To better understand the impacts of drought on the U.S. electric grid, the Pacific Northwest National Laboratory (PNNL), with funding from the U.S. Department of Energy’s Water Power Technologies Office (WPTO), conducted a multi-regional study of drought’s impact on 21st century hydropower generation in the western United States. The report is the most comprehensive look into the effects of drought on hydropower generation in the United States this century.

The analysis reveals that though drought does raise concerns for hydroelectric generation, the overall hydropower fleet sustained 80% of its average generation for the years 2001-2021. Also during this time period, hydropower could still be relied upon to supply flexible power during periods of high energy demand—even during the most severe droughts of the past two decades.

The western United States has always been a region of extreme climate variability, with large fluctuations in rain and snowfall precipitation from year to year. Climate change, and especially extreme weather events such as droughts and floods, is only increasing those fluctuations. This has enormous implications for the energy grid, sanitation, drinking water, food and agriculture, and more.

In a wet year, hydroelectric power can meet 30% of annual western electricity demand, but that number can drop below 20% when less rain and snowfall lead to lower water levels in rivers and reservoirs.

The PNNL study looked at eight climate sub-regions across 11 western states, including Arizona, California, Colorado, Idaho, Montana, Nevada, New Mexico, Oregon, Utah, Washington, and Wyoming. This area, which the U.S. Census Bureau defines as the “West,” contributes more than 60% of U.S. hydropower capacity. All data used in the study is publicly available and taken from the U.S. Energy Information Administration, the U.S. Geological Survey National Water Information System, the U.S. Drought Monitor, the National Centers for Environmental Information of the National Oceanic and Atmospheric Administration, and Oak Ridge National Laboratory.

That data indicated 2021 to be the second worst year for drought this century, with overall hydropower generation 16% lower than the average since 2001. The 2021 drought most severely impacted hydropower generation in California (48% below average) and Oregon (16% below average). However, the large sizes of western states, and wide range of weather across the West means drought rarely impairs hydroelectric power across all climate sub-regions simultaneously. Consequently, the overall hydropower fleet remains reliable even if certain plants or sub-regions produce less power. Washington and Idaho, for example, only experienced an average drop in hydropower generation of about 12% in 2021, and the entire western hydropower fleet was still able to maintain 84% of its average expected generation.

2001 remains the worst year for western hydropower generation this century, owing to extreme drought in the Pacific Northwest where two-thirds of western hydropower capacity is located. Despite the drought, the hydropower fleet still provided 150 TWh of renewable-generated electricity. This means that, even in its worst year this century, the overall hydropower fleet sustained 80% of its average expected generation.

Multi-year drought does mean that sub-regions of the West, such as particular states or parts of states, can experience a significant decline in hydropower generation. In California, we saw evidence of significant curtailments in monthly output as reservoirs drop each time the region experienced two or more years of drought. Yet even within these regions, local hydropower is still reliable and an important energy source: data shows that hydroelectric power in California continued to ramp up in the evening through summer 2021, supplying essential power to the grid as demand increased and solar generation decreased.

Annual hydropower generation across most sub-regions has a strong correlation with yearly rainfall totals, allowing researchers to further create hydropower generation estimates for years lacking energy generation data. Based on these estimates in combination with rainfall outlooks for the remainder of 2022, the researchers expect U.S. hydropower generation in 2022 to rebound from 2021, despite continual drought across large areas of the West. This rebound is driven by favorable water conditions in the Northwest, which contributes the lion’s share of the West’s hydropower generation and is no longer considered to be in severe drought.

Unlike western droughts experienced in the 21st century, the drought of 1976–77, which was the most severe drought ever recorded for the West, affected hydropower output from all major generating regions of the West. As a result, a repeat of this historical event would likely cause a greater reduction in overall hydropower generation than seen in the last two decades. These states are part of the Western Interconnection which is made up of 136,000 miles of transmission lines that carry power between states. As a result, even if these drought conditions repeated, overall hydropower generation in the West would still produce about three-quarters of its average annual output.

Learn more about drought impacts on hydroelectric power generation in the western United States.