Climate and surface properties: hydrological response of small arid and semi-arid watersheds

A positive relationship between annual rainfall and geomorphic processes (runoff and erosion rates) and environmental factors (water regime, soil and vegetation cover) is often assumed for arid and semi-arid areas with an annual rainfall in the range of 100–300 mm. This assumption disregards the fact that changes along a climatic gradient, at desert margins, are not limited to purely climatic factors. They are often accompanied by a parallel change in surface properties; especially the relative extent of rocky or soil covered surfaces that differ greatly in their response to rainfall. This raises the issue whether the change in surface properties along a climatic gradient enhances the assumed positive effects of rainfall increase or limits it. The hypothesis advanced in this paper is that runoff generation and rate in arid and semi-arid areas are primarily controlled by surface properties rather than by the absolute amounts of storm and annual rain amounts. Hydrological data collected at two instrumented watersheds, located one in an arid rocky area, and the second in a semi-arid soil covered area, support this hypothesis. The implications of data obtained for runoff generation and flow continuity under changing climatic conditions are analyzed. They point to the fact that the same regional climatic change may have different, and even opposite effects, on the hydrological response of different adjoining surface units. This response is expected to be strongly controlled by the specific local surface conditions that prevailed in the area prior to the climatic change.