The Relationship Between Modern Rainfall-Runoff Measurements and Soil Leaching in a Small First-Order Drainage Basin, Northern Negev, Israel

Harrison, J.B.J.

Department of Earth and Environmental Science,

New Mexico Tech, Socorro, NM 87801

Kosovsky, A., Yair, A., and Morin, Y.

Institute of Earth Sciences

Hebrew University of Jerusalem, Israel, 91904

GSA MEETING, 1994

SEATTLE

Geochemical soil catenas record the characteristics of hydrological processes over long periods of time. We compared the modern, detailed hydrological records of a small semi-arid watershet with soil catenary relations.

For the past 3 years, rainfall and runoff relations have been measured in a small first-order drainage basin in the hills near Lehavim in the northern Negev, Israel. Average storms consist of a number of short duration (<1 hour) and low intensity (less than 10 mm/hr) events. Under the observed conditions storm runoff from the entire slope is less than 1 percent of the rainfall. No through flow occurs along the entire slope and there is no channel flow. However, during the largest storm recorded in the region (100 mm rainfall in 24 hrs) runoff increased by an order of magnitude and a flow was observed in the first- order channel.

Three catenas of soils have been described from ridge crest to valley floor in an area where there is no channel. The upper slopes have a very thin and patchy regolith cover (<40 cm), whereas on the lower slopes the regolith is continuous and increases to a depth of more than 2 m in the center of the basin. The soils on the upper slopes are strongly leached as are the soils at the contact between the regolith and the continuous colluvium. With increasing distance towards the center of the valley, there is a concomitant decrease in the degree of leaching. However, after a distance of 5 m, the soils are very strongly leached even though soils depth is greater than 2 m. The caternary influence does not extend to the center of the valley where the soils develop in response to different hydrological conditions. This suggests that the wataer must have reached the center of the basin by overland flow and that significant through flow occurred, neither of which has been recorded in this area to date. The soil landscape records an extreme hydrological event greater in magnitude than the storm of last winter.