Estimation of evaporation and irrigation return flow in arid zones using stable isotope ratios and chloride mass-balance analysis: Case of the Euphrates River, Syria

Stable isotope (18O and 2H) ratios were measured together with electrical conductivity (EC) values and chloride concentrations on a monthly basis at 12 stations along the Syrian portion of the Euphrates River during the period 2004–2005. Stable isotope ratios systematically increased with distance downstream with the most significant enrichment occurring at Al-Assad Lake. This systematic increase could be due to: (1) evaporation from the river and its tributaries and (2) drainage return flows of irrigation water via the groundwater system. Based on the integral enrichment of stable isotopes in the river water, the amount of water lost by evaporation from Al-Assad Lake was estimated to be about 1.93–2.15 billion m3 using 18O and 2H, respectively. On the basis of the chloride budget, the computed component of groundwater input through the riverbed from drainage return flow varies spatially from 70% to less than 1%, depending on the extent of irrigation exercised along the river course.