Runoff generation from successive simulated rainfalls on a rocky, semi-arid, Mediterranean hillslope

Runoff generation in a Mediterranean carbonate rock environment-the Judaean Mountains. West Bank and Israel-was studied by a 2 day sprinkling experiment on an 18 x 10 m² plot. During the first day, three spells of 24.1 mm h^-1 (12.3 mm), 27.8 mm h^-1 (12.3 mm) and 14.4 mm h^-1 (12.5 mm) were applied, almost saturating the runoff plot. On the second day, 20 h later, two spells of 18.7 mm h^-1 (34.3 mm) and 35.2 mm h^-1 (25.4 mm) followed. Surface runoff was a combination of infiltration excess runoff from rocky portions of the plot and saturation excess runoff from areas covered by soil. Soil saturation was accelerated by lateral runoff from adjacent rocky areas. Once the plot was saturated, 80-90% of the applied rainfall became surface runoff. Significantly different concentrations of chloride, sulphate, and nitrate ions in the sprinkled water on two successive days served as tracers. During the second day a two-component hydrograph separation pointed to the importance of shallow reservoirs playing active parts in runoff generation. About 14% of the flow collected during the second day originated from water applied during the first day. During the second day of sprinkling, both water sources obviously mixed in saturated soil reservoirs and contributed in variable percentages to surface flow. It is hypothesized that, at least during high-magnitude rainfall, the steep rocky slopes investigated might be regarded as flood-generating zones rather than areas of pronounced recharge into the underlying Yarkon-Taninim aquifer.