Projecting the impacts of end of century climate extremes on the hydrology in California

Abstract. In California, it is essential to understand the evolution of water resources in response to a changing climate to sustain its economy and agriculture and build resilient communities. Although extreme conditions have characterized the historical hydroclimate of California, climate change will likely intensify hydroclimatic extremes by the End of Century (EoC). However, few studies have investigated the impacts of EoC extremes on watershed hydrology. We use cutting-edge global climate and integrated hydrologic models to simulate EoC extremes and their effects on the water-energy balance. We assess the impacts of projected driest, median, and wettest water years under a Representative Concentration Pathway (RCP) 8.5 on the hydrodynamics of the Cosumnes river basin. High temperatures (> 2.5 °C) and precipitation (> 38 %) will characterize the EoC extreme water years compared to their historical counterparts. Also, precipitation, mostly in the form of rain, is projected to fall earlier. This change reduces snowpack by more than 90 %, increases peak surface water and groundwater storages up to 75 % and 23 %, respectively, and makes these peak storages occur earlier in the year. Because EoC temperatures and soil moisture are high, both potential and actual evapotranspiration (ET) increase. The latter, along with the lack of snowmelt in the warm EoC, cause surface water and groundwater storages to significantly decrease in summer, with groundwater showing the highest rates of decrease. Besides, the changes in the precipitation phase lead the lower-order streams to dry out in EoC summer whereas the mainstream experiences an increase in storage.