Comment on hess-2022-160

Abstract

The Heihe River Basin in Northwestern China depends heavily on both manmade and natural storage (e.g., surface reservoirs, rivers, and groundwater) to support economic and environmental functions. The Qilian Mountain cryosphere in the upper basin is integral to recharging these storage supplies. It is well established that climate warming is driving major shifts in high elevation water storage through loss of glaciers and permafrost. However, the impacts on groundwater-surface water interactions and water supply in corresponding lower reaches are less clear. We built an integrated hydrologic model of the middle-basin, where most water usage occurs in order to explore the hydrologic response to cryosphere trends. We simulate watershed response to loss of glaciers (Glacier scenario), advanced permafrost degradation (Permafrost scenario), both responses (Combined scenario) and projected temperature increases in the middle basin (Warming scenario) by altering streamflow inputs to the model to represent cryosphere melting processes, as well as by increasing the temperature of the climate forcing data. Net losses to groundwater storage in the Glacier scenario and net gains in Permafrost and Combined show the potential of groundwater exchanges to mediate streamflow shifts. The result of the Combined scenario also shows that permafrost degradation has more of an impact on the system than glacial loss. Seasonal differences in groundwater-surface water partitioning are also evident. The Glacier scenario has the highest fraction of groundwater in streamflow in early spring. The Permafrost and Combined scenarios meanwhile have the highest fraction of streamflow entering the subsurface in late spring and summer. The Warming scenario raises the temperature of the Combined scenario by 2C. A reversal in trend to net groundwater storage loss, and large seasonal changes in evapotranspiration and stream network connectivity relative to Combined show the potential for warming to overpower changes resulting from streamflow. Our results demonstrate the importance of understanding the entire system of groundwater-surface water exchanges to assess water resources under changing climatic conditions. Ultimately, this analysis can be used to examine the cascading impact of climate change in the cryosphere on the resilience of water resources in arid basins downstream of mountain ranges globally.