Loss Rates from Lake Powell and Their Impact on Management of the Colorado River

Abstract

As demand for water in the southwestern United States increases and climate change potentially decreases the natural flows in the Colorado River system, there will be increased need to optimize the water supply. Lake Powell is a large reservoir with potentially high loss rates to bank storage and evaporation. Bank storage is estimated as a residual in the reservoir water balance. Estimates of local inflow contribute uncertainty to estimates of bank storage. Regression analyses of local inflow with gaged tributaries have improved the estimate of local inflow. Using a stochastic estimate of local inflow based on the standard error of the regression estimator and of gross evaporation based on observed variability at Lake Mead, a reservoir water balance was used to estimate that more than 14.8 billion cubic meters (Gm3) has been stored in the banks, with a 90% probability that the value is actually between 11.8 and 18.5 Gm3. Groundwater models developed by others, observed groundwater levels, and simple transmissivity calculations confirm these bank storage estimates. Assuming a constant bank storage fraction for simulations of the future may cause managers to underestimate the actual losses from the reservoir. Updated management regimes which account more accurately for bank storage and evaporation could save water that will otherwise be lost to the banks or evaporation.

Synoptic climatology of the early 21st century drought in the Colorado River Basin and relationships to reservoir water levels

“…Water inflow at both Lakes Mead and Powell is subject to upstream reservoir regulation and diversions. Consequently, the United States Bureau of Reclamation (USBR), which operates both reservoirs, calculates different measures of inflow at Lake Powell to account for different aspects of water regulation (USBR, ) and losses to evaporation, which can be substantial in the arid Southwest [1.22 m year −1 , according to one estimate (Myers, )]. ‘Inflow’ at Lake Powell is calculated as:…”

Elevation-area-capacity relationships of Lake Powell in 2018 and estimated loss of storage capacity since 1963

“…The reservoir and surrounding Glen Canyon National Recreation Area are classified as arid desert, with an average annual precipitation of 6 inches and summer temperatures up to 110 degrees Fahrenheit (°F; National Park Service, 2021b). Because of the climate and its porous sandstone substrate, losses to evaporation and bank storage are substantial (Myers, 2013;Friedrich and others, 2018). Stable isotope analysis of surface water entering Lake Powell indicates that little evaporation occurs in the upper Colorado River but increases moving downstream, particularly in Lake Powell and Lake Mead (Guay and others, 2006).…”

“…Annual evaporation estimates at Lake Mead, using the eddy-covariance method, were 74.07-81.65 inches between March 2010 and February 2012 (Moreo and Swancar, 2013) and 74.65 inches between March 2010 and April 2019 (Earp and Moreo, 2021). Cumulative water loss to bank storage in Lake Powell is estimated at approximately 15,000,000 acre-feet through 2011 (Myers, 2013) or 300,000 acre-feet per year since Glen Canyon Dam closed in 1963. Releases from Glen Canyon Dam to Lake Mead account for approximately 90 percent of the flow into the Lower Colorado River Basin (Ostroff and others, 2017;Lukas and Payton, 2020;McCabe and others, 2020;Tillman and others, 2020).…”