Reservoir evaporation

Overview

Evaporation is driven by the steep vapor pressure gradient between a water body and the atmosphere, and the available energy to liberate water molecules from the water body. Higher air and water temperatures, greater solar radiation, higher winds, and lower humidity each lead to greater evaporation.

Figure 1: Monthly mean evaporation at Lake Mead (blue) and Lake Mohave (red), Nevada and Arizona between May 2013 and April 2019, showing a strong seasonal trend. Evaporative losses at Lake Mead are generally higher than Lake Mohave due to the energy difference between the two reservoirs’ climates. (Adapted from Earp and Moreo, 2021).

While direct measurements of evaporation can be obtained from an evaporative pan about one yard 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 reservoir evaporation) have measured evaporation data collected on site and 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 midwinter are 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 widely used in the Lower Basin. Arizona stores much of the CAP project water in aquifers.

Relevance

Evaporation from reservoirs is of particular importance in the Colorado River basin given the magnitude of water storage and the surface area of the reservoirs; the ten largest reservoirs in the Colorado River basin, when full, boast a combined 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 system-wide natural flow.

Data and tools

Global Lake Evaporation Volume (GLEV)

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.

Additional resources

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.