However, the state’s water resources are extremely vulnerable to climate change–in part because much of historical precipitation falls at or near the freezing line, so that small increases in temperature will have big hydroclimatic impacts–but also because California receives much of its water from only a handful of storms per year. Scientists also predict the snowpack in the Sierra Nevada mountain range–a significant source of Western water–– could largely decrease or even disappear in the coming decades. Understanding and predicting how water resources will be affected by climate change is crucial to creating and implementing water-resource management strategies that take into account a future with different hydrology.

EESA scientists Alan Rhoades and Erica Siirila-Woodburn recently contributed to a study published by European Geosciences Union in Hydrology and Earth System Sciences that investigates how water may be impacted by predicted end-of-century extreme dry and wet years. The team paired cutting-edge global climate and hydrology models to simulate these extremes in the Cosumnes Watershed, which contains one of the last naturally flowing rivers, to study an environment without large-scale water management. The researchers showed that, in all water year types–wet, average, and dry–end-of-century temperature and precipitation increased.

“Unlike other climate change projection studies which try to predict the impacts on the hydrologic cycle with so-called “pseudo-climate change” or “delta” approaches that assume some fixed amount of atmospheric warming,” Siirila-Woodburn explained, “we wanted to drive a hydrologic model with atmospheric forcing that considers global atmospheric patterns and processes, without any implicit assumptions on the outcome of those changes in greenhouse gas emissions. Historically, there has been a mismatch between the scale at which hydrologic and earth system models are run–making their pairing difficult. Now, with advancements in high performance computing and variable-resolution models, we’re able to produce outputs suitable for hydrologic models and to make predictions like the ones from this study.”

The study demonstrated that the impact of high emissions through the end of the century on this northern California watershed–and likely others like it–is higher temperatures, more precipitation in the form of rain instead of snow, and significant changes in the timing of surface water discharge and seasonal groundwater storage trends. Peak streamflow may also be much higher, a consideration for flooding and reservoir management. They also showed that, with higher end-of-century temperatures and soil moisture, evapotranspiration–the combined flux of water from transpiration and evaporation–also increased. These changes are projected to lead to a decrease in both summer surface water and, more substantially, groundwater storage.

Interestingly, the model also predicts shifts in hydrologic spatial patterns. End-of-century streams may run dry during parts of the year, with more focused streamflow in the mainstem of the river network during the summer. This is an important consideration for ecosystem habitats, streamflow chemistry, and water resource availability.

This study provides a new and detailed understanding of how watersheds may respond to climate extremes. Better predictions of how water resources may be altered in the future allow for more proactive management, adaptation, and the opportunity to ensure water is available, both for the millions of California residents and the crucial agricultural industry that the state supports.