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A thin microbial crust covers the lower part of longitudinal dunes in the western Negev, where average annual rainfall is 95 mm. In order to study the effect of the microbial crust on rainfall-runoff relationships under natural rainfall conditions, runoff plots equipped with pressure gauges were established on opposite north-and south-facing slopes that differ in their vegetal cover and crust properties. The study covered four years (1990-94). The first two years were wet and the following two years relatively dry, One to five flow events were recorded per year. No correlation was found between runoff yield, rain amount and rain intensity. Unlike many microbial crusts reported in the literature, the microbial crust in the western Negev is not hydrophobic. Infiltration rate is high under dry surface conditions and of the order of 9-12 mm h(-1) when the crust is saturated. The high final infiltration rate is explained by the occurrence of large pores that do not seal when the crust is saturated. Typical hydrographs have very steep rising and falling limbs, pointing at a Limited contributing area. Most flows last less than 10 min and runoff volumes collected are, on the whole, very small. Owing to differences in crust properties, runoff is higher on north-than on south-facing slopes. (C) 1997 John Wiley & Sons, Ltd.