California could lose up to 20 percent of its hydropower generation as climate change causes high-elevation reservoirs to shrink, finds a new report from the California Natural Resources Agency and the California Energy Commission.

“Climate change is expected to affect the quantity and timing of water flow in the state,” said Kaveh Madani, a former postdoctoral researcher at the University of California, Riverside’s Water Science and Policy Center who led the science team that authored the report.

“Under dry climate warming, the state will receive less precipitation, with most of it as rain instead of snow, impacting hydropower supply and operations,” said Madani. “The big problem is that hydropower will be less available when it is most needed and expensive – in the summer months.”

Madani projects that under those conditions hydropower operators could suffer a drop in revenues of eight to 18 percent.

Madani’s report is one of several conducted by 26 research teams that together comprise the state’s third climate change assessment released since 2006.

The assessment covers current and projected climate change impacts on the state’s energy, water, agriculture, coastal regions and public health. Areas of focus include the San Francisco Bay Area, the Sacramento-San Joaquin Delta and Santa Barbara.

“We know that climate change will significantly affect the state’s energy supply and demand,” said Energy Commission Chair Robert Weisenmiller. “This groundbreaking research gives us the data and analytical tools we need to better plan, forecast and prepare to meet the state’s energy needs as we face climate challenges.”

This assessment follows up on the Governor’s Conference on Extreme Climate Risks and California’s Future, held last December in San Francisco. The new studies provide a foundation for the 2012 Climate Adaptation Strategy, with completion expected in December.

“The governor is committed to rigorous climate science and understanding the impacts of climate change on California so that we can respond, adapt, and continue to prosper,” said Ken Alex, senior policy advisor to Governor Jerry Brown, and director of the Office of Planning and Research. “Wise investment in our state’s future depends on the science, and is key to strengthening California’s economy and protecting the health of our citizens.”

The assessment studies find that California will continue to get hotter. Statewide average temperatures increased by about 1.7 degrees Fahrenheit from 1895 to 2011. Temperatures are expected to rise by 2.7 degrees F. above 2000 averages by 2050.

Temperatures will rise more in inland areas than at the coast. Historically, in Sacramento, temperatures of 101 degrees F. or higher have occurred four times a year on average. There may be as