Factors controlling stable isotopes variability in precipitation in Syria: Statistical analysis approach

Abstract

The major characteristics of the stable isotopes (18O and 2H) and deuterium excess (d-excess or d) of rainfall samples collected from 16 stations during two periods (1989–1993 and 2003–2006) were statistically analysed to understand the factors controlling the temporal and spatial isotope variability in precipitation (P) in Syria. Rainfall waters at the mountainous regions are remarkably the most depleted in heavy stable isotopes, with also high d-excess values (d> 20 ‱). The enriched rainfall waters (lower d-excess values), mostly referring to coastal and some inland areas, are subject to the evaporation of falling raindrops. Rainfalls at the southern stations have impressively the highest d-excess values (d> 22 ‱), likely because of intensive interactions with the Mediterranean Sea (MS) moistures. The temperature (T) effect, shown by an average δ18O gradient of ≈0.34±0.03‱/∘C, is very close to the Global Network for Isotopes in Precipitation value (0.3 ‱ / ∘C). The altitude effect, represented by a δ18O gradient of −0.18 ± 0.03 ‱ / 100 m, fits well with the range given for most Mediterranean countries. Relative humidity (RH) and P-amount effects are characterised by an isotopic depletion, with a notable rise in the d-excess value with P and RH increases. The seasonal effect is shown by depleted δ18O and δ2H values during December–February and enriched δ18O and δ2H values at the end of April, with frequently low d-excess values (< 8 ‱). T parameter seems to be the major factor that strongly controls the variability of the stable isotope compositions of P in Syria. The moisture from the MS and secondary evaporation conjunctly play additional roles, but of less importance than the T effect.