Reservoir evaporation is of great importance in the Colorado River basin due to the enormous magnitude of water storage and the surface area of the reservoirs; the ten largest reservoirs in the Colorado River basin, when full, have an aggregate surface area of about 650 square miles. Estimates of annual evaporation from all reservoirs in the basin are on the order of 1.5 million acre-feet (maf), or 10% of the river's annual average natural streamflow.
Evaporation from reservoirs is driven by the steep vapor pressure gradient between the water surface and the (usually dry) atmosphere, and the available energy to liberate water molecules from the water. Higher air and water temperatures, greater solar radiation, higher winds, and lower humidity all lead to greater evaporation rates.
While direct measurements of evaporation can be obtained from an evaporative pan about 3 feet across, quantifying evaporative loss from a large reservoir is extremely challenging. Traditionally, pan evaporation readings have been used as a proxy for the larger water body (after adjustment by a coefficient), but this has been shown to be the least accurate method. More accurate methods use varying sets of in situ observations (specific humidity, air and water temperature, winds, net radiation) in equations that approximate the complex physics of evaporative loss in a natural setting; these methods include Eddy Covariance (EC), Bowen Ratio Energy Balance (BREB), and mass transfer. Reclamation has partnered with USGS (at Lake Mead and Lake Mojave; Figure 1) and with the Desert Research Institute (Lake Powell; ongoing) to compare these methods. In the Upper Basin, 11 reservoirs--accounting for 80% of the estimated annual Upper Basin reservoir evaporation--have measured evaporation data collected on site, while 849 reservoirs do not have on-site measurements.
Lake Powell, located in the Upper Basin’s warmest and most arid region, is estimated to lose approximately 70 inches of water to evaporation per year, while Lake Mead, 2000 feet lower in elevation and even warmer and drier, loses about 80 inches per year. While evaporation rates at Mead are highest during summer, high rates persist long into the fall, and even in mid-winter are still about half of the summer peak. The combined evaporative losses from Lake Mead and Lake Powell, when full, are an estimated 1.135 (maf) annually. This is roughly equivalent to the annual beneficial consumptive use of Colorado River water (agricultural and municipal) within both Utah and Nevada.
Given the magnitude of reservoir evaporation losses in the basin, and around the West, a number of ‘geoengineering’ methods for reducing these losses have been proposed, including covering reservoirs in thin films of organic compounds, or covering reservoirs in plastics or shades. Storing water in groundwater aquifers is a strategy for avoiding evaporation losses that is widely used in the Lower Basin. Much of the Central Arizona Project (CAP) water is stored in aquifers.
Data and tools
The GLEV app is hosted on Earth Engine and provides evaporation volume time series estimates for lakes around the world. The app is generated by Gang Zhao (Stanford university) and colleagues, and has an open source GitHub repository and a peer reviewed paper describing the app models and generation.
State of the Science Report
Chapter 5 of the State of the Science report details hydrology observations. Section 5.5 covers evaporation, evapotranspiration, and evaporative demand within the Colorado River.
Additional details about reservoir evaporation in the Colorado River Basin can be found in a journal article by Friedrich et al, 2018: Reservoir evaporation in the Western United States: Current Science, Challenges, and Future Needs
In situ monitoring data for Lake Powell evaporation in recent years can be found in a journal article by Holman et al., 2022: Evaporation from Lake Powell: In-situ monitoring between 2018-2021