California is now in a three-year drought period. The state is experiencing less total rainfall, less snowpack in the mountains, and earlier snowmelt. This means that less water is available to generate hydroelectricity. Also, less snowpack that is melting earlier could reduce the amount of water that generates hydropower during the summer months when electricity demand and prices are highest. 

There is a natural variation in year-over-year hydroelectricity production. It currently supplies between 14 to 19 percent of the state's electricity, down from closer to 60 percent in the 1950s. This is due in part to an increase in renewable energy that has come online. In 2013 alone, 3,300 megawatts (MW) of renewable energy capacity became operational. 

Three snowshed areas have been most affected by the drought: Northern Sierra/Trinity, Central Sierra and Southern Sierra. The Sierra Nevada snowpack typically melts in the spring and summer. It collects in reservoirs and provides about one-third of the water Californians use each year. Even with recent storms, reservoir storage is significantly below average. As of April 2014, Shasta was at 62 percent of its historical average. Similarly, Oroville storage was at 65 percent and Folsom Lake was at 71 percent. 

California imports an estimated 3 - 4 percent of its total hydroelectricity from the Pacific Northwest—which is projecting surpluses through 2018—and the Hoover Dam in the Pacific Southwest, where conditions for hydroelectric generation appear stable through 2015. 

With less water being available to generate hydroelectricity, natural gas and renewable energy supplies will be used to make up the difference. Given that hydropower is one of the least expensive types of energy and has zero emissions, other types of energy used to make up the difference will have additional costs and emissions. Natural gas is more expensive and produces greenhouse gas emissions. While renewable energy does not produce greenhouse gas emissions, it is more expensive than hydropower. Some utilities rely more on hydroelectricity than others. Most have resources spread through more than one snowshed or watershed, which could mitigate shortfalls in one area if greater production is realized in another.

The effects of the drought and additional power replacement costs will not be immediately known. 

The California Energy Commission (CEC) is part of the state's Drought Task Force that is monitoring and assessing drought impacts on hydropower generation, and by extension, the California electricity supply. 

The Energy Commission is also part of the interagency Electricity Working Group comprised of staff from the State Water Resources Control Board, Department of Water Resources, California Public Utilities Commission and the California Independent System Operator. The group is an extension of the Drought Task Force and is lead by a representative from the Governor's Office. 

The Group's purpose is to develop actions that are necessary and prudent to protect California's energy supply and service reliability that could be impacted by the drought. It is tasked with: 

1. Developing tools to illustrate the impact of drought so that decision-makers can better evaluate hydropower generation and electricity reliability. 
2. Monitoring hydropower generation impacts. 
3. Monitoring natural gas plants using water. 
4. Reviewing and activating emergency contingency plans for electricity shortages if needed. 

For more information consult the CEC website: www.energy.ca.gov