The Collison Floating Evaporation Pan, the Next Generation of Evaporation Pans

Accurate tracking of open-water evaporative losses, one of the largest consumptive uses of water in the arid southwest, will become increasingly important in the future with anticipated climate shifts toward longer, more-severe droughts. The current methods for estimating evaporation on reservoirs are known to have uncertainties ranging from ± 20 to 40 percent. This uncertainty in evaporation rates needs to be reduced in order to give water-resource managers a better understanding of current and future water supplies.

This study will investigate an improved method for determining open-water evaporation rates by deploying the Collison Floating Evaporation Pan (CFEP) (U.S. Patented), with built-in wave-guard and adjustable freeboard, which will measure continuous open-water evaporation rates at a fixed location within a reservoir. The CFEP will be semi-submerged to minimize the difference in water temperature between the CFEP and the reservoir. Additionally, the CFEP design should have minimal influence on the atmospheric boundary layer overlying the pan relative to the reservoir. Establishing these two conditions will provide a more accurate evaporation quantification. The CFEP's accuracy will be verified through the use of a hemispherical evaporation chamber designed to measure the actual evaporation rate adjacent to the CFEP.

The first of four CFEPs was deployed on Cochiti Lake, NM, during fall 2017 with the second CFEP to be deployed on Lake Powell during fall 2018. Results from the Cochiti Lake CFEP will be compared to other evaporation models (Priestly-Taylor, Penman's Equation, Hamon Equation, and Hargreaves's Equation) and to the on-site Class A Evaporation Pan managed by the U.S. Army Corps of Engineers.

Through innovative design and extensive field measurements, this study aims to develop a more accurate, robust, automated, and real-time technique for measuring near-actual reservoir or lake evaporation, leading to effective long-term monitoring and management of our nation's reservoir and lake water resources.