Runoff-induced sediment yield over dune slopes in the Negev desert. 1: Quantity and variability

Although extensive data exist on runoff erosion and rates for non-sandy hillslopes, data for arid dune slopes are scarce, owing to the widespread perception that the high infiltrability of sand will reduce runoff. However, runoff is generated on sandy dunes in the Hallamish dune field, western Negèv Desert, Israel (P ≈ 95 mm) due to the presence of a thin (usually 1-3 mm) microbiotic crust. The runoff in turn produces erosion. Sediment yield was measured on ten plots (140-1640 m²) on the north- and south-facing slopes of longitudinal dunes. Two plots facing north and two facing south were subdivided into three subplots. The subplots represented the crest of the active dune devoid of crust, the extensively crusted footslope of the dune, and the midslope section characterized by a patchy crust. The remaining plots extended the full length of the dune slope. No runoff and consequently no water-eroded sediments were obtained from the crest subplots devoid of crust. However, runoff and sediment were obtained from the mid- and footslope crusted subplots. Sediment yield from the footslope subplots was much higher than from the midslopes, despite the higher sediment concentration that characterized the midslope subplots. The mean annual sediment yield at the Hallamish dune field was 432 g per metre width and was associated with high average annual concentrations of 32 g 1^-1. The data indicate that owing to the presence of a thin microbiotic crust, runoff and water erosion may occur even within arid sandy dune fields.