Generally the low rainfall in arid and semi-arid areas is distributed with a high variability in space and time, which hinders profitable agriculture and creates instability in production. However, concentrating the scarce rainwater into smaller areas by means of water harvesting techniques allows economic agricultural production and can also improve drinking water supplies in arid regions. Surface water harvesting techniques that are well adapted to the ecological and social context have the potential to improve the productivity of arable and grazing land by increasing the yields and reducing the risk of crop failure. They are relatively cheap and can therefore be a viable alternative in regions where irrigation water from other sources is too costly. Water harvesting can reduce the use of groundwater, which is a valuable water source and needs energy for its exploitation. The collection of rainwater can even contribute to the recharge of groundwater tables (Prinz, 1999). Very often rainwater harvesting techniques help to prevent floods and to control soil erosion. Thus the systems allow not only a suitable irrigation in regions with scarce water sources but also an improvement of the hydrological equilibrium and the soil quality (Alaya et al., 1993).