Isotopic composition as a tracer of different source contributions to stream flow in the glacierized catchments of Central Asia

Abstract

The mountains of Central Asia are water towers servicing the arid downstream regions and maintaining irrigation and food production. There are several sources of runoff: liquid precipitation, snowpack, glacier ice, ground ice (including rock glaciers and permafrost), and ground water. The relative contributions of different water sources to stream flow are poorly quantified and its improved understanding will reduce uncertainty in hydrological modelling and projections of changes in water resources. In 2019-21, an extensive sampling programme was conducted to quantify the relative contributions of water sources to stream flow in the Tien Shan and Pamir-Alai using stable water isotope tracers (SWI) of oxygen and hydrogen. Samples of the event-based precipitation, river discharge taken daily or twice-daily at the designated sampling points and every fortnight along the river courses, and water sources were collected in the glacierized catchments in Kazakhstan (Ulken Almaty and Kishi Almaty catchments), Kyrgyzstan (Ala-Archa and Chon-Kyzyl Suu), Tajikistan (Varzob and Kafornihon), and Uzbekistan (Chirchik). The samples were processed using Picarro isotope analyser. A data set of SWI ratios from approximately 6000 samples has been produced and analysed. It is the first comprehensive SWI database in Central Asia contributing to understanding of regional and global isoscapes and water resources. The local meteoric water line (LMWL) was developed from the event-based precipitation samples. It is approximated as δD = 7.6δ18O + 8.7. The values of SWI in precipitation exhibit a clear annual cycle and depend on precipitation type (rain, snow, and mixed). The derived seasonal SWI values are different from those available from the Water Isotopes Database being nearly twice as high in winter. Snow, glacier ice and permafrost exhibit distinct isotopic signatures although these vary between the basins. Glacier ice in the Chirchik basin appears to be more depleted than elsewhere. Rock glaciers were sampled in the Ulken Almaty basin showing SWI ratios similar to those of glacier ice but both are distinct from permafrost. These results point at the feasibility of the application of the mixing model and end-member mixing analysis approaches to the partitioning of runoff and quantifying relative contributions of different water sources in the Tien Shan and Pamir-Alai. This is a policy-relevant task under the conditions of climate change.