Estimation Of Phreatic Evaporation In Irrigation Agriculture Using Stable Isotopes

Agriculture in the Aral Sea basin is the main consumer of water resources and due to the current agricultural management practices inefficient water usage causes huge losses of freshwater resources. There is huge potential to save water resources in order to reach a more efficient water use in irrigated areas. Therefore, research is required to reveal the mechanisms of hydrological fluxes in irrigated areas. This paper focuses on estimation of one of the crucial components in the water balance of irrigated areas - phreatic evaporation (Ep), i.e. evaporation from (shallow) groundwater - using stable isotopes of water. Our main objective was to estimate the rate of phreatic evaporation on sites with different soil texture and ground water tables (GWT) and investigate the relationship between these environmental parameters and the Ep rate. Soil samples were collected in various soil depths from irrigated areas in Ferghana Valley (Uzbekistan). The soil water from these samples was extracted via a cryogenic extraction method and analyzed for the isotopic ratio of the water isotopes (2H and 18O) based on a laser spectroscopy method (DLT 100, Los Gatos USA). A total of 18 soil profiles in fields under cotton have been analyzed. Estimations of phreatic evaporation rates were evaluated in dependence of soil texture and groundwater table. Annual amounts of water losses via phreatic evaporation were calculated between 104 to 349 mm, accounting for 35.1 % of mean irrigation water. Ep rates significantly increase with decreasing depth to GWT. There also exist difference of Ep rate between different soil texture classes with lower rates on sandy and loamy soils as and higher rates on clay. We conclude that site specific groundwater level managing can reduce phreatic losses substantially, providing an efficient and easy adaptable way to improve irrigation and leaching practices.