Precipitation in the mountains of Central Asia: isotopic composition and source regions

Abstract. Isotopic composition of precipitation in the mountains of four Central Asian countries (Kazakhstan, Kyrgyzstan, Tajikistan and Uzbekistan) was measured using 908 event-based precipitation samples collected at eight sites in 2019–2021, and 7 monthly samples from Dushanbe (Tajikistan) thereby filling a gap in stable isotope data for the region. Regional and seasonal patterns of δ18O, δD and D-excess were investigated. Local Meteoric Water Lines (LMWL) derived using seven regression methods using both non-weighted and weighted precipitation. It is recommended that the non-weighted Ordinary Least Squares Regression (OLSR) and Reduced Major Axis Regression (RMA) methods can be applied across the region except in summer, when the Precipitation-Weighted Least Squares Regression (PWLSR) method is recommended. An atmospheric back trajectory analysis and a mixing model were applied in combination for the first time, using the δ18O, δD and D-excess data, to identify the atmospheric moisture source regions and quantify the relative importance. The main distant sources were the Black and Caspian Seas region, Iran – eastern Mediterranean, and northern Kazakhstan – Siberia. The recycled moisture from the irrigated lower reaches of the Amu Darya and Syr Darya rivers, and from the study catchments, accounted for 29–71 % of the atmospheric moisture reaching the observation points. In spring, summer and winter, in the Chon-Kyzyl-Suu catchment, up to 85 % of the precipitation was estimated to be derived from local re-evaporation, most likely from Lake Issyk Kul. These findings highlight the importance of moisture from terrestrial sources, especially irrigated land, in precipitation formation in Central Asia.

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