Estimating Open Water Evaporation

Open-water evaporation is one of the largest losses of water in the Southwestern United States, especially during hot dry summers. Evaporation will increase as temperatures rise. Through collaboration with local partners, Reclamation is piloting innovative methods to track and reduce these losses. Modernized, floating evaporation pans that mimic reservoir conditions and eddy covariance towers are being deployed at several reservoirs where evaporation losses affect water operations. These real-time estimates are being used to support reservoir operations that can reduce total evaporative losses.

"These projects are a result of the mindset central to Reclamation’s work: every drop of water matters—even the upward drops we cannot see.”

--Andrew Gelderloos, Hydrologist with Reclamation’s Albuquerque Area Office.

For each location on the map below, navigate through the photos to explore the instrumentation platforms and surrounding reservoir environments. Continue scrolling to the text below to learn more about estimating evaporation.

Note that although there are no reliable methods for directly measuring reservoir evaporation, various methods exist to estimate reservoir evaporation with onsite weather station measurements. Most historical reservoir evaporation estimates are based on changes in evaporation pan water depth, but these estimates have been proven to be inaccurate in many cases.

More recently available instrument types and platforms can provide greater accuracy. Eddy covariance instruments for estimating reservoir evaporation are best placed on floating platforms versus along shorelines to ensure continuous data collection regardless of wind direction. 

In total, these suites of instruments provide important information to track and reduce reservoir water supply losses based open water evaporation.