Climate Model Projections Agree: Flooding to Increase in Sierras by Century's End

The source of much of California's water is more likely to experience increased flooding as the 21st Century progresses and climate change trends continue, according to a study led by Scripps Institution of Oceanography, UC San Diego, researchers.

The study lead author, Tapash Das, said that studying flood behavior in future decades will provide crucial information to water management, since California's reservoir system is designed to provide not only water storage but also flood protection. Thus, more severe floods would require maintaining reserve levels in key reservoirs, meaning that valuable water might have to be released.

Two recent papers on this subject will be presented by Das at a poster presentation at the 2012 American Geophysical Union Fall Meeting (2012 American Geophysical Union Fall Meeting (GC21C-0980 · Tuesday, Dec. 4, 8 a.m - 12:20 p.m. · Hall A-C, Moscone South)

California is expected to warm between 1.5 and 4.5 degrees C (2.7 and 8.1 degrees F) between now and 2100. Over the same time period, the atmosphere will hold more water vapor as global evaporation rates accelerate. Even though the overall frequency of precipitation events may decrease in many areas of California, researchers say there may be opposite effects upon the largest precipitation events. On occasion the higher moisture could beef up some of California's most extreme precipitation episodes, which usually occur in winter months, when it coincides with active North Pacific storms. On top of the effects on storm intensity, warmer mountain watersheds from climate warming would amplify runoff because more of a given basin receives rain instead of snow, and snowmelt would intensify.

Das, a researcher at engineering firm CH2M HILL in San Diego and a visiting assistant research hydrologist at Scripps, and colleagues at Scripps, Santa Clara University and the U.S. Geological Survey, ran scenarios using 16 different downscaled global climate models that were subsequently run through a hydrologic model calculation over the California watersheds. This ensemble of climate models provides a better estimate of future changes than any single model because every model has, to differing degrees, its own representation of various climate processes and thus somewhat different results regarding future changes. The analysis was done while Das was a postdoctoral researcher at Scripps. The study was supported by both the CALFED Bay-Delta Program funded-postdoctoral fellowship grant provided to Das and the California Energy Commission-funded California Climate Change Center.

The frequency of unusually heavy "50-year floods" increased in all but two of the model simulations. A 50-year flood is a large flood that has only a one in 50 chance of occurring in any given year. The researchers used the 50-year flood magnitude because of the limited time period provided by the climate model runs and the observed datasets, and it corresponds to the capacity limit of many components of California's water management infrastructure. This type of flood also exceeds the capacity of many roadways and storm runoff systems to deal with floodwaters.

Investigating the ensemble of model simulations, the researchers found broad agreement showing more likelihood of intense floods even though half of the models projected a net decrease in the average amount of rain and snow falling in California. More than one factor was at play--there was an increase in the number of intense precipitation events, and more precipitation fell as rain instead of snow, according to the scientists.
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RELATED PRESENTATION:

GC21C-0980. Tuesday, Dec. 4, 8 a.m - 12:20 p.m. Hall A-C, Moscone South "INCREASES IN FLOOD MAGNITUDES IN CALIFORNIA UNDER WARMING CLIMATES"

Scripps Institution of Oceanography: scripps.ucsd.edu

Scripps News: scrippsnews.ucsd.edu

About Scripps Institution of Oceanography
Scripps Institution of Oceanography at University of California, San Diego, is one of the oldest, largest and most important centers for global science research and education in the world. Now in its second century of discovery, the scientific scope of the institution has grown to include biological, physical, chemical, geological, geophysical and atmospheric studies of the earth as a system. Hundreds of research programs covering a wide range of scientific areas are under way today in 65 countries. The institution has a staff of about 1,400, and annual expenditures of approximately $170 million from federal, state and private sources. Scripps operates robotic networks, and one of the largest U.S. academic fleets with four oceanographic research ships and one research platform for worldwide exploration. Learn more at scripps.ucsd.edu.