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

5 The Colorado River’s two largest reservoirs are drawing down because releases exceed inflows and 6 releases adapt to reservoir elevations instead of elevation and inflow triggers. To help slow reservoir 7 drawdown and sustain target elevations, we introduced a new rule that adapted basin depletions to 8 available water. We simulated inflow-based operations and validated existing operations in a new open9 source exploratory model for the Colorado River Basin. We developed the exploratory model to more 10 easily adapt Upper and Lower Basin depletions to available water, reduce run time, and lower costs to use 11 compared to the proprietary RiverWare Colorado River Simulation System (CRSS) model. We simulated 12 adaptive and existing operations for (a) the 2000 to 2018 period, and (b) scenarios of steady Lees Ferry 13 natural inflow each year of 17.3 down to 6.2 billion cubic meters (bcm, or 14 to 5 million acre-feet, maf) 14 per year. We found the following: (1) the existing rules drew down Lake Powell and Lake Mead to their 15 critical storages of 7.4 bcm (6.0 maf) in less than 5 years when Lees Ferry natural flow was less than 15.3 16 bcm/year (12.4 maf/year; 2000 to 2018 period average). (2) The adaptive rule sustained both reservoirs 17 above their critical levels for long periods of time by requiring Upper and Lower Basin users to conserve 18 up to 1.2 bcm (1.0 maf) per year more water than existing operations. We see next steps as testing the 19 adaptive rule in CRSS and devising conservation programs that can adapt and scale to available water. 20 1 Jian Wang, corresponding author, Dept. of Civil and Environmental Engineering and Utah Water Research Laboratory, Utah State Univ., 8200 Old Main Hill, Logan, UT84322-8200. Email: jian.wang@usu.edu 2 David E. Rosenberg, Dept. of Civil and Environmental Engineering and Utah Water Research Laboratory, Utah State Univ., 8200 Old Main Hill, Logan, UT84322-8200. Email: david.rosenberg@usu.edu