The Bureau of Reclamation operates more than 300 reservoirs and 8,000 miles of water delivery canals throughout the West. Developing reservoir operation protocols and water accounting strategies requires estimates of open water evaporation, which constitutes a substantial water demand. Historically, evaporation from lakes and reservoirs has been estimated using pan evaporation information. However, evaporation pans can overestimate lake or reservoir evaporation by 25 to 100% when compared to water balance or energy balance estimates of evaporation, because heat storage in reservoirs can alter both the rate and timing of evaporation. A simple aerodynamic mass transfer approach, which estimates evaporation from near-surface wind speed and vapor pressure gradients measured from buoys, has been shown to avoid problems associated with estimating energy storage within water bodies.
Because of the scarcity of on-water weather data, BOR and the Desert Research Institute (DRI) are estimating evaporation using the aerodynamic mass transfer approach by coupling data from a reservoir meteorological network with remotely sensed imagery and gridded climate data. Landsat and Moderate Resolution Imaging Spectroradiometer (MODIS) imagery provide surface temperature data, which are critical to establishing vapor pressure gradients. Climate data include hybrid gridded weather data (GridMET) derived from Parameter elevation Regression on Independent Slopes Model (PRISM) and North American Land Data Assimilation System (NLDAS). Initial applications show that combining remote sensing with gridded weather data can yield fairly reliable monthly evaporation estimates and capture the heat storage effect causing a seasonal lag in evaporation.
Aerodynamic bulk mass transfer monthly evaporation estimates derived from Landsat thermal data and NLDAS gridded weather data compared to U.S. Geological Survey eddy covariance estimates and American Society of Civil Engineers Penman Monteith alfalfa reference evapotranspiration (ETr) for Lake Mead, AZ/NV.