Stable Isotope Mass Balance Method to Find the Water Budget of a Lake

Authors: VEHBİ ÖZAYDIN, UYGUR ŞENDİL, DOĞAN ALTINBİLEK

Abstract: Determination of a lake water budget is essential for water resources engineers. Some of these elements, such as surface inflow, precipitation, evaporation, surface outflow, and variation of lake level, can be measured easily at the site. However, it is difficult to determine the groundwater inflow and outflow. Although these quantities could be calculated as a residual term of the water budget equation, they yield information only on absolute contributions of groundwater flow. At least one additional equation is needed to solve the unknowns separately. Stable isotopes (oxygen-18 and deuterium), exist in water naturally, provide additional equations, and simultaneous solutions of these equations make it possible to quantify the groundwater inflow into and the outflow from the lake. In this paper, the stable isotope mass balance method, which is an expensive technique, together with the conventional water budget method are applied to Mogan Lake, located south of Ankara, Turkey, to determine the groundwater contribution to the lake. It was found that the average groundwater inflow to Mogan Lake is 20.42 million m^3 \pm 17.22% while the average groundwater outflow is 16.44 million m^3 \pm 24.95% for the 1994 water year.

Keywords: Stable Isotope, Water Budget, Isotope Mass Balance, Mogan Lake

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