Differential water distribution over dune slopes as affected by slope position and microbiotic crust, Negev Desert, Israel

Runoff is one of the main water sources responsible for water redistributio n within a given ecosystem. Water redistribution is especially important in arid regions, and may be of great importance an sandy dunes, where the lik elihood of runoff is low owing to the high infiltration rates of sand. Redi stribution of water may significantly affect plant and animal distribution, and may explain vegetation patterns within an ecosystem. Runoff yield over sandy dune slopes in the western Negev Desert was measure d under natural conditions during 1990-1994. The magnitude of runoff yield on different slope sections and on north and south exposures was establishe d. The results demonstrate that while slope position controlled the microbi otic crust cover, crust cover and crust biomass controlled the amounts of r unoff obtained. Whereas no runoff was measured on the upper dune sections d evoid of crust, only meagre quantities were measured on the midslope sectio ns, characterized by discontinuous crust cover. Substantially larger amount s were, however, obtained at the bottoms of the slopes, characterized by co ntinuous crust cover. North-facing slopes, usually characterized by a chlor ophyll a content of 29-41 mg m(-2), yielded on average 3.2 times more runof f than south-facing footslopes, characterized by a 17 mg m(-2) chlorophyll a content. Whereas microbiotic crust was found to be responsible for runoff generation, additional water supply owing to runoff may also explain the o ccurrence of a high biomass crust and the dense vegetation belt at the dune -interdune interface of the northern exposure, where runoff tends to collec t. Thus, whereas crust may reduce in filtration in certain habitats, runoff generated by crust may also be responsible for the promotion of crust grow th in other habitats. Runoff may also be used to promote vegetation growth at the dune footslopes. The possibility of using runoff to facilitate agrof orestry is discussed. Copyright (C) 1999 John Wiley & Sons, Ltd.