Climatic factors controlling chemical and isotopic characteristics of precipitation in Syria

The chemical and isotopic composition of monthly composite rain samples collected at 13 meteoric stations in Syria during two hydrological cycles from 1991 to 1993 have been measured. The chemical analysis of the samples revealed at a number of stations pollution due to industry and sand storms. The temporal and spatial variation of the isotopic composition has been found to be comparable with one of the neighbouring countries such as Jordan and others. The average weighted oxygen-18 and deuterium values are - 7·5 and - 39·11[per thousand], respectively, and the deuterium excess is 21[per thousand] . The individual isotope values can be divided into two groups. One group is represented by winter precipitation and fits closely the Mediterranean Meteorological Water Line (MMWL). Thus, for winter precipitation, condensation of Mediterranean water vapour appears to be the dominating isotope fractionation process. The other group represents spring precipitation and is spread along an evaporation line below the MMWL, thus indicating the influence of sub-cloud evaporation. The d-excess has been found to be lower in the north of Syria (19·9[per thousand] at Tartous, 18·1[per thousand] at Jarablous) than in the south of the country (23·4[per thousand] at Sweida, 24·1[per thousand] at Izra) where Mediterranean air mass dominates. The d-excess of precipitation in neighbouring countries is also close to the average value of the eastern Mediterranean basin of 22[per thousand] , e.g. for Jordan the value is 23[per thousand] , which signifies that Mediterranean water vapour is, for all these countries, the dominant source of precipitation. The tritium content of precipitation was found to increase with distance from the coast (5·3 TU at coastal station Tartous, 8·8 TU at continental station Palmyra). Low tritium content and high d-excess at coastal stations clearly indicate a Mediterranean air moisture source.