Water Resources Assessment of the Mountainous Upper Syr Darya Catchment

Water is a strategic and highly contested resource in Central Asia. This is exasperated by a highly uneven distribution of this resource within the region as most of the water originates from the high-mountainous regions. In this study we evaluate the current water resources of the highly mountainous headwaters of the Syr Darya River. Trends in hydrometeorological data are investigated and the dominant hydrological processes are studied using hydrologic modeling. The Syr Darya is one of the two tributaries of the Aral Sea, which has been of high interest due to its drastic decrease. The headwaters investigated in this study include the Naryn and Karadarya Rivers, which originate in the mountainous regions of Kyrgyzstan and flow into the Ferghana Valley where they form the Syr Darya River. The discharge regime is dominated by nivo-glacial processes and therefore highly susceptible to climate change.

Streamflow and climatological data spanning from 1889 until 2018 is statistically analyzed and evaluated with regard to trends and change points. For example, at the gauge in Naryn City, an increase in streamflow can be observed, while precipitation peaks tend to shift to earlier months. The observed temperature increase is above the global average.

For a more comprehensive understanding of the water resources of the region the fully revised version of the Soil Water Assessment Tool (SWAT+) is used to represent the hydrological cycle of the catchments. The model is calibrated using daily streamflow gauges at several locations. In addition, evapotranspiration is calibrated using remotely sensed data from the Global Land Evaporation Amsterdam Model (GLEAM). The model is driven by three different sets of reference data from three ensembles of General & Regional Circulation Models (GCM and RCM, respectively). In detail, we used one RCM, (REMO) and the two sets of GCMs from ISIMIP2 (Inter-Sectoral Impact Model Intercomparison Project) and ISIMIP3. The ISIMIP data is based on the 5^th and 6^th phase of the Coupled Model Intercomparison Project (CMIP5 and CMIP6), respectively. The different driving climatological data sets are investigated with respect to multiple variables essential for an understanding of the water resources, such as streamflow, evapotranspiration, snow and soil moisture. Besides, observed trends and signals are investigated with respect to their dominant physical controls.