Evaporation from seven reservoirs in the Denver water-supply system, central Colorado

Abstract

Seven reservoirs in central Colorado, operated by the Denver Board of Water Commissioners, were studied to determine evaporation losses. These reservoirs, Elevenmile Canyon, Oil Ion, Gross, Antero, Cheesman, Williams Fork, and Ralston, are located on both sides of the Continental Divide. The period of study was 19&7-73. Evaporation was computed by the energy-budget and masstransfer methods, and from evaporation-pan relationships. For three reservoirs, Elevenmile Canyon, Oil Ion, and Gross, mass-transfer coefficients were calibrated by energy-budget studies. At the remaining reservoirs, an empirical technique was used to estimate the mass-transfer coefficient. The energybudget-calibrated methods give the most accurate evaporation values; the empirical coefficients give only a best estimate of evaporation. The pan method of computing evaporation is the least reliable method because of problems of advected energy through the sides of the pan, representative pan exposure, and the variability of ratios of reservoir to pan evaporation. Total evaporation for the seasons is not known because instrumentation rafts were not operated during the entire open-water season. Calculation of evaporation (sublimation) amounts from the ice cover was not attempted, but the amounts are believed to be small. Amounts of evaporation during the longest single period of record at each of the three reservoirs for which energy budgets were determined are: