Rain events in an arid environment — Their distribution and ionic and isotopic composition patterns: Makhtesh Ramon Basin, Israel

Abstract

Forty-six individual rain events and successive fractions of these events were studied with a network of instruments in the arid Makhtesh Ramon Basin, southern Israel, during 1981/1983 Annual rainfall varied from 47 to 107 mm, number of rain events varied from 8 to 20 per year, start of the rainy season varied from September to January, termination of the rainy season varied from March to May and length of the rainy season varied from 4 to 9 months. About 85% of the rain events were recorded at more than one station, indicating an aerial distribution exceeding 20 km. A cliff amount effect was observed — the rain on the cliff (800 m) was 73% more than the rain at the bottom of the Makhtesh (500 m). Dust samples revealed the following soluble ions (in equivalents): Ca 2+ ⪢ Mg 2+ > Na + ⪢ K + and HCO − 3 > Cl − > SO 2− 4. The dust also contained CaCl 2. Rain composition of 61 analyzed samples revealed (in equivalents): Ca 2+ > Na + > Mg 2+ ⪢ K + and HCO − 3 > Cl − > SO 2− 4, neutral pH of 7.1–7.6, and presence of CaCl 2. A similar pattern was observed in 54 samples collected during the same period at Sde Boker, 30 km north of the Ramon, thus typifying the Negev Heights. Two distinct sources of dissolved ions were inferred: dust, providing mainly Ca(HCO 3) 2, and cloud-borne sea spray, providing mainly Na +, Mg 2+, Cl − and SO 2− 4. A chemical and isotopic effect was observed — the rain of the rainier year contained 34% less dissolved ions and was isotopically lighter in δ 18O by 54% than the rain of the less rainy year. A chemical and isotopic front effect was observed — the first fraction of the individual rain events contained more dissolved ions (32–69%) and was enriched by more δ 18O (31%) than the subsequent rain fractions. The observed rain distribution and chemical isotopic effects are discussed in terms of input sources, evaporation processes and altitude effects. The obtained data define rain and salt inputs into the hydrological systems.