The climate of South Africa varies from desert and semi-desert in the west (rainfall less than 200mm mean annual) to sub-humid along the eastern coastal areas (more than 1000 mm mean a1mual) with an average annual rainfall of approximately 500 mm. As a consequence of the topography and rainfall distribution, the natural availability of water across the country is very unevenly distributed, with more than 60% of the river flow arising from only 20% of the land area.
The Directorate Hydrology of the Department of Water Affairs and Forestry is responsible for the supply of all required information on the quantity and availability of surface water in the RSA. Hydrology maintains and operates the national hydrological network distributed throughout the country. Eight regional offices collect this data in the field and supply it to the Directorate Hydrology in Pretoria for final data processing, quality assurance and availability to all data users. Flow is measured on a continuous basis at more than 1000 stream-flow stations and at approximately 200 flow meters on pipes. Water levels are monitored at about 250 dams. Rainfall data is collected at more than 360 sites and evaporation at about 350 sites. The combined average run-off for South African rivers totals 53 500 million m3 annually – less than half the run-off of the Zambezi River. Of this, large quantities are lost due to evaporation or wastage during floods, and only some 33 000 million m3 can be economically used each year. Groundwater also delivers another 5 400 million m3 annually. A total storage capacity of about 27 000 million m3 has been created by the construction of large dams, holding more than half of the mean annual run-off of the country. City councils, irrigation and water boards also operate a small number of flow stations.