Sensitivity of Western U.S. river basins to seasonally varying climate warming

Abstract:

Climate warming over the Western U.S. is projected to have a seasonally varying signature, generally with more warming in summer than winter. Our previous work has shown that the same amount of warming in different seasons can result in much different changes in annual streamflow. However, the mechanisms that control the responses to cold vs. warm season warming remain poorly understood, especially for Western U.S. river basins, runoff from many of which is dominated by winter snow accumulation and spring melt. Here, we investigate the factors that control streamflow response to seasonal warming for the 616 HUC-8 river basins across the Western U.S., mostly west of the Continental Divide. We focus on the relationship between runoff response asymmetry and basin characteristics, including basins’ temperature, gross incoming water, elevation, runoff coefficients, vegetation height, snow thickness, and snow coverage, over the period 1950 to 2018 using four land surface models (CLM4.5, VIC, Noah-MP, and SSiB). We apply a water balance framework to associate the runoff response asymmetry with the asymmetry of seasonal evapotranspiration (ET)-temperature (T) sensitivity. We use the Penman-Monteith framework to quantify the primary ET drivers’ contribution to ET-T sensitivity as a function of the above basin characteristics. Our results suggest an increasing contribution of snowmelt-season radiation change to ET-T sensitivity as warmer snow-covered basins warms, which is synergistically supported by the variation of elevation, snow coverage, runoff coefficient, and vegetation height.