Three aspects of the hydrologic cycle are critical to climate in the Kunene River basin:
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Precipitation;
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Evaporation; and
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Temperature variability.
Precipitation and evaporation are described below. Temperature variability across the basin is discussed in the chapter on Climatic Variability.
Precipitation
Mean annual precipitation varies significantly across the basin, with well over 1 000 mm/yr falling on the plateau in the north east of the Upper Kunene and decreasing dramatically to below 100 mm/year at the coast in the south western reaches of the Lower Kunene. Around 75 % of the entire flow of the Kunene is generated in the Upper Kunene sub-catchment.
Precipitation across the Kunene River basin.
Source: AHT GROUP AG 2010, after WorldClim 2009
( click to enlarge )
Rainfall is seasonal with around 90 % of the annual volume falling in the five month rainy period from December to April, with the main season occurring between February and March.
Towards the coast, fog is a major source of moisture for the ecosystem in the desert-ocean interface where the cool Benguela current in the Atlantic Ocean comes into contact with the hot desert air. An annual average of 50 to 75 fog days per year is observed in this region.
Evaporation
Across the entire basin, average annual evaporation totals are very high. They are lowest in the upper reaches of the catchment at around 2 500 mm/yr, rising through the Middle Kunene and reaching around 4 000 mm/yr around Calueque. From Calueque towards the coast the evaporation rate falls to less than 2 400 mm / year.
Most of the annual total evaporation occurs between the months of September and December, corresponding to the dry period across the basin, with monthly averages of around 300 mm to 350 mm. The months of June and July have least evaporation with around 180 mm.
