For the 2020 water year, the northern half of the Columbia River Basin should see above-average water supply while the southern half sees below-average supply. This below-average supply could reduce electricity available from hydropower in the lower Columbia River Basin this summer, says the U.S. Energy Information Administration.
This data comes from the National Oceanic and Atmospheric Administration’s Northwest River Forecast Center (NWRFC), which released its final Pacific Northwest water supply projection for the 2020 water year last month. This water year runs from October 2019 to September 2020.
More than a third of the nation’s hydroelectric capacity is in the Pacific Northwest, so the water supply forecasts for the region are closely monitored, EIA says. Grand Coulee, the largest hydropower facility in both the Columbia River Basin and the U.S. at 6,809 MW, generates enough electricity to power 4.2 million households. Changes in water supply, and subsequently hydroelectric generation, in the Pacific Northwest can have implications for the use of other electricity-generating fuels in the region and electricity trade with neighboring areas.
Snowpack and seasonal precipitation levels are the main factors in the water supply forecast. Snowpack, or accumulated winter snowfall, can indicate how much water will be available to power hydroelectric generators throughout the year. In addition to precipitation and winter temperatures, soil moisture in the months preceding snowfall contributes to water supply. In September 2019, the majority of reporting stations in the Columbia River Basin recorded precipitation levels at more than 130% higher than the 30-year normal. The wet September drove up soil moisture levels, contributing to the preservation of the region’s snowpack in the following months.
Seasonal precipitation levels drive the NWRFC forecast. To date, seasonal precipitation in the northern half of the Columbia River Basin is approaching near-normal levels, after strong storms at the beginning of the year. In January, atmospheric rivers drenched parts of the northern Columbia River Basin with rain and snow, which increased water supply.
Conversely, the southern half of the Columbia River Basin is experiencing below-normal seasonal precipitation levels, and most of the reporting stations in Oregon were lower than 70% of normal. These differences in seasonal precipitation are reflected in the regional differences in the water supply forecast for April through September.
The Dalles Dam is a run-of-river hydroelectric plant near the mouth of the Columbia River. Because of its location, hydro operators consider water flow at The Dalles an indicator for the entire upstream Columbia River system. However, water flow at The Dalles it does not capture regional variation in water supply across the basin. As of June 4, the NWRFC projects The Dalles Dam water supply forecast for April through September to be 100.1 million acre-feet, about 8% higher than the 30-year normal (1981–2010).
Because water supply and the subsequent hydroelectric generation can vary widely from year to year, water supply forecasts are closely monitored and used as inputs to EIA’s Short-Term Energy Outlook (STEO). The July 2020 STEO forecasts that hydroelectric generators will provide 285 million MWh in 2020. About half of that total comes from hydro plants in the Northwest. Overall, hydroelectricity is expected to account for 8% of U.S. electricity generation in 2020.