Estimating Open-Water Evaporation from Lake Powell

Abstract:

The Bureau of Reclamation (Reclamation) manages over 300 reservoirs located across the western United States and requires estimates of reservoir evaporation for variety of activities, including water storage and accounting, water supply and demand planning, and climate impact assessment studies. Although necessary for a variety of applications, accurate estimates of evaporation can be difficult and expensive to obtain. For example, at Lake Powell, a major reservoir of the Colorado River system located on the Arizona/Utah border, current estimates of evaporation used for operational and planning purposes are based data collected and analyzed during the 1970s. In an effort to improve current understanding and estimates of evaporation from Lake Powell, Reclamation has partnered with the Desert Research Institute and deployed two floating platforms, one in Padre Bay, the other in Warm Creek Bay, outfitted with instruments to estimate evaporation using three different methods: the eddy-covariance (EC) method, the bulk aerodynamic method, and the Bowen-ratio energy balance method. During this talk, we present findings from a two-year period between November 2018 and November 2020. A data processing workflow has been developed to post-process the EC data that includes correction observations for platform motion and gap-filling using a technique called multiple imputation. We apply multiple imputation to the 30-minute EC and bulk aerodynamic evaporation estimates. Preliminary results suggest that the vapor pressure difference between the air and water peak during August and September, along with the vertically integrated heat content of the water (measured between 0.5 and 20 m) and evaporation estimates from the three methods. This finding (a late-summer, early-fall peak in evaporation) differs from previous estimates, including the Class A pan. Landsat satellite-measured water skin temperature, along with gridded weather data, have been used to develop spatial estimates of evaporation across the lake, using records from the two platform sites as calibration locations. Preliminary results from this spatial analysis will also be presented and used to discuss the development of a new remote-sensing based approach for estimating open-water evaporation.