Reclamation Information Sharing Environment (RISE)

Evaporative losses from reservoirs are not well understood due to climatic conditions, the size and shape of the reservoir, fluctuation of water levels, and reservoir operations. For many decades, water management agencies have relied on Class “A” evaporation pans and area–capacity tables to estimate and account for evaporative losses for individual reservoirs. This project aimed to enhance our comprehension of the evaporation losses occurring from Elephant Butte and Caballo reservoirs. Infrastructure was set in place to allow the mounting of sensors to measure weather variables offshore and onshore during the regular operation of the two reservoirs. Data was collected from 2021 to 2023. Several methods were used to estimate evaporation losses from the two reservoirs, including bulk aerodynamic, eddy covariance, and remote sensing. The analysis of the data and results are presented in this report. Reservoir evaporation using the bulk-aerodynamic method can be estimated using weather data measured at an onshore weather station near the reservoirs. Direct evaporation measurements at both reservoirs by the eddy-covariance method were lower than those estimated by the bulk-aerodynamic method. In contrast, evaporation by Class “A” evaporation pan was 2 to 3 times higher than both methods.