The water resources of the Kunene River are substantial, being a perennial river with a mean annual flow of 50 m³ per second at Gove Dam and 160 m³ per second at Ruacana, with an annual discharge of about 5.5 km³ at the river mouth.
The hydrology of the Kunene river basin is discussed in terms of
The Kunene River basin is conventionally considered as consisting of three sub-basins - the Upper, Middle and Lower Kunene (for more information on this see the chapter on Watersheds). In general terms, the climate in the Upper and Middle Kunene can be classified as a tropical wet-and-dry savannah climate, influenced by the high altitudes, while the lower section has a semiarid to arid climate with tropical steppe and desert vegetation. The majority of the runoff is generated from rain falling between October and March over the highlands in Angola in the Upper Kunene, with the main river being fed by a dense network of tributaries. In the highlands, the rainfall is in the order of 1 300 mm/yr , which steadily decreases to a mere 20 mm/yr at the mouth of the river.
Kunene River downstream of Gove Dam.
Source: Kellner 2010
( click to enlarge )
Due to the variation in climate from humid in the northern highlands to arid at the coast, between 75 and 90 % of the flow of the river is generated north of Matala in the Upper Kunene. Flows in the Kunene River are usually close to zero by the end of the dry season, although flow regulation by storage dams upstream (at Gove and Matala) will make flows more regulated.
Interannual rainfall variability is high in the Upper Basin and results in large differences between dry and humid years. The highly variable annual flow volumes differ as much as 14-fold between high and low years (PJTC 2009) while within a given year, variation in flow can be as much as 11-fold between high flow in April and low flow in October. The chart below shows the mean, maximum and minimum hydrographs for the Kunene River at Ruacana, and illustrates the high variability of mean annual rainfall across the basin. The chart also shows that the mean flow patterns are much closer to the recorded minimum than to the maximum (DRWS 2001).
A detailed discussion of hydrological monitoring in river basins is contained under Resource Monitoring.
Monthly flow volumes (1933-1997) at Ruacana.
Source: DRWS 2001
( click to enlarge )
The high year to year and annual variation is illustrated in the chart below. Annual runoff values as recoded or estimated from 1933/34 to 2000/2001 show relatively long periods of drought put on record lasting for several years.
Mean annual runoff (MAR) at Ruacana from 1933 to 2001.
Source: DRWS 2001
( click to enlarge )
Pitman and Midgley (1974) modelled the sub-basins as single units, based on runoff records collected from gauging stations in the period 1961-1972. The majority of the mean annual runoff (MAR) is generated in the Upper Kunene sub-basin where the average rainfall is 1300 mm/year, contributing 75 to 90 % of the total runoff in the basin. The Middle Kunene contributes 10 to 25 % of the total runoff while the contribution of the ephemeral tributaries in the Lower Kunene is commonly less than 5 %.
Hydrologic Characteristics of the Kunene Sub-basins
|
|
Gauging Station |
Area (km²) |
MAR (Mm³/year)
|
MAR (mm)
|
|
Upper Kunene |
Matala |
27 950 |
4 884 |
167 |
|
Middle Kunene |
Ruacana |
56 200 |
6 012 |
67 |
|
Lower Kunene* |
- |
24 400 |
- |
- |
* Information for the lower Kunene is not available since no gauging station exists downstream of Ruacana.
The geomorphical development and the climatological differences in the basin have created an environment that is particular in terms of runoff, sediment production, transport and deposition.
The Upper and Middle Kunene belong to a well-developed ancient drainage system. The Middle Kunene is characterised by low gradients, annually flooded plains and natural deposition environments. In contrast, the Lower Kunene is a young river characterised by steep gradients, rapids and a channel controlled by bed-rock structure. These differences have created two distinct sediment delivery systems: In the upper and middle part of the basin, fine grained suspended material dominates in combination with chemically dissolved load. In the lower reaches, the river beds indicate a higher amount of sediment load.
The Kunene River is relatively unpolluted and the Water Quality is considered to be good, with a low concentration of phosphorus as well as other nutrients.
