THE CHEMICAL-COMPOSITION OF PRECIPITATION AND RUNOFF WATER ON AN ARID LIMESTONE HILLSIDE, NORTHERN NEGEV, ISRAEL

The study deals with the chemistry of precipitation and runoff water on an arid limestone hillside (467 m2) the upper part of which is rocky and the lower part soil covered. Rainfall was measured with a rain recorder and sampled with a sequential collector which samples consecutive fractions of 2 mm. Runoff rate was measured with a stage recorder and runoff water was collected immediately after each flow from a collecting tank. Rain and runoff samples were analysed for their major constituents: Na+, K+, Ca2+ Mg2+, Cl-, SO42- and HCO3-. Rainfall during the study year (1981/1982) amounted to 75 mm. The salt input by rainfall was 8 g m-2. The salt output by runoff was 1.2 g m-2 for the rocky area and 0.5 g m-2 for the soil-covered area, indicating a substantial net gain for the different areas. Both rainwater and runoff water have a calcium bicarbonate composition. These two ions represent some 55% of the dissolved ions. Runoff water is enriched by a factor of 2 in comparison with rainwater. Finally, the chemistry of runoff water from very small hillslope areas is quite similar to that of flood waters in the major channels of the Negev, pointing to a very limited chemical enrichment with increasing drainage area.