to first obtain a naturalised flow
record which can be used in the system analysis. This process is briefly described in
Section 3.1. There are still some areas, located mainly in the drier arid catchments,
where human development had little or no effect on the runoff from the sub-catchment.
In such cases the observed records can be used to represent the flow under natural
conditions.
Observed flow data that were selected in previous studies as the best and most
suitable records to be used for calibration purposes or as a natural flow record where
no or little human development occurred in the catchment, were included in the data
base and rated according to the confidence ranking system. Results of the rating are
summarised in Table 3-3. Although there are lots of other gauging stations with flow
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Orange IWRMP
Task 4: Surface Hydrology
records available in the study area, they were not included as they did not contribute to
the final natural flow sequences generated for the Orange River Basin.
Table 3-3: Rating of observed flow data
Main Sub-catchment
Rating
Description
Upper Vaal
A4 to A5
Data was checked & verified & patched and accuracy
regarded as good to excellent
Vaal Barrage
A4 to A5
Data was checked & verified & patched and accuracy
regarded as good to excellent
Middle Vaal
A1 to A5
Data was checked & verified & patched and accuracy
regarded as poor to excellent, although most were given a 3
or 4 accuracy rating which is reasonable to good.
Lower Vaal
A3 to A4
Data was checked & verified & patched and accuracy
regarded as reasonable to good.
Riet/Modder
A2 to A4
Data was checked & verified & patched and accuracy
regarded as use with caution to good, but most were
regarded as reasonable.
Senqu
A2 to A4
Data was checked & verified & patched and accuracy
regarded as use with caution to good.
Upper Orange
A3 to A4
Data was checked & verified & patched and accuracy
regarded as reasonable to good.
Caledon
A3
Data was checked & verified & patched and accuracy
regarded as reasonable.
Lower Orange RSA
No data
No detail hydrology study was carried out for this area.
Lower Orange Botswana
No data
No detail hydrology study was carried out for this area.
Lower Orange Namibia
C1 to B4
Limited data was available and cover only a small portion of
the total sub-catchment area. Most of the data was rated as
B4 which means data was checked & verified and accuracy is
good.
Fish River Namibia
C2 to B4
Most of the data rated B3 to B4. This means that data was
checked & verified and accuracy is regarded as reasonable
to good.
From Table 3-3 it is evident that the observed flow data in the upstream catchments is
generally rated as very good and the rating decreases as one progress downstream
into the more arid areas.
As natural flows is most widely used in hydrological and system analyses, the
remainder of this section focuses on the natural flows or flows under virgin conditions
as it is also referred to. The total natural flow generated in the Orange River Basin in
excess of 11 500 million m³/a. The natural runoff generated in large parts of Namibia
and Botswana is not available from existing reports and is not included in the total of
11 500 million m³/a given in Table 3-1. Most of the current unknown flow volumes
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Orange IWRMP
Task 4: Surface Hydrology
from these areas in Namibia and Bostwana are, however, not expected to reach the
Orange River and is of local importance only.
As result of non-contributing or endoreic areas (localised areas where runoff flows into
pans and evaporate and is therefore not contributing to the river flow) al the runoff
generated in a sub-catchment will not contribute to river flow. Transmission losses in
the Fish River (Namibia) are very high and result in a significant reduction in the runoff
volume that will eventually reach the Orange River. To provide an indication of the
runoff volume that is expected to eventually reach the Orange River the "MAR
adjusted" column was added to Table 3-4.
From this data it is evident that
approximately 11 400 million m³ is expected to reach the Orange River. Due to the
extremely dry conditions in the Lower Orange River with very little incremental flow
reaching the Orange River in this area, large volumes of the flow in the Lower Orange
River is utilised to satisfy river requirements such as evaporation, evapo-transpiration
and seepage. This will result in a further reduction of approximately 615 million m³ in
the flow, so that under natural conditions it is expected that only approximately
10 800 million m³ will reach the Orange River mouth.
Table 3-4: Summary of natural flow data
Name
MAR (million
MAR
Percentage of
Unit runoff
m³/a)
adjusted
total runoff
(mm/a)
Vaal River Catchment
Upper Vaal
1 977
1 977
17.3
51.2
Vaal Barrage
257
257
2.2
29.7
Middle Vaal
1 076
1 076
9.4
17.8
Lower Vaal
191
191
1.7
3.6
Riet/Modder
406
406
3.5
14.7
Sub-total
3 907
3 907
34.1
20.6
Orange River Catchment
Senqu
4 038
4 038
35.3
163.2
Upper Orange
1 450
1 450
12.7
29.8
Caledon
1 217
1 217
10.6
79.8
Lower Orange RSA
b) 420
b) 330
2.9
1.0
Lower Orange Botswana
No data available
Lower Orange Namibia
c) 29
c) 7
0.1
1.5
Fish River Namibia
d) 706
d) 494
4.3
5.2
Sub-total
7 860
7 536
65.9
10.1
Total
a) 11 767
a) 11 443
Total Orange & Vaal
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Orange IWRMP
Task 4: Surface Hydrology
Notes: a) Due to river evaporation, evapo-transpiration and seepage losses along the Lower Orange River the total
volume of 11 443 million m³/a will not be able to reach the Orange River mouth.
These river requirements are
dependant on the flow in the river and can vary between 575 to 989 million m³/a for average flow rates of between 50
m³/s to 400 m³/s respectively. Under the current operating conditions with an average flow of approximately 70 m³/s
the river requirements is estimated to be in the order of 615 million m³/a.
b) The hydrology for the Lower Orange RSA was produced at a lower level of detail due to the fact that area, although
large in size, contributes to less than 3% of the total flow in the Orange. In the current yield models a total flow of 218
million m³/a is used, which is not the natural flow in the true sense of the meaning, but rather represents the inflow to
the Orange River from this area, under current conditions. The natural flow generated in this area is 420 million m³/a,
but a large amount of this is not reaching the Orange River, due to non contributing areas as well as large pans in the
Sak and Brak rivers, from where extensive evaporation losses occur. Flow from the Molopo is not reaching the Orange
River as sand dunes near Noenieput have blocked its course for at least the last 1000 years. The natural flow still
reaching the Orange River has not been determined in detail, but is expected to be in the order of 330 million m³/a.
c) The only flow data available for this area is for Oanob Dam in upper reaches of the Auob, Otjivero and Daan Viljoen
dams in the upper reaches of the Nossob as well as Dreihuk and Tsamab dams in the Hom and Ham rivers
respectively, which is located relatively close to the main Orange River. These areas represent a very small portion of
the total Lower Orange Namibia sub-catchment and will therefore not reflect the total volume of runoff generated in this
area. The Auob and Nossob rivers are both tributaries of the Molopo River, and flow from these rivers will not reach the
main Orange River. Under natural conditions it will only be the flow from the Dreihuk and Tsamab dam catchments that
can reach the main stem of the Orange River. The 7 million m³/a from those areas, however, do not represent all the
natural runoff that will enter the main Orange River from the Lower Orange Namibia sub-catchment, as all the small
tributaries close to the main Orange River will to a certain extent contribute to the runoff in the Orange River, although
small.
d) The total volume of runoff generated in the Fish River catchment (706 million m³/a) is quite substantial. Due to the
erratic nature of the run-off, the low rain fall and high evaporation in this catchment, large volumes of transmission
losses occur along the river reaches, and it is estimated that only 494 million m³/a will on average reach the Orange
River under natural conditions.
It is interesting to note that although the Lower Orange (Fish River Excluded) sub-
catchment area represents approximately 35% of the total Orange River catchment
area, only 3% of the runoff reaching the Orange River is generated in this sub-
catchment, while 35% of the runoff is generated in the Senqu Catchment, which in turn
represents as little as 2.6% of the total Orange River catchment area.
The runoff
contributions from different main sub-catchments in the Orange River Basin are given
in Table 3-4 and shown on Figure A-7 of Appendix A.
Statistics of the natural flows are summarised in Table 3.5, and it is evident that in sub-
catchments with a high unit runoff such as Senqu (163 mm/a) the coefficient of
variance (CV) is typically low (0.35). In arid areas where CV's as high as 3 to 5 are
common, the unit runoff is as low as 1mm/a. The change in the CV over the entire
basin is clearly evident from Figure A-8 of Appendix A where it slowly changes from
relatively low values in the higher rainfall areas to high values it the arid areas. Some
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Orange IWRMP
Task 4: Surface Hydrology
lower CV values are found in the Boskop and Klerkskraal sub-catchments which seem
like outliers. These values are however strongly affected by base flows from dolomitic
springs in these sub-catchments and are therefore not reflecting the characteristics of
rainfall runoff on it's own.
Table 3-5: Natural flow statistics
Name
MAR (million
STD. DEV
CV *
Unit runoff
m³/a)
*(million m³/a)
(mm/a)
Vaal River Catchment
Upper Vaal
1 977
1 570
0.79 (0.73 ­ 0.84)
51.2
Vaal Barrage
257
209
0.81 (0.74 ­ 0.92)
29.7
Middle Vaal
1 076
967
0.90 (0.47 - 0.93)
17.8
Lower Vaal
191
400
2.10 (1.51 ­ 3.46)
3.6
Riet/Modder
406
617
1.52 (1.17 ­ 3.04)
14.7
Orange River Catchment
Senqu
4 038
2 015
0.50 (0.35 ­ 0.58)
163.2
Upper Orange
1 450
1 507
1.04 (0.86 ­ 1.71)
29.8
Caledon
1 217
885
0.73 (0.37 ­ 1.58)
79.8
Lower Orange RSA
330
637
1.93 (1.43 ­ 5.32)
1.0
Lower Orange Botswana
No data available
Lower Orange Namibia
1) 7
16
2.24 (2.01 ­ 3.67)
1.5
Fish River Namibia
494
835
1.69 (1.25 ­ 1.78)
5.2
Note: * - The values in brackets give the range of the CV for the sub-catchments within the
main
sub-catchment.
The single value outside the brackets is the weighted average CV for the main sub-catchment. The standard deviation
given for each main sub-catchment was calculated backwards from the CV and therefore also represents a weighted
average value. Standard deviation and CV values were only available from the reports for the sub-catchments and not
for the main sub-catchments. Details of the standard deviation and CV values for the sub-catchments are given in the
data base.
The confidence one has in the natural flow sequences generated from the different
studies and which were finally included in the system analysis of the total Orange
River System, is of great importance, as it will directly dictate the confidence one has
in results from the system analyses. The ratings based on the confidence ranking
system as given to the natural flow sequences, are summarised in Table 3-6 and
shown in more detail in Figure A-9 of Appendix A. Ratings given for sub-catchments
in the Vaal River, Upper Orange, Caledon and Senqu are in general very good with
some exceptions such as for the Sterkfontein Dam sub-catchment in the Upper Vaal
and the Aliwal Noord sub-catchment in the Upper Orange.
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The large transfer volumes into the Sterkfontein Dam from the Tugela made it almost
impossible to obtain an accurate naturalised flow and calibration for this sub-
catchment and the generated natural flow was therefore based on regional
parameters. The runoff from this sub-catchment is relative small (18 million m³/a) and
it represents less than 1 percent of the total flow generated in the Upper Vaal. The
lower rating for the Sterkfontein Dam sub-catchment has therefore almost no effect on
the larger system.
As result of the high flows in the Orange River relative to the incremental flow from the
Aliwal Noord sub-catchment, it was not possible to obtain an accurate indication from
the observed flow for the incremental catchment only. The flows for the Aliwal Noord
sub-catchment was therefore simulated using the calibrated parameters of the
neighbouring Kraai River catchment.
Although the total flow from this incremental
catchment is not that small (229 million m³/a) it still represents only 3.4 percent of the
total natural inflow to Gariep Dam. Inaccuracies in the simulated flow for the Aliwal
Noord sub-catchment will thus have a small impact on the larger system.
Ratings for natural flows from the Lower Orange are in general not very good. No
natural flow data is available for Botswana and large parts of Namibia. Very little of the
runoff generated in these areas is, however, expected to reach the main Orange River.
It was therefore decided to include an additional column in Table 3-6 to indicate the
importance of the runoff from a sub-catchment to flow in the Orange River and
therefore also to water supply schemes depended on water from the main Orange
River.
Table 3-6: Rating of available natural flow data
Main Sub-
Data importance
Rating
Description
catchment
to flow in Orange
River
Upper Vaal
5
1)A to C
Most of sub-catchments received an A to B
rating which means good rainfall data and
good to reasonable calibrations
Vaal Barrage
5
A to B
Good rainfall data and good to reasonable
calibrations
Middle Vaal
5
A to B
Good rainfall data and good to reasonable
calibrations
Lower Vaal
4
A to B
Good rainfall data and good to reasonable
calibrations
Riet/Modder
5
A to B
Good rainfall data and good to reasonable
calibrations
Senqu
5
2)A to B
Most of sub-catchments received an A rating
indicating good rainfall data and good
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Orange IWRMP
Task 4: Surface Hydrology
Main Sub-
Data importance
Rating
Description
catchment
to flow in Orange
River
calibration.
Upper Orange
5
3)B to C
Most of sub-catchments received a B rating
which means good rainfall data and
reasonable calibrations
Caledon
5
B
Good rainfall data and reasonable calibration
Lower Orange RSA
3
C
Based on flows generated using regional
parameters and reasonable rainfall data.
Lower Orange
2
E
No data is available
Botswana
Lower Orange
2
4)E to F
The largest portion of this area received an E
Namibia
rating which means there is no data available
Fish River Namibia
4
B to D
Hardap and Naute dam catchments received
a B rating with a C for Seeheim and D for
Lower Fish.
Notes: 1) ­ It is only the Sterkfontein Dam sub-catchment flow which received a C rating. This sub-catchment is very
small relative to the remainder of the Upper Vaal catchment.
2) ­ It is only the smaller sub-catchments from the Lesotho Lowlands Study that was rated as B's
3) ­ It is only the Aliwal Noord sub-catchment that was rated as C, the rest of the sub-catchments was all rated as
B's.
4) ­ Data were only available for the small sub-catchments just upstream of exiting storage dams in Namibia. These
sub-catchments represent less than 9% of the Lower Orange
Namibia sub-catchment. The natural flows available for
these catchments are simply the observed flows as the effect of development in the catchment on the observed flows is
regarded as negligible. The rating given for these sub-catchments were therefore a F, which means that reasonable
observed data were used as natural flow due to the low level of development.
The importance of the runoff from the Lower Orange Botswana and Namibia areas are
shown as a 2, which means that it will be nice to have this type of information, but will
add very little value to the overall hydrology. The natural runoff from these areas will
however be of more value for the local area, specifically in some areas of Namibia
where existing dams are already utilizing runoff from the sub-catchment.
The flow from the Lower Orange RSA was given a slightly higher importance value of 3
as it contributes in the order of 218 million m³/a to the main Orange River. This will
have an effect on the flows in the Orange, and will in particular be valuable in cases
where a dam such as the proposed Vioolsdrift Dam is considered in the Lower Orange
River.
The flow from the Fish River (Namibia) is regarded as important to the main Orange
River, as the total volume reaching the Orange River is quite substantial,
approximately 494 million m³/a under natural conditions. Although the existing users
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Orange IWRMP
Task 4: Surface Hydrology
from the Orange River downstream of it's confluence with the Fish River is limited, the
flow still provide a valuable contribution to the environmental requirements in the river
and at the river mouth. Locally within Namibia the Fish River is of high importance and
two of Namibia's largest storage dams are located in this catchment, namely Hardap
and Naute dams.
The natural flows with the highest ratings (B) in the Fish River
catchment are those for the Hardap and Naute Dam sub-catchments, which are for
local purposes the most important flow data. Although a B rating was given for these
two sub-catchments the natural flow for these two sub-catchments should rather be
regarded as a low B as it is not at the same level as the other B's. The flows from
these two sub-catchments already represent approximately 34% of the total runoff
generated in the Fish River catchment. The Seeheim sub-catchment has a C rating
which should also be regarded as a low C and represents approximately 47% of the
runoff generated in the Fish River catchment. The remaining 19% of the flow data has
a low rating (D) and includes the Konkiep and Lower Fish sub-catchments.
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Orange IWRMP
Task 4: Surface Hydrology
4
CONCLUSIONS AND RECOMMENDATIONS
4.1
Conclusions
For system analysis purposes one would in general require natural flow sequences
with a relative high confidence rating of an A or at least and B. Confidence ratings of
C and lower should preferably not be used. The natural flow data for the total Vaal
River catchment as well as the Orange River catchment from Vanderkloof Dam and
upstream falls in the A and B confidence rating with the exception of two small areas
which will have a negligible affect on results. In the remainder of the Orange River
catchment only two sub-catchments (Hardap and Naute) within the Fish River sub-
catchment has a B rating which is also regarded as a low B rating. All the rest of the
sub-catchment has ratings of a C and less.
This means that on a catchment area basis approximately 32 percent of the area has
natural flow records with a confidence rating that is high enough to be used in system
analysis. From this 32 percent of the catchment area, approximately 94 percent of the
natural flow reaching the Orange River, is however generated. This brings along a
total different perspective, showing that on a volume basis only 6% of the natural flow
records for the Orange River are not at an acceptable confidence level.
This 6%
represents the runoff volume that is expected to reach the Orange River main stream
and expressed as a volume it would be in the order of 680 million m³/a. This volume
was not neglected in previous analyses, but is just not at the desired confidence level.
This means that the margin of error is most probably higher than the 10% to 15%
generally accepted for hydrology and might be in the range of 25% to 30% (170 to 200
million m³/a error) for 68% of the catchment.
Improving these flow records will
contribute to an overall improvement in accuracy of approximately 1% which is not
much. Considering only the Lower Orange and Fish River inflows the improvement
might be in the order of 12%, which need to be taken into account when the focus is
on developments in the Lower Orange and Fish River sub-catchment.
Areas where no natural flow records are currently available amount to a total of
220 500 km². Although this area is significant in size, it includes mainly arid and semi
arid areas. If the unit runoff for this area is between half to equal that of the Lower
Orange RSA, which is also a arid to semi arid area, it means that the runoff generated
from the 220 500 km² can be in the order of 110 to 220 million m³/a, of which only
approximately 16% (17 to 35 million m³/a) is expected to reach the Orange River. The
remainder will only be useful for local use close to the area where it was generated.
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Orange IWRMP
Task 4: Surface Hydrology
Most of this runoff currently evaporates and/or is contributing to the recharging of
groundwater resources.
No data was available on small dams in the Senqu and Lower Orange Sub-
catchments. It is however expected that small dams in the Senqu sub-catchment will
be negligible. In the Lower Orange River catchments the effect of the smaller dams
will quite possibly be significant and should be taken into account.
4.2
Recommendations
From the findings and conclusions given in this report the following recommendations
are made.
· Upgrade the hydrology for the areas contributing to flow in the main Orange
River to at least a B rating. Effect of small dams need to be included.
· Develop and upgrade hydrology for the areas (Lower Orange Namibia and
Botswana) that is not contributing to flow in the Orange River main stem, to a
confidence level representing at least a C rating. Effect of small dams and
endoreic areas need to be included.
· Discrepancies in the rainfall isoline between the different counties need to be
resolved (mainly RSA & Botswana).
· Discrepancies in evaporation isoline between the different counties need to be
resolved and which evaporation data should be used A-pan or S-pan or both.
· Agreement on the Orange River Basin boundary specifically between Namibia
and Botswana need to be reached.
· Standardise the approach that need to be followed to develop hydrology of an
acceptable confidence level between the four countries.
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Task 4: Surface Hydrology
5
REFERENCES
DWAF (1994)
Vaal Augmentation Planning Study. Orange Vaal Transfer
Scheme: Hydrology. Report no.
PC 000/00/13194, PD 000/00/3894 by Oorvaal Consortium for
the Department of Water Affairs and Forestry, Pretoria, South
Africa, August 1994. Prepared by Project Team.
DWAF (1994)
Vaal Augmentation Planning Study. Orange Vaal Transfer
Scheme: System Yield. Report no
PC 000/00/13194, PD 000/00/3894 by Oorvaal Consortium for
the Department of Water Affairs and Forestry, Pretoria, South
Africa, August 1994. Prepared by Project Team.
DWAF (1999)
Vaal River System Analysis Update. Lesotho Highlands
Hydrology. Report no PC 000/00/16996 by BKS (Pty) Ltd,
Stewart Scoot Inc and Ninham Shand (Pty) Ltd for the
Department of Water Affairs and Forestry, Pretoria, South
Africa, January 1999. Prepared by R S McKenzie.
DWAF (1991)
Orange River System Analysis. Upper Orange River:
Hydrology. Report no P D 000/00/0691 by BKS Incorporated
Consulting Engineers and Stewart Sviridov & Oliver Consulting
for the Department of Water Affairs and Forestry, Pretoria,
South Africa, December 1991. Prepared by R S McKenzie, N
W Schäfer, and P G Van Rooyen.
DWAF (1996)
LHDA / DWAF Joint Study of Lesotho Highlands
Hydrology: Executive Summary. Report no WR/Les/0039/96
by LHDA and BKS Water Resources Associates and SS&O for
the Department of Water Affairs and Forestry, Pretoria, South
Africa, November 1996. Prepared by Project Team
DWAF (1986)
LHDA / DWAF Joint Study Of Lesotho Highlands
Hydrology; Supporting Report A: Rainfall. Report no
PC 000/00/5286 by BKS Water Resource Associates and
SS&O for the Department of Water Affairs and Forestry,
Pretoria, South Africa, February 1986. Prepared by Project
Team.
DWAF (2004)
Internal Strategic Perspective. Lower Orange Water
Management Area. Report no P WMA 14/000/00/0304 by
PDNA, WRP Consulting Engineers (Pty) Ltd,
WMB
and Kwezi-V3 for the Department of Water Affairs and
Forestry, Pretoria South Africa, July 2004. Prepared by Project
Team.
DWAF (2004)
Internal Strategic Perspective. Upper Orange Water
Management Area. Report no P WMA 13/0000/00/0304 by
PDNA. WRP Consulting Engineers (Pty) Ltd, WMB and Kwezi-
V3 for the Department of Water Affairs and Forestry, Pretoria,
South Africa, April 2004. Prepared by Project Team.
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Orange IWRMP
Task 4: Surface Hydrology
DWAF (1997)
Orange River Development Project Replanning Study.
Hydrology and System Analysis - Orange River Basin.
Report no PD 000/00/4697 by BKS (Pty) Ltd and Ninham
Shand (Pty) Ltd for the Department of Water Affairs and
Forestry, Pretoria South Africa, May 1997. Prepared by R S
McKenzie and H G Maré.
DWAF (2002)
Orange River Continuous Studies Task 15. Caledon -
Modder River System Planning Analysis: Effect of Updated
Water Requirement Projections based on the National
Water Resources Strategy Study. Report no P876615 by
BKS Engineering Management for the Department of Water
Affairs and Forestry, Pretoria, South Africa, February 2002.
Prepared by H G Maré and R Dube.
DWAF (2004)
Internal Strategic Perspective. Orange River System:
Overarching. Report no P RSA D000/00/0104 by PDNA, WRP
Consulting Engineers (Pty) Ltd, WMB and Kwezi-V3 for the
Department of Water Affairs and Forestry, Pretoria, South
Africa, February 2004. Prepared by Project Team.
DWAF (1991)
Orange River System Analysis. Lower Orange River:
Hydrology. Report no P D000/00/1992 by BKS Incorporated
Consulting Engineers and Stewart Sviridov & Oliver Consulting
Engineers for the Department of Water Affairs and Forestry,
Pretoria South Africa, November 1992. Prepared by R S
McKenzie and N W Schäfer.
WRC (1994)
Surface Water Resources of South Africa 1990: Volume III.
Report no WRC 298/3.1/94 by Steffen, Robertson and Kirsten
Inc, Stewart Scott Inc, Watermeyer Legge Piésold & Uhlmann
for the Water Research Commission, South Africa, 1994.
Prepared by D C Midgley, W V Pitman and B J Middleton
DWAF(2000)
Vaal River System Analysis Update. Historic and Long-
Term Stochastic Yield Analysis of the Grootdraai Dam and
Bloemhof Dam sub-systems. Report no PC 000/00/17696 by
BKS (Pty) Ltd, Stewart Scott Inc and Ninham Shand (Pty) Ltd
for the Department of Water Affairs and Forestry, Pretoria
South Africa, February 2000. Prepared by H G Maré.
DWAF(2000)
Vaal River System Analysis Update. Historical and Long-
Term Stochastic Yield Analysis of the Senqu sub-system.
Report no PC 000/00/17796 by BKS (Pty) Ltd, Stewart Scott
Inc and Ninham Shand for the Department of Water Affairs and
Forestry, Pretoria, South Africa, February 2000. Prepared by P
G Van Rooyen, H G Maré and O Viljoen.
DWAF (1999)
Vaal River System Analysis Update. Hydrology of the
Middle Vaal Catchment. Report no PC 000/00/16496 by BKS
(Pty) Ltd, Stewart Scott Inc and Ninham Shand (Pty) Ltd for the
Department of Water Affairs and Forestry, Pretoria South
Africa, October 1999. Prepared by W V Pitman, C E Herold
and A K Bailey.
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Orange IWRMP
Task 4: Surface Hydrology
DWAF(1999)
Vaal River System Analysis Update. Hydrology of the
Upper Vaal Catchment. Report no PC 000/00/16296 by BKS
(Pty) Ltd, Stewart Scott Inc and Ninham Shand (Pty) Ltd for the
Department of Water Affairs and Forestry, Pretoria South
Africa, January 1999. Prepared by R S McKenzie and FGB de
Jager
DWAF(1997)
Vaal River System Analysis Update. Hydrology of the
Barrage Catchment (Draft). Report no PC 000/00/16396 by
BKS (Pty) Ltd, Stewart Scott Inc and Ninham Shand (Pty) Ltd
for the Department of Water Affairs and Forestry, Pretoria
South Africa, January 1997. Prepared by A K Bailey.
DWAF (1991)
Orange River System Analysis. Riet and Modder River:
Hydrology. Report no P D 000/00/0591, P C 000/00/0291 by
BKS Incorporated Consulting Engineers and Stewart Sviridov &
Oliver Consulting Engineers for the Department of Water
Affairs and Forestry, Pretoria, South Africa, December 1991.
Prepared by Project Team.
DWAF (1993)
Orange River System Analysis. Caledon, Modder and Riet
Rivers: Yield Analysis. Report no P D 000/00/1092, P D
200/00/0192, P C 500/00/0792 by BKS Incorporate