Namibia is the driest country in sub-Saharan Africa and depends largely on groundwater. With an average rainfall of about 350 mm per annum, the highest rainfall occurs in the Caprivi in the northeast (about 600 mm per annum) and decreases in a westerly and southwesterly direction to as little as 50 mm and less per annum at the coast. The only perennial rivers are found on the national borders with South Africa, Angola, Zambia, and the short border with Botswana in the Caprivi. In the interior of the country surface water is available only in the summer months when rivers are in flood after exceptional rainfalls. Otherwise, surface water is restricted to a few large storage dams retaining and damming up these seasonal floods and their runoff. Thus, where people don’t live near perennial rivers or make use of the storage dams, they are dependent on groundwater. The advantage of using groundwater sources is that even isolated communities and those economic activities located far from good surface water sources such as mining, agriculture, and tourism can be supplied from groundwater over nearly 80% of the country.