Forecast Informed Reservoir Operations

FIRO is a reservoir-operations strategy that better informs decisions to retain or release water by integrating additional flexibility in operation policies and rules with enhanced monitoring and improved weather and water forecasts (American Meteorological Society; 2020).

FIRO is being developed and tested as a collaborative effort in the Russian River Basin (Lake Mendocino), the Santa Ana River Basin (Prado Dam), and the Yuba-Feather Rivers Basin that engages experts and stakeholders in civil engineering, hydrology, meteorology, biology, economics and climate from several federal, state and local, universities and others. There is significant interest and support for developing FIRO at other appropriate locations in the Western U.S. and elsewhere.

Lake Mendocino
Prado Dam

Greg Woodside

(Orange County Water Distrcit)

F. Martin Ralph

(Center for Western Weather and Water Extremes at Scripps Institution of Oceanography)

Jay Jasperse

(Sonoma Water)

Michael Anderson

(California Department of Water Resources)

Cary Talbot

(US Army Corps of Engineers)

Alan Haynes

(Caifornia Nevada River Forecast Center, NWS)

Rene Vermeeren

(US Army Corps of Engineers)

Jon Sweeten

(US Army Corps of Engineers)

James Tyler

(Orange County Public Works)

Ken Corey

(US Fish and Wildlife Service)

Adam Hutchinson

(Orange County Water District)

Arleen O’Donnell

(Eastern Research Group)

Robert Hartman

(RKH Consulting Services)

Forest Cannon

(Center for Western Weather and Water Extremes)

Prado Dam Work Plan

Prado FIRO Fact Sheet

Prado Dam

Figure 1. Map of the Santa Ana River Watershed.

To increase the efficiency of stormwater capture at Prado Dam, CW3E is collaborating with the Orange County Water District (OCWD) and U.S. Army Corps of Engineers (USACE), Los Angeles District to see if the lessons learned implementing FIRO at Lake Mendocino can be transferred and applied to the Santa Ana River Watershed. Prado Dam is located in Riverside County near the City of Corona at the upper end of the Lower Santa Ana River Canyon, about 30.5 miles (49 kilometers) upstream of the Pacific Ocean (see figure 1). This natural constriction controls 2,255 square miles (5,840 square kilometers) of the 2,450 square mile (6345 square kilometer) Santa Ana River watershed.

Prado Dam, constructed in April 1941 by the USACE, Los Angeles District, is the downstream element of the Santa Ana River flood control system and provides flood control and water conservation storage for Orange County, California. Since its construction, OCWD and the USACE have worked together to maximize the capture of stormwater behind the dam, an important source of water supply to the Orange County Groundwater Basin. Currently, OCWD is allowed to temporarily conserve up to 10,000 acre-feet (AF) of water behind the dam during flood season (Oct-Feb.) and 20,000 acre-feet of water in the non-flood season (March-Sept) (figure 2). FIRO is being studied to assess the feasibility of allowing 20,000 AF of water to be conserved behind the dam year-round.

Figure 2. Current water conservation pool program at Prado Dam. Source: OCWD

The purpose of Prado Dam is to collect runoff from uncontrolled drainage areas upstream along with releases from other storage facilities. Generally, when the water surface elevation in the reservoir pool is below the top of the buffer pool elevation (498.0 feet during the flood season, 505.0 feet during the non-flood season), water conservation releases are made to help fill the groundwater recharge facilities. When the water level in the reservoir exceeds the buffer pool, flood control releases are made in an effort to drain the reservoir back to the top of the buffer pool as quickly and safely as possible. Recently, the existing dam and reservoir have been enlarged and the release capacity of the outlet has been increased to provide additional capacity for storage of flood waters and sediments and to take advantage of the increased downstream channel capacity. Once the ongoing construction of the Corps of Engineers’ Santa Ana River project (Reach 9 Project) is completed, the new outlet works will allow for increased flood control releases and greatly improve the level of flood protection to the communities of Orange County in the Santa Ana River flood plain.

Over the past 25 years, OCWD has captured and recharged an average of 55,000 AF of stormwater per year, with a maximum of 117,000 AF in 1995. For planning purposes, OCWD estimates that 40,000 AF of stormwater will be captured and recharged in an average year. This represents enough water for 320,000 people annually. For comparison, the cost to purchase imported water to replace this supply is approximately $40 million. Further, the decreasing reliability of imported water due to the fragile Bay-Delta, oversubscribed Colorado River, and changes in weather patterns make the capture of local stormwater even more important.

Figure 3. Precipitation over the Santa Ana River Watershed (lower blue line), outflow (orange), inflow (green), and water surface elevation (upper blue) at Prado Dam during February 2017. Source: OCWD

In February 2017, a strong atmospheric river made landfall over the Santa Ana River Watershed and produced significant precipitation. Prior to the event, the USACE released approximately 6,500 AF of previously captured stormwater to create additional storage. This water was released at a rate more than 4,000 cfs, meaning most of the water was not captured by groundwater recharge plants and flowed into the Pacific Ocean. When the AR made landfall the peak inflow to Prado Dam reached 7,000 cfs. After the event, about 900 AF on unused storage space remained, representing approximately $1 million worth of water that was not captured (figure 3).

American Meteorological Society, cited 2020: Forecast-informed reservoir operations. Glossary of Meteorology. [Available online at]