scenarios. In addition, the 2025 Study reaffirmed that IRWD can manage risks associated with the range of import disruptions and changing regional conditions, as well as risks to local groundwater supplies. B. Historic Drought Analysis IRWD completed a historic drought analysis to evaluate customer water use with respect to dry, wet, and normal year conditions. Indicators for “wet”, “dry”, and “normal” year conditions were compiled from DWR annual hydroclimate and water year reports and supplemented by data from the California Irrigation Management Information System (CIMIS) and the federal drought monitor where applicable. IRWD selected the period from 2010-2025 for the analysis. During this period, there were several single-year and multi-year droughts. Additionally, use of more recent years for the analysis was deemed more representative of changes in hydrology and climate change impacts. The first year of a five-year consecutive drought is assumed to have the same change in demands as a single dry year. After the first year of a multi-year drought, IRWD customer demand experiences a 5-10% increase. During the 2012-2016 drought, upwards of a 10% reduction was seen during the period of the statewide mandate and increased conservation efforts. Based on these results, IRWD projects potable demands to increase by 5% during the first year of a drought, increase by 1% in years 2 and 3, and then decrease by 1% in year 4, and decrease by 5% in year 5. Non-potable water demands are projected to increase 10% in the first year, 5% increase for in the second year, decrease 1% in the third year, decrease 2% in the
fourth year, and decrease 5% in the fifth year. C. Water Supply Reliability Projections
The following section provides IRWD water supply and demand projections from 2030 to 2045 for normal, single-dry, and multiple dry year (5 consecutive dry years) perspectives. Projections were developed for both potable and non-potable supplies and separated accordingly for results in this section. For additional combined analyses tables see Appendix D – DWR Submittal Tables. Solely for the purpose of its 2025 UWMP, IRWD utilized MWDOC’s water demand forecast model outputs specific to IRWD for normal or average years. This demand forecast is reported in the 2025 Orange County Water Demand Projection Model TM (MWDOC, 2025), see Appendix G . As previously discussed in Section 4, there are some differences between the demand forecast outputs from the MWDOC model and the IRWD WRMP due to different methodologies, but they are not significant enough to change the conclusions. IRWD does not utilize the MWDOC adjustments to average year demands for single dry year and five consecutive dry year projections. Instead, IRWD modifies the average year projections developed by MWDOC based on an analysis of changes in demand within IRWD’s service area in response to single and multiple dry years. Throughout the UWMP, IRWD relies on its own supply forecasts.
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IRWD – 2025 Urban Water Management Plan
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