El Niño Rains Increase Orange County Water Supplies

January brought El Niño rains to arid Orange County and the Orange County Water District (OCWD; the District) did everything it could to capitalize on rains to increase local water supplies. The District has captured more than 4.8 billion gallons (15,000 acre feet) of stormwater from the January rains, enough water to serve 133,000 people annually. OCWD uses this 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 natural incidental recharge, recycled water, Santa Ana River water, and imported water.

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. Regular investment in building and maintaining recharge basins helps maximize stormwater capture, which is significantly less expensive than purchasing imported water from Northern California and the Colorado River. Fifteen thousand acre‐feet of imported water would cost approximately $15 million.

The 4.8 billion gallons of recent stormwater was captured behind Prado Dam, which is operated by the United States Army Corps of Engineers (Corps). Since the 1960s, OCWD and the Corps have worked cooperatively to capture stormwater to increase the region’s water supply. In anticipation of the early January storms, the Corps approved a temporary change in the operation of Prado Dam that allowed OCWD to capture more water behind the dam. This translates into more water for eventual recharge into the groundwater basin and less water lost to the ocean. During medium‐sized storms, little rain goes to the ocean, but when back‐to‐back heavy rains hit, most is lost to the ocean and can’t be captured.

Currently, during flood season, October through February, the Corps allows stormwater to be stored at elevation 498 feet and for the duration of the year at 505 feet. OCWD and the Corps are working on a long‐term plan called the Prado Feasibility Study to restore ecosystems in Prado Basin and permanently increase the amount of water that could be stored behind Prado Dam to a year‐round elevation of 505 feet. This would potentially provide an additional 30,000 acre‐feet (9.7 billion gallons) of water annually, enough for 60,000 households.

Accelerated by the drought, many California agencies are looking for ways to capture more stormwater and the State Water Resources Control Board just approved a plan to help accomplish this.

In addition to stormwater, the rains bring incidental recharge, which is rain that falls on OCWD's area (229,000 acres) and percolates into the basin through permeable surfaces. It is too soon to accurately measure this quantity, but for OCWD it means more water. The groundwater basin typically receives about 19.5 billion gallons of incidental recharge during an average rainfall year.

Incidental recharge and stormwater account for about 23 percent of the water that goes into the Orange County Groundwater Basin. Other primary sources, accounting for nearly 60 percent, are imported water and reused water from the Groundwater Replenishment System (GWRS).

Over the years, OCWD has made significant investments to ensure water reliability for the region. It has maximized stormwater capture, expanded its recharge system, enhanced groundwater management, built water infrastructure projects, and increased the production of recycled water. The District also continues to explore all possibilities for alternative water supplies. It will take multiple years of above‐average rainfall to recover from current drought conditions, but OCWD is always prepared to maximize stormwater capture.

Pictured: Water flows from Weir Pond 2, over the newly constructed Weir 2, and into Weir Pond 3.  With the completion of the Weir Pond Rehabilitation Project, the District was able to divert Santa Ana River water into the recharge ponds during this winter’s storms