Where water comes from

Dr. P. James Dennedy-Frank

Did you wash your hands today? Chances are that water started as Sierra Nevada snowmelt. It flowed through streams, reservoirs, and pipes from the Sierra across the Central Valley, and through your faucet. However, as climate change shifts snow to rain, California’s water system may no longer provide reliable water to the world’s 5th largest economy and farms that grow 1/4 of America’s produce.

To adapt our water system, we need to better understand how water cycles over and through Earth, which integrates weather, water use by plants, manmade structures, and complex subsurface geology. Researchers like me use computer models to better understand and predict this hydrologic system. We’ve found that properly representing water’s complex subsurface flow paths is key to correctly predict the system’s response to a changing climate. But past models represented only the bulk flow of water in historic conditions, so do not provide reliable predictions.

To do better, our group simulates detailed flow paths for nearly a billion virtual water “particles”, going from Sierra snow to the Central Valley. In field work, we measure tracers that can distinguish these flow paths to confirm our simulations are correct.

What are tracers? In the classic example, scientists learned about ocean currents by modeling the flow paths of 28,000 rubber ducks that spilled from a ship in the Pacific and made landfall around the world.

We couldn’t find rubber ducks small enough to fit through soil pores, so we test our simulations by measuring water with slightly different weights—called isotopes. These tracers tell us the elevation of precipitation (where the duck starts) and the water age (how long that duck spends in transit). Together, this when and where information also tells us whether the water starts as snow or rain.

What have we found? In our simulations, snow from small parts of the Sierra provides a surprisingly large portion of summer streamflow in the Valley—shifting that snow to rain will have a large effect. If the tracers confirm these simulations, this will inform how much water we need to store for the dry summer months; our work will also help determine where in the subsurface we might store water *for a not-so-rainy day*.

By helping California manage its water in a changing climate, we can help ensure a home for some of the world’s most productive agriculture, some of the world’s most beautiful landscapes, and many of the world’s most innovative companies (and laboratories).

*And please use some of that water to wash your hands*

By helping California manage its water in a changing climate, we can help make it a home for productive agriculture, beautiful landscapes and thriving ecosystems, and innovative people.

*And please use some of that water to wash your hands*