A water balance is used to describe the flow of water in and out of the Kunene River system and can be used to help manage the water supply and to predict where there may be water shortages. Given the general lack of quantitative data for the Kunene Basin, a very simple and generic approach is presented here using the following water balance equation:
P = Q + E + dS
where
P is precipitation
Q is runoff
E is evapotranspiration
dS is the change in storage (in soil or the bedrock)
Precipitation (P) in the basin is known from interpolated historical data. Actual river runoff is known for the Ruacana hydrological station: This station is located in the Lower Kunene, with almost all the run-off from the basin passing this point due to the low contribution from the arid Lower Kunene. For a large period of time, the variation of the underground storage dS can be disregarded and considered zero. The only water coming into the system is then precipitation.
Kunene River near Epupa.
Source: Wagner 2006
( click to enlarge )
This approach is of course a gross oversimplification: Runoff characteristics in the densely vegetated areas with high rainfall on the plateau are very different compared to the southern parts of the basin along the border with much lower rainfall and different vegetation cover and soil characteristics.
Average Precipitation (P)
Precipitation decreases from 1 500 mm/yr in the northern parts of the Upper Kunene to less than 50 mm/yr at the mouth of the river. In the Lower Kunene, water resources are severely limited, with annual evaporative losses far exceeding annual rainfall. Studies by Gois (1970) estimated the average precipitation north of Ruacana at 958 mm/yr, which is considered here as a long term average for the Upper and Middle Kunene sub basins.
Actual Runoff (Q)
Actual runoff at Ruacana station during the same study period was estimated at 6 457 Mm³/year (Pitman and Midgley 1974). For a catchment area of 89 600 km² (north of Ruacana), this corresponds to a flow of 72 mm/year.
Evaporation (E)
Application of the above formula gives an evaporation (E) of 886 mm/year, which includes evaporation from soil and vegetation, evaporation from the river itself and from artificial lakes at the various dams in the river (Gove, Matala, etc), from the extensive wetland areas in the Middle Kunene during the rainy season, and water extraction for domestic, agricultural and industrial use. The evaporation of 886 mm/year corresponds to 79 000 Mm³/year.
Water use and abstraction
Information on water abstraction for domestic, agriculture and industry use does not exist for the Angolan parts of the basin. The small-scale irrigation in the Middle Kunene (Huila province) along the Kunene and some of its tributaries can be considered as the most significant water withdrawal in the Angolan part of the basin. Industrial water extraction is limited to the mining of red and black granites in Middle and Lower Kunene.
Quantitative data on water use is only available for the transfer of water from the Calueque dam into the North Central Region of Namibia. The 1990 agreement on the Kunene River includes the provision to abstract 6 m³/s at Calueque for diversion to the Cuvelai. However, due to lack of pumping capacity, Namibia can only divert about 2-3 m³/s at present. This water is used for domestic purposes and for the 600 ha Etunda irrigation scheme.
