The national renewable groundwater resources in Namibia are estimated at 2.1 km³ / year. Groundwater sources meet approximately 57 % of total the water consumption demand, with the remaining 43% being supplied from surface water sources (FAO 1995).
The percentage of mean annual precipitation that ends up as river flow in ephemeral systems in Namibia varies from as little as below 1 % up to around 12.5 % for parts of the Fish River basin. The remainder goes directly to evaporation and evapotranspiration, with the latter being by far the greatest component. Some of the runoff recharges alluvial aquifers on its way downstream, and in so doing the majority of ephemeral river floods eventually disappear entirely into the sand. Namibia’s ephemeral rivers are ‘effluent’ systems. This means that the river feeds the groundwater table, rather than having its flow sustained by a high groundwater table, as is the case with ‘influent’ rivers. Namibia’s groundwater occurs in a wide range of rock types making groundwater management a complex process. It provides a buffer against drought in many regions of the country, but it does remain inherently vulnerable to over-exploitation and pollution.
Within the Namibian part of the Kunene River basin groundwater reserves have generally a very limited potential, with the slight exception of the dolomitic formation on the eastern margin of the basin. To the east of the basin, north of the Etosha pan in the Oshana and Oshikoto Regions, groundwater is saline and the 700 000 people living here rely on the Calueque-Oshakati Water Supply Scheme for their drinking water needs. Desalination of this brackish water for drinking purposes is at an experimental stage and groundwater is not used for supplies. Further to the east the groundwater quality and potential improves again.
Groundwater reserve potential in Namibia.
Source: NamWater 2007
( click to enlarge )
Aquifers occurring in Namibia are classified as porous aquifers (such as continuous sandstone aquifers and the unconsolidated Kalahari and alluvial aquifers), and fractured / karstic aquifers comprising a localised groundwater potential along the structures and karstic features. Parts of the Grootfontein-Otavi-Tsumeb Karstland aquifer have been subject to thorough investigations and modelling, and for the Otavi Mountain Area the following recharge conditions were identified:
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The recharge rate amounts to 2 percent of the long-term mean annual rainfall after a sequence of rainy seasons in each of which the long-term annual rainfall is exceeded;
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The recharge rate amounts to 1 percent of the long-term mean annual rainfall after a single rainy season in which the long-term mean annual rainfall is exceeded;
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The recharge rate amounts to 0 percent if the rainfall does not exceed the long-term mean annual rainfall (FAO 2010b).
