Quantification of Evaporation and Seepage Losses with a Floating Evaporation Pan: Lee Valley Reservoir, Arizona

Abstract

During the summer of 1989, a water balance study was conducted at Lee Valley Reservoir, located approximately 19 miles southwest of the town of Eagar in Apache County, Arizona. The objectives of this study were to quantify evaporation and seepage losses from the lake and substantiate the use of a land-based evaporation pan to estimate lake evaporation. Lake level, inflow, controlled releases, precipitation and evaporation were measured on a twice weekly basis for six months from May 1 to October 31. Evaporation was measured in a Class A evaporation pan designed to float in the reservoir. Evaporation was also measured at a Class A land pan near the town of Eagar. A 1.0 inch difference in the estimated rate and the measured rate for a 147-day period of common record represents a 3.6% error between the land-based pan and the floating pan. Total losses from the lake over the six -month period were 224 acre-feet. Of this loss controlled releases accounted for 8.2 acre-feet, and evaporation was measured at 116 acre-feet. The remaining loss of 100 acre-feet is due to seepage, much of which can be seen as seeps rising within 800 feet downstream from the dam. Using average data from past years, an annual water balance for the lake was also calculated.