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Over any time period, the inflows to minus the depletions from any controlled area must equal the change of storage. When users' aspirations to deplete water exceed inflows, water managers must either a) draw down reservoir storage to meet some or all of users’ demands, or b) cut back customary deliveries to adapt to inflows and stabilize reservoir storage. We use a new open-source, Python reservoir model for the Colorado River Basin to simulate new demonstrative adaptive reservoir operations across many short-duration, severe flow sequences observed or reconstructed between 1416 and 2020. Results show: 1) the existing rules to operate Lake Powell and Lake Mead will draw down both reservoirs to their critical storages of 6.0 million acre-feet (3,525 and 1,025 feet) in 3 to 5 years. 2) Triggering the new rule at a Lake Mead elevation of 1,060 feet to adapt basin wide depletions to inflow can sustain both reservoirs above their critical levels for long periods of time. The new rule asks or requires Lower and Upper Basin users to conserve from 0.5 maf per year less to 1.0 maf per year more water than the largest mandatory cutback of 1.375 maf per year. To adopt these adaptive operations, the parties will need to creatively combine five water conservation principles to convert lose-lose conflicts into more positive processes.

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