8.3.6  Conservation/Curtailments

Tacoma has developed a multistage water curtailment plan for droughts and operating system emergencies. From an operations perspective, Tacoma's curtailment plan is designed to reduce demand in four stages, based on the severity of the predicted duration and magnitude of the drought. The assumptions listed below reflect "best guesses" as to how Tacoma may implement the goals of the curtailment plan and will likely be refined.

• Stage I, implemented when storage in HAH or available groundwater supply is projected to be below normal, relies on voluntary conservation and public education to reduce weekly demand by 7%. This stage is triggered in CRYSTAL when the last four weeks of HAH inflow have been less than 90% of the average HAH inflows or when groundwater levels in the South Tacoma aquifer are below 18,000 acre-ft.
• Stage II is implemented when the ability to meet the Corps IFR of 110 cfs at Palmer is uncertain or groundwater supply is limited. Stage II initiates mandatory municipal conservation and voluntary industrial conservation measures to reduce weekly demand by 21% (including the 7% achieved through Stage I). This stage is triggered in CRYSTAL when the last four weeks of HAH inflow have been less than 75% of the average HAH inflows or when groundwater levels in the South Tacoma aquifer are below 16,000 acre-ft.
• Stage III is implemented when stream flows below HAH fall below the Corps IFR or the groundwater supply becomes contaminated. Relying exclusively on either source limits the yield of the system to 70-75 mgd and would require eliminating municipal outdoor water use, curtailing industrial water use, and limiting purveyor supply. This stage is triggered in CRYSTAL when HAH inflows are below 110 cfs or when groundwater levels in the South Tacoma aquifer are below 12,000 acre-ft. Stage III measures results in reducing weekly demand by 45% (including the 21% achieved through Stage I and II).
• Stage IV, resulting from a catastrophe such as reservoir failure, implements severe water rationing. This scenario is not modeled in CRYSTAL.

All trigger levels can be adjusted in CRYSTAL to determine if conservation / curtailment can be used to optimally allocate scarce water during low flows. The conservation period is limited from June 1 to November 1 so that lower than average winter flows do not trigger conservation or curtailment. CRSYTAL implements conservation / curtailment measures for at least 4 weeks if a stage is triggered. During this period more severe stages can be implemented, and then a new 4-week period begins. The total volume of water conserved or curtailed is tracked, and the number of weeks where conservation / curtailment is implemented is counted.