The ongoing record drought in California has caused significant changes in the state’s power mix as water available for hydroelectric generation becomes increasingly scarce, according to a recent report from Oakland-based nonprofit the Pacific Institute.

The California Independent System Operator warned last year that water shortages were likely to substantially impact the state’s generation, with new gas-fired plants (most of which use dry cooling) making up most of the 1,300 MW to 1,600 MW of hydro generation that would be forced offline.

The Pacific Institute report, released on March 17, makes clear how dramatic the shift has been since the drought began in 2011.

On average between 1983 and 2013, the report notes—though few years are “average” with California’s erratic precipitation patterns—the state has gotten 18% of its electricity from hydropower. From October 2011 through October 2014, however, hydro’s share fell to under 12%, a drop of around 34,000 GWh. At its most recent peak in 2011—an unusually wet year—hydro accounted for 21%.

Despite California’s rapidly growing wind and solar generation capacity, the lost hydro generation has mostly been replaced by natural gas. This has resulted in about $1.4 billion in additional costs to state ratepayers for the extra fuel, and has increased carbon dioxide emissions from the state’s power plants by 8%, around 14 million tons.

More ominously, however, the current drought appears to be part of a long-term pattern of reduced precipitation and hydroelectric generation going back at least to 2007. With the state’s reservoir levels and snowpack far below normal, the chances of a rebound in the near term appear very slim.

The challenge has implications for state energy policy as Gov. Jerry Brown moves to expand the state’s renewable portfolio standard from the current 33% in 2020 to 50% in 2030 and plots a path toward its carbon emissions reduction target of 80% below 1990 levels by 2050. Another study by San Francisco–based think tank Energy Innovation released in March suggests the state will need even more aggressive shifts toward renewable energy to meet the 2050 goals.

As the Pacific Institute report notes, California’s installed hydroelectric capacity has been flat for decades. Prospects for further expansion are very limited given the few remaining unexploited resources and substantial political and environmental opposition that a significant new dam project would encounter.

Natural gas’s share of the California energy mix, meanwhile, has jumped from 45% in 2011 to 61% in 2013, though a portion of this increase is attributable to the premature retirement of the San Onofre nuclear plant in 2012.

—Thomas W. Overton, JD is a POWER associate editor (@thomas_overton, @POWERmagazine)