Large amounts of water are lost from reservoirs and lakes by evaporation. Thus, measurements are being made of evaporation from free-water surfaces throughout the world. In the Western United States the storage of stream flow from mountain watersheds in reservoirs has made possible the development of much of the irrigated agriculture. These reservoirs help to prevent floods, conserve a water supply that otherwise might be wasted, and make water supply available for the production of power, irrigation of crops and for municipal purposes. Usually the evaporation rate cannot be measured directly from large water areas. Thus, it is a common practice to measure the evaporation rate of water from pans, and, using coefficients, a reduction is made of the pan-evaporation measurement to the lake-evaporation value. At high altitudes it is seldom possible to measure the evaporation rate during the winter months because the water in the pans freezes. Evaporation data from data of high-mountain areas are limited. The paper presents data concerning evaporation of water in several western states, and develops a method of estimating monthly evaporation for the entire year from temperature measurements and other data.