Section 1 – Analysis of Supply Reliability and Seismic Risk Assessment
IRWD’s approach to manage risk and to maintain a resilient water supply relies on diversifying water supplies, establishing groundwater banking programs, and maintaining a robust planning process that anticipates and prepares for a variety of risks such as extended drought, climate change, and infrastructure disruptions at both the regional and local level. In 2016, IRWD prepared a Water Supply Reliability Evaluation (Evaluation) which provided an understanding of how projected conditions, such as imported water supply shortages, climate change, and facility outages could impact water supply. The 2016 Evaluation included an analysis of IRWD’s ability to maintain a minimum level of service under reasonably foreseeable hydrologic and system outage conditions and emergency scenarios, or combination of such scenarios, based on a rigorous and transparent probability analysis. In 2025, IRWD prepared a Water Supply Reliability Study Update (2025 Study), a comprehensive update to the 2016 Evaluation. The 2025 Study evaluated IRWD’s ability to reliably manage supply shortages and enhance system resilience in response to emergencies and evolving water supply conditions over a 38-year planning horizon from 2022-2059. The 2025 Study incorporated updated potable water demand forecasts, revised supply assumptions, and a detailed analysis of system performance. 1.1 Supply Reliability Scenario Planning The 2025 Study assessed IRWD’s ability to mitigate supply shortages and enhance system resilience in response to emergency disruptions and local and regional water supply conditions. The impact on IRWD’s potable water supply was evaluated using the Integrated Resources Planning Distribution System Model (IRPDSM), which applies a probabilistic Monte Carlo modeling approach, over a 38-year planning horizon (2022–2059). The following planning disruption scenarios were simulated: • Delta Levee Failure. The Delta is located where the Sacramento and San Joaquin Rivers converge. Water must flow through the Delta’s complex network of levees and islands to convey water from Northern California to Southern California. It supplies water to over 25 million Californians and millions of acres of farmland throughout the SWP and Central Valley Project. A seismic event in the Delta region that causes levee failure can flood Delta islands with saline water, severely impair water quality, and interrupt SWP exports. The extent of disruption depends on the number and location of levee breaches, the season of occurrence, and the capacity to implement emergency repairs.
• Delta Levee and Colorado River Lower Feeder Failure: The Delta Levee Failure is paired with a seismic event in Southern California that damages the Metropolitan Water District
WSCP - 5
IRWD – 2025 Water Shortage Contingency Plan
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