Asymmetry of Western U.S. River Basin Sensitivity to Seasonally Varying Climate Warming

Abstract

AbstractFuture climate warming over the Western U.S. (WUS) is projected to be greater in summer than winter. Previous model‐based studies of large river basins in the WUS showed much different annual streamflow responses to warming in warm versus cool seasons. However, it remains unclear how the annual streamflow relative responses to seasonal warming (asymmetry) and drivers of the response asymmetry vary across the entire WUS at the catchment‐scale, and how the simulated results compare with observations. Here, we investigate the asymmetry of annual streamflow responses to warm versus cool season warming at the HUC‐8 level across the entire WUS using model simulations and observations. We also examine the asymmetries' relationship with land surface and hydroclimate characteristics, and the primary contributor to the response asymmetry for each HUC‐8 basin. The HUC‐8 level results reveal more complexity than do earlier analyses of much larger river basins. Over 25% of WUS area has annual streamflow increases in response to warming in at least one season (mostly cool season). Annual streamflow is most sensitive to warm season warming in cool, inland basins, especially the northern Columbia River basin and most of the Upper Colorado River Basin, and most sensitive to cool season warming in warm, coastal basins. This bi‐directional pattern is enhanced by vegetation coverage but weakened by long‐term snowpack decline. In coldest basins with short snow‐free seasons, net radiation changes dominate the streamflow response asymmetry. For basins with cold‐to‐intermediate temperatures, vapor pressure deficit changes dominate. For warmest basins, surface resistance changes dominate.