ABSTRACT
In humid regions there is an obvious excess of precipitation over the seasonally integrated water need for an abundant plant cover. Different runoff generation processes (e.g. runoff from saturated areas, piston-flow effects, macropore flow and the slow outflow of large groundwater bodies) sustain the flow of perennial rivers. On the other hand, semi-arid zones can be viewed as those where a favourable water balance is achieved only seasonally. During the wet season most precipitation infiltrates to refill underground storages emptied during the long dry period. Humid runoff generation processes, dependent on the abundance of water, lose significance; runoff is increasingly generated as infiltration excess overland flow following the ideas of Horton (1933). Conditions for this process are even more favourable in arid areas, mainly as a result of the absence of a developed soil and vegetation cover and exposure of impervious surfaces. Surface runoff hence achieves renewed significance in desert environments (Gat 1980).
