Rainfall variability is a major constraint to human life in arid and semi-arid areas. The variations can be seasonal as well as interannual. When the average precipitation is very low, fluctuations in rainfall, both as droughts and as floodings, can have severe consequences for a country. Many developing countries are situated in the arid and semi-arid belt on the Earth, e.g. in the Sahel belt of Africa. The rainfalls in these regions mostly appear as showers. Due to the dry soil most of the rainwater will go into surface run off and evaporate. Lakes and other natural basins for the collection of rainwater are very rare. To overcome this unequal distribution of rain it is necessary to find some kind of storage, natural or artificial, in which the rainwater, infiltrated into the ground, can be stored to be used during the dry season. The usual artificial type of storage is surface dams. However, they are very unfavourable in dry areas due to intensive evaporation and sedimentation. Underground storages in soil or rock do not have these disadvantages. On the other hand it is a problem to find necessary storage capacity in these media for the infiltrating water. Deep layers of alluvials have been used since long as aquifers. The storativity of these deposits is dependent on the extent of the strata and its porosity. Sedimentary rock, e.g. sandstones and limestones, have a natural porosity. They mostly are good aquifers. Hard rocks on the other hand have a secondary porosity, a fracture porosity, which can be waterbearing. Due to the tectonic type of the fractures, shear or tensile, they have quite different storage capacities. The authors show how special tensile fractures systems have a remarkable storativity. A tectonic model of this type of fracture is demonstrated. The rain water, which infiltrates the rock via the soil, is drained downstream the fracture system by gravity. A method is shown how to stop this drainage by means of an underground dam. This is achieved by an injection screen transvers to the main direction of the fractures. The authors show how the ground water storage in the subterranean dam and how the necessary leakage downstream are controlled by a simulation model. In areas, where drinking water is very costly, the storativity of the tensile fracture system can be increased considerably by the construction of an underground rock storage in close connection to the fractures. A preliminary model of this arrangement is discussed.
Strong climatic fluctuations, seasonal as well as interannual are characteristic for the arid and the semi-arid areas of the Earth. In large parts of Africa the mean deviation in annual rainfall is more than 25 % of the average. This means that rainfall of less than half the average or more than twice the average is a real risk in arid and semi-arid areas (Morales, 1977). The rainfalls of this type of climate mostly fall as showers. Due to the dry soil most of the rainwater goes into surface run off and finally evaporates.