Estimation of mean water residence times using tritium measurements in the Euphrates River Basin, Syria

The data of tritium, as an environmental radioisotope tracer, was used to estimate the mean water residence times of the Euphrates River water at a number of stations (Jarablous, Al-Raqqa, Deir-Ez-Zor), distributed along its course in Syria The estimation process of mean residence time for this hydrological system is based on the application of mathematical modeling approach, starting with the determination of the input function for this system, dealing with this water system as a black box, and treating the input function to become similar with that of the output function, i.e. the observed tritium content in the river water at the monitored stations. The available tritium data of rainfall in both stations, Adana in Turkey and Jarablous in Syria, was used to buildup the tritium input function. The calculations of tritium output values were done by applying the so-called MODEL program, and by adopting the completely mixed reservoir model, linked with an exponential time distribution function. The results show that the mean residence time of the Euphrates River water ranges from 0.8 year (≅300 days) to 1.5 year (≅550 days). This result is in a good agreement with the mean residence time of water in Al-Assad Lake (250 days), calculated by adopting a lake volume of about ≅11.9 billion m3, and an average river discharge rounded 550 m3/s. (author)