OCWD manages the Basin to allow utilization of up to 500,000 AF of storage capacity of the Basin during dry periods, acting as an underground reservoir and buffer against drought. OCWD operates the Basin to keep the target dewatered Basin storage at approximately 200,000 AF as an appropriate basin operating range. The amount of groundwater that can be produced is a function of basin replenishment, total demands by all producers, and the resulting Basin Production Percentage (BPP) that OCWD sets based on these factors. The OCWD framework for Basin management is through financial incentives based on establishing the BPP each year. The BPP is the ratio of groundwater production to total water demands expressed as a percentage. Groundwater production above the BPP is charged a Basin Equity Assessment (BEA) fee. The BEA is set so that the cost of groundwater pumping above the BPP is greater than the cost of imported water. Each year, OCWD sets a target amount of pumping, the BPP, and assesses a BEA on all water pumped above that limit. OCWD prohibits consideration of recycled water demands when determining the amount of water IRWD can pump from the Basin each year without having to pay the BEA.
A. Basin Replenishment
Replenishment supplies for the Basin include the capture and recharge of Santa Ana River flows, purified recycled water, and purchases of water from Metropolitan. OCWD’s Groundwater Replenishment System (GWRS) has been producing advanced treated recycled water since 2008. The GWRS purifies wastewater (sewage) using a three-step process to produce high- quality water used to recharge the Basin and for injection into the seawater intrusion barrier. The GWRS was expanded in 2015 from 72,000 AF annual production to about 100,000 AF per year, and in 2022 underwent a final expansion to about 134,000 AFY.
6.3 Surface Water
IRWD’s local surface water sources are the drainage tributaries to the Irvine Lake.
Irvine Lake
The local surface water to Irvine Lake from Santiago Creek runoff has historically and solely been a supply to the non-potable water system. On average, approximately 3,000 AFY of local surface water is captured by Irvine Lake for IRWD’s use. During dry years, IRWD’s annual use of local surface water could be as little as a few hundred AF. With the completion of the Baker WTP and IRWD’s new ownership of the Howiler WTP, local surface water in Irvine Lake can be supplied for treatment as a potable water supply source. IRWD owns Irvine Lake and has the right to divert and store up to 28,000 AFY. IRWD intends to implement the proposed Santiago Creek Dam Improvement Project that would replace the Santiago Creek Dam outlet tower and spillway facilities and modify the embankment to meet or exceed current seismic, safety, and design requirements. This work is expected to begin in late 2027 or early 2028 and is expected to be completed by 2032. In the interim, use of surface water from Irvine Lake would not be available. IRWD has access to untreated water from Metropolitan that can be bypassed from Irvine Lake to the Baker WTP.
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IRWD – 2025 Urban Water Management Plan
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