The components of the hydrologic balance, precipitation, discharge and evaporation, interact with each other. The thermal seasons (summer and winter) as well as the hygrical seasons (dry season and rainy season) influence their size. They varies in every area. The interactions of precipitation, runoff and evaporation are examined on the basis of the analysis of the hydrologic balance of an area. Except for the natural influences there are also human-caused influences such as construction of dams, deforestation, irrigation, extraction of ground water and so on.
In large parts of Africa clean water is already scarce good. Basing on the statistics about the expected population development the population will increase. The requirement of food will increase and therefore the requirement of water. To minimize the negative consequences for the environment through too much extraction of ground water as much as possible, it is necessary to know the numbers of hydrologic balance precipitation, runoff, evaporation, retention and operating capacity in its chronological and spatial distribution.