How does drought affect hydropower?

• Hydroelectric dams generate power by releasing reservoir water through turbines. During a drought, less precipitation falls resulting in less water flowing to rivers and then into reservoirs, causing storage levels to drop. With less water to release and with lower reservoir levels from which to release water, hydroelectric power production declines in the regions affected by drought.

How big of a loss in terms of hydropower production have happened in recent drought years?

• It all depends on what scale you’re looking at. If focusing on a single plant during a specific time of year then hydropower losses can be substantial. For example, Edward Hyatt at Lake Oroville in California had to be shut down with no power produced for several months in 2021 due to reservoir levels dropping too low. Oroville is an extreme case. If you zoom out to consider the fleet of hydropower plants across all of California and the Southern Cascades (Oregon), then the impact of drought is less pronounced. An extreme year might result in a loss of 50% hydropower relative to typical conditions. If you zoom out even further to the scale of the Western United States, the impact of drought is even less pronounced, with an extreme year resulting in perhaps 20% less hydroelectricity produced relative to an average year. That’s because drought rarely affects all regions of the West simultaneously. When one area experiences a decline, other plants can ramp up and send power to that area to partially compensate for any loss.

Which areas will see the most severe hydropower impact from droughts?

• Although we cannot answer this question definitively, we can point to regions that are at greatest risk. The Southwest, California in particular, experiences the largest variability in weather conditions from year to year. Sometimes very dry years occur in sequence—which we call multi-year drought. California may see the most severe impacts from drought as a result of the extreme natural variability it experiences—and indeed data show that during the last 20 years California’s hydropower production has varied much more than other regions’ output. The southwest more broadly has also experienced prolonged, multi-decadal spells of below-average rain and snowfall. During these times, the large storage on the Colorado River, including Lake Powell (Glen Canyon dam) and Lake Mead (Hoover dam) has faced years of water level decline. The risk for the desert Southwest is that continued below-average precipitation over several years will lead to plant shutdowns as water levels drop below intakes. Other regions of the West, including the Columbia River Basin, may also experience severe drought in future. However, we have no way of knowing when or where the next severe drought will land.

Will there be enough electricity during droughts?

• Yes. The main impacts of impaired hydropower generation are not necessarily any shortage of electricity, but instead increased electricity sector emissions and increased electricity prices. This is because when hydroelectric dams cannot fulfil their usual output, other generating resources take up the slack. This could mean more imported power from hydro along with more power from gas plants, which emit more carbon than hydro, and are more expensive to run.

Do the recent droughts in the West mean that hydropower is not reliable?

• No. The Western hydropower fleet should be considered a reliable contributor to the electric grid regardless of Western droughts. Below are two reasons why:
• The overall fleet remains reliable even if certain plants or sub-regions produce less power. Drought rarely affects the entire Western region, and hydroelectric dams are distributed across diverse climate zones of the West. Climate diversity in the West has allowed Western hydropower to sustain 80% of its typical generation during the very worst multi-year droughts that afflicted the West during the past two decades.
• Plants that generate less power on average during drought can still be relied upon to supply power when needed most. Multi-year drought does mean that generation in sub-regions of the West can decline significantly from average. However, even within these regions, local hydropower can still be relied upon to fulfil the important role of peaking production in response to prices such as during a heat wave or when solar drops off.

What efforts are being put forward to mitigate drought effects on hydropower?

• Mitigation measures differ by scale. At the drought-impacted watershed scale, mitigation efforts differ depending on the region.
  • Water managers meet seasonally to look through a drought outlook and create a plan to manage how water will be available during the drought to many different stakeholders. Hydropower operators are one of those stakeholders. Researchers are studying ways to alleviate the effects of drought, such as water storage, pumped storage hydropower and aquifer charging.
  • In the Southwest, reservoirs are managed for multiple years. In both drought and non-drought years, hydropower is saved for the times when demand and electricity prices are the highest.
  • In the Northwest, hydropower is typically run at all times because the region has enough water available. In a drought year however, hydropower operations are prioritized to meet the most important grid needs, including grid reliability and resilience, such as responding to a heat wave in California in August 2020.
• At the utility and power grid scales:
  • Utilities perform seasonal resource adequacy exercises, further coordinated at the regional and power grid scales. Based on the drought outlook, utilities secure electricity supply for the depleted hydropower while managing local hydropower day to day and second by second to meet the electricity demand at all times.
  • Seasonal resource adequacy assessments are also performed for a 1-5 year outlook and longer horizons (10 and 20 years) where natural climate variability (wet/dry/average year) and also climate change are considered. 

Is the decrease in hydropower production in 2021 an outlier or something we should expect in the future?

• The year 2021 was a lower-than-average hydropower year, but not unprecedented in terms of total impact on generation. Precipitation in the West can vary significantly from year to year, and sometimes dry years occur in sequence. So, the natural climate of the region creates good years and bad years for hydropower generation and 2021 was just a bad year.
• Nonetheless, because of the warmer temperatures associated with climate change, snowpack levels and snowmelt timing are projected to change. This could lead to reduced hydropower production if this water cannot be stored and used efficiently.

What is the effect of lost hydropower generation for utilities and grid operators?

• When hydropower facilities must reduce generation, utilities must either compensate by generating from their other conventional power plants or buy electricity on the market, e.g., produced by other power plants from outside their immediate region. This leads to higher operating cost for the utilities, which may be passed on to consumers.

What are megadroughts and what do they mean for hydropower generation?

• The Southwest appears to be experiencing a multidecadal drought at present—meaning average precipitation over the last twenty years is significantly below the long-term average going back hundreds of years. However, such droughts still contain relatively wet years, which help replenish water resources. Electric utilities and power producers have therefore been able to sustain healthy levels of hydropower generation despite generally drier conditions on average. This form of multidecadal drought could pose a threat to systems with very large storage and high usage of water—such as the Hoover and Glen Canyon projects in the Lower Colorado. The reservoirs supporting these hydropower generators have been declining over the last two decades and temporary wet periods have been insufficient to refill the reservoirs. Fortunately, most hydropower in the Western United States is generated outside of this region and very few hydroelectric plants face the same unique challenges of those on the Lower Colorado River.

Will drought severity change in future with global warming?

• Yes, but the extent and impacts of those changes remain unclear. For example, we know that the future will bring warmer temperatures, which promote increased evaporation and evapotranspiration—translating to less water in rivers and reservoirs. So, all other things being equal, future droughts mean less water available for generating power. However, there are many other factors to consider. Some climate models project a general trend toward more precipitation in many parts of the West, potentially offsetting any effects of evaporative loss. The nature of the dams, reservoirs, and power plants also matter. For example, systems with large storage are resilient to short, intense periods of drought, but can become vulnerable following many years of below average water availability.