This page should not be necessary. It would not be but for the fact that some members of the BWD Board of Directors and at least one architect in Borrego believe, or at least claim, that surface water evaporation from swimming pools, spas, etc., is not a serious source of water loss in the Anza-Borrego Desert. To refute this pernicious and counter-intuitive notion, and in case anyone else should be foolish enough to subscribe to it, we are providing the following information.
Many factors define an area's climate, but free-water surface evaporation is an excellent indicator of the climatic variables that also partly control transpiration of plants, such as solar radiation, air temperature, and wind. according to the U. S. Geologicla Survey, annual free-water surface evaporation along the Mojave River ranges from about 60 to 85 inches.
One study of rainfall and climate in the California Deserts pointed out that in all areas, the potential evaporation far exceeds the average precipitation. In 1975, at the United States Date Gardens at Indio, for example, total precipitation amounted to only three inches (76mm), while free evaporation (from an open pan) was 112.96 inches (2869 mm) or nearly ten feet; more than forty times the year's precipitation (Lenz, 1981). Most of the Colorado desert (Coachella Valley and related area) normally receives less than five inches (127 mm) of rain per year. http://www.serg.sdsu.edu/SERG/techniques/microcatch.html
A 2002 study by the California Regional Water Quality Control Board, Colorado River Basin Region found that the area in Riverside County around Blythe, California with annual precipitation of approximately 3.6 inches and an average temperature of 73.6 degrees Fahrenheit, has an annual evaporation rate of approximately 90 inches (7.5 feet).
The Salton Sea, about 22 miles east of Borrego Springs, is situated in a closed basin; water flows into it but does not leave except by evaporation. The surface evaporation rate there is about 6 ft/yr (2 m/yr). (Ponce, V.M., et al., Modeling Salinity Balance in Proposed Salton Sea Restoration Using a Diked Impoundment, San Diego State University, San Diego, Calif. 2003) The Salton Sea has no outlet. With an evaporation rate of 5.78 feet per year, the entire volume of the Salton Sea, with its maximum depth currently at about 50 feet, would evaporate within about 10 years if all inflow sources were stopped.