UCLA

Climate models project stronger warming in the warm season than in the cool season over much of the Western U.S. Across much of this region, the widely-accepted hydrologic signature of climate change is reduced winter snow accumulation and earlier streamflow peaks, which are mainly caused by cool season (October-March) warming. However, the warm season (April-September) warming effect receives far less attention. One of the few studies that investigated the warm season warming using a single land surface model showed that while runoff timing shifts consistently under cool season warming, runoff volume changes are mostly attributable to warm season warming in the western U.S. However, what remains unclear are the mechanisms controlling runoff responses to asymmetrical (warm/cool season) warming, as well as the extent to which the previous results are model-dependent. To answer the questions, we expand the earlier work to include experiments with four land surface models over the four major river basins of the West, with a focus on investigating the reasons causing differences in annual and seasonal streamflow response to asymmetric seasonal warming. Our results show that: (i) the general features of seasonal and annual streamflow responses to asymmetric warming are consistent across models, although the magnitudes vary; (ii) basins with higher ratio of warm to cool season gross incoming water, cooler summer, and colder winters have the strongest relative annual streamflow decreases for warm season warming than cool season warming, as a consequence of strongly influenced evapotranspiration-temperature sensitivity.