The appropriate choice of a technique depends on the amount of rainfall and its distribution, soil type and depth, land topography and local socio-economic factors (Oweis et al., 1999). Loess and loess-like soils are ideally suited for rainwater harvesting because even after a small rainfall a crust is formed that promotes runoff. Slope affects the quantity and the quality of runoff. The most efficient water harvest is on small, gently sloping catchments (with a slope preferably between 1 % and 5 %).

The ratio of the surface of the catchment area (runoff producing area or impluvium) to the surface of the cropping zone is expressed by the factor CCR (“catchment to cropping ratio�). In the course of time the respective ratios became apparent for the different systems according to the climatic conditions. The factor can also vary depending on the charac­teristics of the watershed. When planning a water harvesting system the proper establish­ment of the CCR is very important in order to avoid an insufficient water supply or damages to the systems as a result of an outsized catchment area (Alaya et al., 1993).

The CCR is directly related to the mean annual precipitation quantity and the mean runoff coefficient of the region. It also depends on the water requirements of the crops. The knowledge of these three parameters is indispensable for applying rainwater harvesting systems on a serious technical foundation. The relation between CCR and the three factors is the following:

CCR · C · P = WR – P

or:

CCR = (WR – P) / (C · P)

With:

CCR: area impluvium / area cropping zone [-]

P: mean annual precipitation [mm]

C: mean annual runoff coefficient [-]

WR: water requirements of the crops [mm]