California’s Snowpack Is the State’s Biggest Reservoir—and It’s Declining

When most Californians think about where their water comes from, they likely think of the state's dams and reservoirs-and they're largely correct. Most of the state's annual rainfall arrives in a narrow window between October 1st and April 1st; twelve large reservoirs and over a hundred smaller reservoirs (scattered throughout the state) capture and store this water to control floods and keep our taps running in the dry season. But another natural reservoir is also essential to the state: snowpack.

At the start of spring, California's snowpack has historically contained about 70% as much water, on average, as all the state's reservoirs combined. That's an astonishing service, provided completely free of charge. Snowpack water storage is critical for a variety of downstream needs.

In the wet season, snowpack helps reduce the chance of flooding by retaining winter and spring precipitation. As it melts, snow slowly replenishes reservoirs in the dry season. Snowmelt also helps keep our trees and other vegetation supplied with water through the dry season. This makes it harder for wildfires to start, and makes them less severe when they do occur. Snowpack also helps provide a steady flow of cool, clean water, which many aquatic wildlife species depend on to survive the summer.

Snowpack is expected to become less reliable

Surprisingly, average annual precipitation is not expected to decline as the climate changes. But warmer temperatures will result in more precipitation falling as rain instead of snow, and snowpack will melt earlier. As more water runs off our mountains-rather than being stored as snow-during the rainy season, floods become more difficult to control and those steady supplies of cool water during the hot summer season become less common.

What tools can help California prepare for changes in snowpack?

Climate models are among the most important tools we have to prepare for changes in snowpack water storage and other impacts of our changing climate. These are models that allow us to estimate what future conditions will look like.

However, outputs from global climate models are not that helpful to local and regional decisionmakers on their own. "Downscaling" is a process that translates global climate model data into a form that's relevant to these decisionmakers. In recent years, these models have gotten better at capturing the actual physical processes occurring in the atmosphere at regional and local scales.

Researchers now have access to downscaled climate model data for California that checks both boxes-the scale of the data is relevant to local and regional decisionmakers and the models behind these data are highly physically realistic. These new data improve our ability to anticipate and prepare for the impacts of reduced snowpack water storage.

But are these models accurate? My coauthors and I asked this question in new research that examines how well these refined models are doing. By comparing model predictions to real-world measurements of snowpack in the southern Sierra Nevada mountains, we found that these models are good at capturing the timing of peak snowpack. However, less water is actually stored in snow than the models predict. And the snowmelt season is ending earlier than the models indicate.

These are important findings, since they can help water managers consider how to use model projections. Water managers need to understand the strengths and weaknesses in these data that may inform their decisions. Use of conservative estimates when planning is crucial to reduce the risk of impacts from extreme floods and droughts.

We also looked at future changes in California's snowpack water storage, and our findings are in keeping with the consensus of scientific community: snowpack will store less water, and snowmelt seasons will be earlier and shorter. The good news is that we have strategies to adapt to these changes. Managing groundwater aquifers conjunctively with surface reservoirs and using forecast-informed reservoir operations can help maintain water supplies and reduce flood hazard. And these new-and-improved climate model outputs can help us prepare for what the future holds.