Refilling the Orange County Groundwater Basin – Inch by Inch
Since July 2016, the start of the Orange County Water District’s (OCWD; the District) “water year,” north and central Orange County has received a total of 10.0 inches of rain. Storm events from December 15 through January 16 alone brought in 7.9 inches. Rather than big flashes of heavy rain, slow and steady rainstorms, spaced out by one to two weeks are optimal, since this allows temporary capture of stormwater behind Prado Dam in Riverside County and subsequent release of the captured stormwater to flow down the Santa Ana River and be diverted into groundwater recharge basins in Orange County, owned and managed by OCWD.
A key component to capturing stormwater is OCWD’s close collaboration with the U.S. Army Corps of Engineers (USACE; the Corps), which operates Prado Dam for flood risk management and conservation. The water conservation program at Prado has been extremely valuable to the Orange County region in capturing water that would otherwise flow to the Pacific Ocean. The existing water conservation program allows for capture of stormwater up to elevation 498 feet above mean sea level (amsl) during the flood season, which is from October 1 to February 28, and up to elevation 505 feet amsl during the non-flood season.
Due to drought conditions in California, OCWD requested the USACE evaluate a deviation from the Prado Water Control Manual to capture additional stormwater above the 498 feet amsl. In anticipation of a storm, the USACE’s Los Angeles District expedited approval of a Planned Minor Deviation on December 23 to allow water conservation up to elevation 503.9 feet amsl. This deviation allowed OCWD to capture an additional 3,500 acre-feet of stormwater from December 24 to 25 and 7,000 acre-feet (AF) from January 1 to 16. Due to this permitted deviation, OCWD has been able to capture 100% of Prado Dam release flows totaling more than 30,000 AF since December 15, 2016 and put the water back into the Orange County Groundwater Basin. Without the deviation, the water would have been lost to the Pacific Ocean. There is still 18,000 AF stored behind the dam, bringing storage to 38,000 AF so far. Without the minor deviation to capture additional water from the recent rains, more than 6,000 AF would have been lost to the ocean. That is enough water for approximately 50,000 people.
“The capture of stormwater behind Prado Dam and recharge of this water into the groundwater basin is the most economical way for us to replenish local water supplies,” stated Denis Bilodeau, P.E., OCWD President. “We are very grateful to have a long and productive relationship with the Army Corps to maximize the capture of this water, which helps the region become less dependent on imported water supplies from Northern California and the Colorado River.”
The District and the Corps are currently working on a long-term plan called the Prado Feasibility Study that, if successful, could lead to permanently changing the Prado Water Control Manual to allow water conservation up to 505 feet amsl year-round. In addition, the plan would restore ecosystems in Prado Basin which has been critical to the District’s successful recovery of the endangered least Bell’s vireo, a native California songbird, and the Santa Ana sucker.
OCWD uses stormwater as one source of water to fill the groundwater basin it manages. Other sources used to replenish the Orange County Groundwater Basin, which provides 2.4 million people in north and central Orange County with 75 percent of their water supply, include recycled water from the Groundwater Replenishment System, Santa Ana River water and imported water. In addition to stormwater, the rains bring incidental recharge, which is rain that falls on OCWD’s service area (229,000 acres) and percolates naturally into the basin through permeable surfaces. Over the years, OCWD has managed the basin in a sustainable manner to ensure the region’s water future and has more than doubled its annual output. Its primary responsibility is to protect basin water quality, manage pumping and replenish what is taken out.
Average annual rainfall for Orange County is 14 inches. It will take years of above-average rainfall to recover from current drought conditions. By maximizing stormwater capture, expanding its recharge system, enhancing groundwater management, building water infrastructure projects, and increasing the production of recycled water, the Orange County Water District has helped to strengthen water reliability for the region. The District also continues to explore all possibilities for alternative water supplies to help the region weather future dry-spells.