Current and future water balance of a mountain subcatchment of Issyk-Kul Lake, Tien Shan range, Kyrgyzstan.

Snow and ice dominated basins are particularly vulnerable to climate change but estimating their hydrological balance remains challenging in data-scarce regions like the Tien Shan mountains. With the overall aim of modeling of the large Issyk-Kul Lake basin in Kyrgyzstan, this article focuses on the hydrological balance of the Chon Kyzyl-Suu basin, a representative sub-catchment of the lake basin. The study involved two steps: first, calibration/validation of a distributed hydrological snow model, second, assessment of future trends in runoff, evaporation, snow melt and glacier melt under different climate scenarios. Our results show that the balance of the basin is already upset due to glacier mass loss and that groundwater processes play a significant role in generating discharge. Climate projections for the next 40 years (2020-2060) show no significant trend in precipitation under scenario ssp2-4.5 but an 8.9 % decrease in precipitation under scenario ssp5-8.5. at the same time, air temperature will increase by 0.4 °C under scenario ssp2-4.5, and by 1.8 °C under scenario ssp5-8.5. Under the "business as usual" scenario (ssp2-4.5), the annual river flow of the headwater basins should increase by 13 %, or under the "pessimistic" ssp5-8.5 scenario, by 28 %, mainly due to the increase in glacier runoff. These results make it possible to envisage realistic modeling at the scale of the lake at a daily time step.