Corps shows advanced weather forecasts conserve water at Prado Dam

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A report evaluating a pilot program to use advanced weather and streamflow forecasts to enhance water storage capabilities at Prado Dam in California found that enough water could be conserved to supply an additional 60,000 people annually. This is particularly important as drought persists in the state, making the need to increase water supply reliability an essential issue.

The Forecast-Informed Reservoir Operations (FIRO) pilot program, led by research meteorologists from the Center for Western Weather and Water Extremes at UC San Diego’s Scripps Institution of Oceanography, found that 7,000 acre-feet per year of stormwater could potentially be added to groundwater recharge in Orange County. One acre-foot is equivalent to about 325,000 gallons.

FIRO uses data from watershed monitoring and modern weather and hydrologic forecasting, specifically the study of atmospheric rivers, to help water managers selectively retain or release water from reservoirs in a manner that reflects current and forecasted conditions.

The program was supported by a combination of funds from the U.S. Army Corps of Engineers (USACE), Orange County Water District and California Department of Water Resources (DWR). The district manages the Orange County Groundwater Basin, which provides 77% of the water supply to 2.5 million people in north and central Orange County. It also manages a six-mile stretch of the Santa Ana River between Prado Dam and its recharge basins in Anaheim.

“We are excited to partner with Scripps and the USACE on this project, which increases water supply and reliability for the region,” said Orange County Water District President Steve Sheldon. “Local stormwater capture is important because it lessens demand on imported water supplies, which are more costly and less reliable than groundwater.”

USACE constructed Prado Dam in 1941 for flood risk management, with a secondary use of stormwater capture for water supply. Many dams in the west, including Prado, are regulated by USACE-issued water control manuals, which do not take advantage of modern precipitation and streamflow forecasting capabilities.

“The Prado Dam FIRO project is an example of the continued partnerships between state, federal and local agencies. The FIRO program has shown that by better utilizing emerging technologies in observations and forecasts to create an adaptive strategy, we can improve water management, not only during the wet years, but during drought conditions as well,” said Kris Tjernell, DWR’s deputy director for integrated watershed management. “This type of project perfectly aligns with the goals described in the Governor’s Water Resilience Portfolio and is also the type of multi-benefit project that uses common sense approaches, combined with the latest science, to embrace innovation and new technologies, and increase resilience to climate change.”

Research meteorologist Marty Ralph, director of the Center for Western Weather and Water Extremes, explained: “Atmospheric river storms cause 25 to 50 percent of annual precipitation in key parts of the west, which can replenish water supply but can also lead to hazardous and costly flooding. When atmospheric rivers make landfall, they can release a staggering amount of rain and snow. However, their absence can lead to drought.”

The science of forecasting atmospheric rivers has continued to advance. Research conducted by Scripps Oceanography includes atmospheric and soil moisture observations; data collection over the Pacific Ocean, including measurements from buoys and dropsonde deployments into approaching storms; and advanced modeling that allows for better assessment of uncertainty in forecasts.

Using models to simulate reservoir operations under FIRO conditions, the assessment found that temporarily storing water to higher elevations can enhance groundwater recharge. The improvements in atmospheric river forecasts show high reliability at up to five days’ lead time, which allows dam operators to make timely water releases and could enhance flood-risk management.

“Completion of the preliminary viability assessment for Prado Dam is an important milestone for the U.S. Army Corps of Engineers as it builds our understanding of how to safely and effectively implement this important policy change across the agency,” said Cary A. Talbot, a division chief at the U.S. Army Engineer Research and Development Center and FIRO program manager for the Corps. “FIRO pilot sites like Prado Dam add to our agency’s ability to find a better balance between flood-risk management, water supply and ecological benefits, and makes us more resilient for the challenges of a changing climate.”

The final viability assessment at Lake Mendocino earlier this year showed that FIRO operations increased water supply by 20%. USACE and the Center for Western Weather and Water Extremes also are assessing FIRO opportunities in other watersheds where atmospheric rivers are dominant, including New Bullards Bar Reservoir in Yuba County, Calif.; Lake Oroville in Butte County, Calif.; and Howard Hanson Dam near Seattle, Wash.

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Elizabeth Ingram is content director for the Hydro Review website and HYDROVISION International. She has more than 17 years of experience with the hydroelectric power industry. Follow her on Twitter @ElizabethIngra4 .

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