Assessing Future Hydrological Impacts of Climate Change on High-Mountain Central Asia: Insights from a Stochastic Soil Moisture Water Balance Model

Abstract. We use a new set of data available to compute 21^st century climate impacts on the hydrology of 221 catchments in high-mountain Central Asia. For each of these subcatchments, a parsimonious steady state stochastic soil moisture water balance model was set up and the partitioning of available water from precipitation into runoff and evaporation computed for different climate futures using the Budyko framework. Climate change sensitivity coefficients are analytically derived for the first time using the total differential method. Relative changes in discharge for three future periods 2011–2040, 2041–2070, and 2071–2100 were computed in relation to the baseline period from 1979–2011. For the baseline observation period, climate data from a global high-resolution climatology data set (CHELSA V21) were used to extract mean daily subcatchment-specific temperature and precipitation values. Data from the coupled model intercomparison project phase 6 (CMIP6) were used to compute catchment mean future climate data using 4 GCM models with 4 scenario runs each. CMIP6 data were bias corrected with CHELSA V21 observation data. For the spatial distribution of soil parameters, different global products were utilized. The robustness of the soil water balance model results was assessed using a comprehensive sensitivity analysis in relation to variations of these soil parameters over typically observed ranges for each subcatchment.

The analysis of climate change suggests increasing precipitation over the three periods (+4.44 %, +5.89 %, and +8.51 % relative increases in median total precipitation averaged over subcatchment and scenarios). Median values of temperatures changes between periods relative to the baseline are +1.33 °C, +2.44 °C, and +3.55 °C. Results of the hydrological soil water balance model runs suggest a median increase of discharge of +4.71 %, +7.44 % and +10.87 % for the corresponding periods. This is a strong indication of a wetter and hotter future in Central Asia, relative to today’s hydroclimate. Modelling results suggest that decreasing contributions from glacier melt over the course of the 21^st century will be offset by increases in discharge consistently throughout the region, despite increasing potential evapotranspiration. Increases in relative discharge will be most pronounced in the Afghan Murghab-Harirud basin and in the Amu Darya. Changes in precipitation characteristics in terms of frequency and event depth also indicate possible impacts on hydrological extremes which remains a heavily under researched topic in Central Asia.