Development of a Reservoir Meteorological Network for Estimating Evaporation to Aid in Operations, Policies, and Procedures

Effective and accurate management of surface water storage and the development of new storage and water accounting policies and procedures are impeded by lack of accurate estimates of historical and current evaporation. In addition, accurate evaporation estimates are essential for long-term water resources operations and planning under a changing climate. Historical estimation of evaporation using pan evaporation data is known to have significant uncertainty both in magnitude and timing. It has been found that evaporation pans can over estimate lake or reservoir evaporation by 25 to 100% when compared to aerodynamic bulk transfer, and water and energy balance estimates of evaporation. Heat storage in open water bodies can alter both the rate and timing of evaporation depending on the volume, geometry, clarity, and the surrounding environment of the water body. An important requirement for accurately estimating evaporation is the collection of weather data over the water surface, as air temperature is lower, relative humidity is higher, and wind speed is increased over water opposed to land. Having long-term over water weather, water “skin” temperature, and thermal profile data readily available is critical for accurately estimating evaporation and providing benchmark datasets for validating remote sensing and climate models.

Our motivation is to develop a long-term reservoir meterological network that consists of weather station bouys, measuring all required weather variables and reservoir themal properties to accurately estimate evaporation using multiple approaches with near real time data transmission to aid in operations, policies, and procedures, and benchmarking. We present results from a pilot study at Stampede Reservoir, California, where we compare and contrast pan, aerodynamic bulk transfer, Bowen ratio energy balance, and eddy covariance and Bowen ratio combinations, and describe the benefits and weaknesses of each approach for various monitoring needs. Our vision is to deploy additional buoy weather stations on at least 3 more western US reservoirs in the next year, including American Falls, ID, San Luis, CA, Lahontan, NV, for long term evaporation monitoring with near real time data access through a custom webserver.