B.3 Demonstration of Reduced Reliance on the Delta
Some of the key documentation underlying IRWD’s demonstration of reduced reliance include: Data obtained from IRWD’s 2025 UWMP or previously adopted UWMPs for supplies and demands under average or normal water year conditions. All analyses were conducted at the service area level and all data reflect the total contributions of IRWD and MWDOC, in conjunction with information provided by Metropolitan. No projects or programs that are described in the 2025 UWMP as “under development” were included in the accounting of supplies, with the exception of future groundwater supplies. IRWD assumes additional groundwater supplies for the period 2030 through 2045 and also applies these assumptions in the expected outcome calculations for increased regional self-reliance resulting in reduced Delta reliance (see Tables C-2 and C-2B).
Baseline and Expected Outcomes
A baseline water use is needed to compare average current and future water use in order to calculate how Delta use and regional self-reliance have changed over time. The Guidebook Appendix C approach uses 2010 as the baseline year when the Delta Reform Act became effective, which is the baseline IRWD uses in its analysis. Consistent with the Guidebook Appendix C approach, IRWD utilizes forecasted data from its 2005 UWMP to establish the 2010 baseline use. Since supplies and demands vary from year to year, UWMP-reported normal year conditions typically incorporate a range of hydrologic scenarios, to determine average or “normal” conditions, which are used in forecasts of supplies and demands. Since UWMPs generally do not provide normal water year data for the year that they are adopted, each subsequent set of data comes from the previous UWMP normal year projections. IRWD then quantified the expected outcomes for reductions in reliance on supplies from the Delta and compared this to the established 2010 baseline. Based on the Guidebook Appendix C approach, IRWD utilized its 2015 and 2020 UWMPs reported normal water supplies to calculate change in water use between baseline and the current UWMP to demonstrate reduced Delta reliance. Expected outcomes for 2030-2045 are from the 2025 UWMP. Documentation of the specific data sources and assumptions are included in the discussions below. Service Area Demands without Water Use Efficiency In alignment with the Guidebook Appendix C, as well as both the MWDOC and Metropolitan regional reduced reliance on the Delta calculations, this analysis uses normal water year demands to calculate expected outcomes for reductions in reliance on the Delta. Using normal water year demands serves as a proxy for the volume of supplies that would be used in a normal
Appendix B - 5
IRWD – 2025 Urban Water Management Plan – Reduced Delta Reliance
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