Adapting Colorado River Basin Depletions to Available Water to Live within Our Means

The Colorado River's two largest reservoirs are drawing down because releases exceed inflows and releases adapt to reservoir elevations instead of to elevation and inflow triggers. To help slow reservoir drawdown and sustain target elevations, we introduce a new rule that adapts basin depletions to available water. We simulated inflow-based operations and validated existing operations in a new open-source exploratory model for the Colorado River Basin. We developed the exploratory model to more easily adapt Upper and Lower Basin depletions to available water, reduce run time, and lower costs to use compared with the proprietary RiverWare Colorado River Simulation System (CRSS) model. We simulated adaptive and existing operations for (1) the 2000-2018 period, and (2) scenarios of steady Lees Ferry natural inflow each year of 17.3-6.2 billion m3 (bcm) [14-5 million acre-ft (maf)] per year. We found the following: (1) the existing rules drew down Lake Powell and Lake Mead to their critical storages of 7.4 bcm (6.0 maf) in less than 5 years when Lees Ferry natural flow was less than 15.3 bcm/year (12.4 maf/year) (the 2000-2018 period average); and (2) the adaptive rule sustained both reservoirs above their critical levels for long periods by requiring Upper and Lower Basin users to conserve as much as 1.2 bcm (1.0 maf) per year more water than existing operations. The next steps should be testing the adaptive rule in CRSS and devising conservation programs that can adapt and scale to available water.