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CURRENT REQUIREMENTS
Unlike the Vaal River basin where
most of the water is used for urban and industrial purposes, most water used in
the Orange River basin is for irrigation with a relatively small portion being
utilised for urban and industrial purposes. River losses through
evaporation also represent a large portion of the total requirement as can be
seen in the figure below.
It is very difficult to provide accurate information on the water requirements
from the Orange River since they are continually changing and some are dependant
upon both the weather and average flow in the river. In order to provide
an indication of the water requirements supported from the Orange River (ie
excluding those supported by Vaal river water), they have been split into four
broad categories namely:
At 1994 development levels it is estimated that the total water requirement
supported from the Orange River is in the order of 3 500 million m3
per annum. The distribution of the water requirements is provided in the
figure below.
Click on image for full size diagram
The demands are summarised in the following table which provides an indication
of how the demands are split both geographically as well as by user group.
|
Description of water requirement |
Annual requirement
(million m3/a)
|
|
|
|
|
Irrigation |
|
|
Upstream of Gariep (36 400 ha) |
395 |
|
Supplied locally in Riet and Modder (7 100 ha)
|
70 |
|
Directly from Gariep and Vanderkloof (24 300 ha) |
278 |
|
To Eastern Cape through Orange/Fish Tunnel (52 700 ha) |
627 |
|
Lower Orange River in South Africa (63 790 ha) |
751 |
|
Lower Orange River in Namibia (2 270 ha) |
39 |
|
Total from the Orange River excluding the irrigation in the Vaal River basin
upstream of the Vaal/Riet confluence. (179 640 ha) |
2 160 |
|
|
|
|
Urban/Industrial Demands |
|
|
Orange River upstream of Vanderkloof Dam |
70 |
|
Eastern Cape demands through Orange/Fish Tunnel |
20 |
|
Downstream of Vanderkloof Dam including Namibia |
40 |
|
Total urban/industrial demands |
130 |
|
|
|
|
Other Requirements/return flows |
|
|
Environmental |
300 |
|
River evaporation losses (varies with flow) |
960 |
|
Return flows from irrigation |
-50 |
|
Total other requirements |
1 210 |
|
|
|
|
Total Orange River Requirements |
3500 |
As can be seen from the above table, irrigation is by far the largest user of
Orange River water. Most of the high value irrigation takes place along
the lower Orange River
downstream of Prieska and
there are numerous government water schemes along this stretch of river as shown
in the figure below.
Click on image for full size diagram
The seasonal distribution of demands is also an important factor to consider
when examining the water resources of the Orange River and the associated water
requirements. The figure below provides and indication of how the various
demands are distributed throughout the year. It does not include the
demands supplied from Gariep Dam
or those supplied through the main canal at
Vanderkloof Dam.
Click on image for full size diagram
As can be seen from the above figure, most of the consumptive requirements occur
during the summer months from October through until March. This is to be
expected due to the large influence of irrigation on the total requirement.
It does, however, present problems with regards to hydro-power generation at the
Gariep and Vanderkloof dams. ESKOM currently use the two HEP stations for peak
clipping power and would ideally like to generate most of the power during the
cold winter months when demands for electricity are at their highest. The
conflict between the two users is shown in the figure below.
Click on image for full size diagram
As a result of the recent studies into the water resources of the Orange River
it has been decided to generate power in accordance with the downstream
requirements during all periods when water resources are scarce.
Additional power can only be generated when the main storage reservoirs are at
or near full supply level. In this manner, no water is wasted and at the same
time any excess water is used productively to generate power.
FUTURE REQUIREMENTS
Various studies have been undertaken to assess the likely future water
requirements from the Orange River including the Orange River Replanning Study
(ORRS) which is the basis for this web site. For the purpose of the study
it was assumed that the irrigation demands will remain at the current levels
although the urban and industrial demands will increase. in addition, the
first phase of the LHWP is now
operational (January 1998) and by the year 2005 the transfer will approach 900
million m3/a. The most recent estimates indicate that the
transfer is likely to be slightly lower than this initial estimate due to
environmental considerations, however, for the purpose of the ORRS it was
decided to use the original value until a new value is finalised.
The figure below provides details of the expected water requirements in the year
2005 following completion of Phase 1 of the
LHWP.
Click on image for full size diagram
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