Hydrological Response of Desert Margins to Climatic Change: The Effect of Changing Surface Properties

Abstract

Arid and Semi-arid ecosystems are regarded by ecologists as highly resistant to stress due to their adaptation to the extreme variability in the climatic conditions over a time scale of decades. Under such conditions a rather extreme change in climate, mainly rainfall, would be required in order seriously to affect natural semi-arid and arid environments. The above approach disregards the fact that one of the forms of land-surface change that may result from climatic change in deserts, and especially at a desert fringe, is not limited to purely climatic variables such as precipitation and temperature. It is always accompanied by quite rapid alteration of surface properties, connected to deposition of loess or sand. In subtropical semi-arid and arid areas loess deposition, at a given site, is often attributed to wet periods; while sand deposition to dry periods. The new surface properties can be expected to exercise strong influence on infiltration, runoff and soil moisture. An aspect not yet answered is how much sand or loess deposition is required to affect the hydrological regime and related water resources. In order to check the effect of thin topsoil sand or fine-grained layers on infiltration and runoff sprinkling experiments were conducted in the laboratory, at various rain intensities and duration. Data obtained show that a slight change in surface properties has a rapid and significant hydrological effect. A sand layer 1–2 cm thick is enough to eliminate runoff generation; whereas a fine-grained layer 1–2 mm thick has an opposite effect, significantly increasing runoff generation. One can therefore conclude that arid and semi-arid environments, although highly adapted to extreme variations in rainfall, may be extremely sensitive to slight changes in their surface properties, which alter their hydrological regime quickly and efficiently.