Precipitation Whiplash and Climate Change Threaten California’s Freshwater

Imagine the snow in the Sierra Nevada mountains as a giant reservoir providing water for 23 million people throughout California. During droughts, this snow reserve shrinks, affecting water availability in the state.

Researchers fear global warming will cause the Sierra Nevada snowpack to lose much of its freshwater by the end of the century, spelling trouble for water management throughout the state.

The California Department of Water Resources found last month that the water content in the Sierra snowpack was about half its historical average for the beginning of April despite late winter storms. One year before, the water content had been measured at over 160 percent of the historical average. This swing is not new and continues California’s recent trend of climate shifts, following the 2011-2015 drought.

Scientists from the University of California, Los Angeles (UCLA) expect to see an increase in ‘precipitation whiplash’ events in the region, with rapid transitions between extreme wet and extreme dry periods.

These extreme precipitation events pose a risk to dams, levees and canals, few of which have been tested against intense storms such as those that caused the Great Flood of 1862. By the end of the 21st century, the frequency of floods of this magnitude across the state is expected to increase by 300 to 400 percent.

Dams are at risk

During the winter of 2017, record snowfall in the Sierras caused a spillway to fail on the Oroville Dam, sending water spilling over the dam. Nearly 190,000 residents were forced to evacuate their homes. The man-made lake is the linchpin of California’s government-run water delivery system. It provides water for agriculture in the Central Valley and for homes and businesses in Southern California.

After the damage to the dam’s spillways, DigitalGlobe, a satellite imagery company, released images showing the extent of the damage.

Melting brings trouble

Scientists from UCLA’s Institute of the Environment and Sustainability and the Center for Climate Science predicted increased warming in the region will cause snow to melt faster. Also, more of the precipitation will fall as rain rather than snow.

Currently, half of the total water in the Sierra reservoir runs off by May. If greenhouse gases are not mitigated, by the end of the century we will see the water reserve halved 50 days earlier. This could pose problems in managing water in the reservoir system, which serves a dual purpose: it stores water for use in dry seasons, but also protects downstream communities against flooding.

The future of snowpack in the Sierras

If the pace of global warming remains unchanged, there will be 64 percent less snow in the Sierra by the end of the century, scientists said.

If the global community takes measures to curb climate change in line with the 2015 Paris Climate Agreement the loss in average springtime snowpack volume would be 30 percent.

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