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WATER RESOURCES OF THE ORANGE RIVER

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The natural water resources of the Orange River are estimated to be in the order of 12 000 million m3/a and Table 1 provides an indication of the distribution of the water resources within the basin.

 

Description

Catchment Area (km2)

Natural run-off (million m3/a)

1. Vaal River basin 196 290 4300
2. Senqu/Orange to SA/Lesotho border

24 680

4 010

3. Caledon River basin to Welbedacht Dam 15 270

1 240

4. Remainder of Orange River upstream of Orange/Vaal confluence ~59 400*

1 300

5. Remainder of lower Orange basin excluding Fish River basin ~670 000+ 420
6.Fish River basin

76 000

 

480

Total

~1 000 000

11 750

*Based on 1994 Water Research Commission Reports. (WRC, 1994)

+Estimate including pan and other non-contributing areas

The run-off figures given in Table 1 relate to natural conditions and represent the flows which would occur if there were no developments in the basin. Obviously major developments have taken place, particularly in the Vaal and upper Orange catchments and as a result of these developments, the flow reaching the Orange River mouth is now estimated to be less that 50 % of the natural flow i.e. ~6 000 million m3/a. To highlight the current situation and the variability of the flow that can be expected in the Orange River, the average flows reaching the river mouth between 1976 and 1987 can be considered.

  • The average annual flow reaching the river mouth for the 12 year period between 1976 and 1987 is estimated to be   ~5 700 million m3.

  • The flow reaching the river mouth during the wettest year which includes the major flood of February 1988 was estimated to be ~26 000 million m3.

  • The average annual flow for the two driest years on record was estimated to be only ~1 100 million m3. It is interesting to note that during this period there was no inflow from the Vaal River and very little flow from the Fish.

Another interesting point to be considered when dealing with the natural and developed flows in the Orange River basin concerns the contribution from the area downstream of the Orange/Vaal confluence (i.e. item 5 in Table 1). Although this portion of the basin has a very large surface area, it contributes very little to the total run-off. The run-off estimate of ~420 million m3/a given in the table is in fact misleading since in reality the area often makes no contribution to flow in the Orange River. In normal years there is a net loss of water between Vanderkloof Dam and the river mouth. This is due mainly to high evaporation from the river surface which is estimated to be in the same order of magnitude as the natural inflows from areas 5 and 6.

It is only during infrequent and extreme rainfall events that the lower Orange catchment (excluding the Fish) makes a noticeable contribution to the Orange River. Such events may occur at intervals of many years or even decades and during such periods it is likely that the Orange River will already have above average or flood flows. The average natural inflow of ~420 million m3/a is therefore of limited use since it occurs sporadically in large volumes when it is least required and cannot be stored since there are no storage reservoirs downstream of the Orange/Vaal confluence. The losses on the other hand occur each year assuming that there is flow in the river (which is generally the case) and therefore have a very significant and detrimental influence on overall water resources.

When describing the water resources of the Orange River it should be noted that the sum of the individual natural run-offs does not necessarily give the total run-off for the whole basin. In reality, the natural run-off at the Orange/Vaal confluence is estimated to be in the order of 10 850 million m3/a as given in Table 1 (items 1, 2, 3 and 4).  The natural resources to the river mouth, however,  should rather be quoted as  approximately 11 000 million m3/a and not 11 750 million m3/a since the evaporation losses downstream of the Orange/Vaal confluence tend to exceed the combined natural inflows from areas 5 and 6 in the long-term.  In the short-term, the situation may appear worse since the natural inflows are likely to be insignificant while the losses will remain high. The confusion arises since the losses are linked more to the flow originating upstream rather than downstream of the Orange/Vaal confluence.