(2) A determination of the reliability of each source of supply under a variety of water shortage conditions. This may include a determination that a particular source of water supply is fully reliable under most, if not all, conditions. (3) A comparison of the total water supply sources available to the water supplier with the total projected water use for the drought period. (4) Considerations of the historical drought hydrology, plausible changes on projected supplies and demands under climate
Every urban water supplier is required to include a Drought Risk Assessment (DRA) as part of their 2025 UWMP. The CWC requires a DRA to be based on the driest five-year historic sequence for the supplier while also considering plausible changes to projected supplies and demands due to climate change or regulatory changes. Specifically, a DRA evaluates whether a water shortage condition due to extended drought is expected to occur over the next five consecutive years (2026 to 2030). This DRA analysis is designed to work in conjunction with the Water Supply Reliability Assessment (WSRA, Section 7.1) and Water Shortage Contingency Plan (WSCP Section 8 in Appendix E ) to present a comprehensive view of IRWD’s water reliability, preparedness, and response actions for drought.
Drought Risk Assessment Data Inputs and Methods
The five-year drought period for IRWD’s DRA is evaluated in the same manner as the multiple dry year analysis in the Water Supply and Demand Assessment in Section 7.1. This same hydrology is used to project supply and demands for the five-year DRA period from 2026 to 2030. IRWD’s DRA assumes that single-dry year conditions are in effect for “Year 1” of the drought (2026), and that consecutive drought conditions occur in Years 2 through 5 (2027- 2030). IRWD’s DRA presented here analyzes potable water supplies and demands. Recycled water is considered to be a drought-resistant supply source and is not addressed in this section. A separate DRA for recycled water was prepared and is included in Appendix D . The results of IRWD’s DRA are presented in Table 7-5A . This table presents the calculations for IRWD’s water supplies and demands under assumed drought conditions lasting the five consecutive years described above. Following is an overview of the data inputs and methods used to calculate the values corresponding to each line of Table 7-5A . A. Total Water Use (Demands ) The historic drought analysis methods described in Section 7.1B were used to determine water use inputs in the DRA. Historic demand data from 2010 to 2025 was analyzed for each fiscal year and normalized for population growth over time. The year over year percent change in historic customer usage from 2010-2025 was averaged for periods with consecutive years of drought to determine a percentage change in demands for the following year. Based on this analysis, in a single dry year, potable demands increased by 5%. Over multiple dry years, demands are adjusted +1%, +1%, -1%, and -5% for years two through five respectively. It should be noted that the District experienced demand decreases exceeding 10% for the fifth year of the 2012 to 2016 drought, but for the purposes of creating a conservative assessment the fifth year
7 - 13
IRWD – 2025 Urban Water Management Plan
Powered by FlippingBook