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-42- EVAPORATION Introduction Evaporation is defined as.thephysical process 'by which water vapor escapes from a water surface. The rate of evaporation depends funda- mentally on many factors including ~adiation (both short- and long-wave)" vapor pJ:'essure, air temperature, wind 'Movement and atmospheric pressure. It is also affected by impurities such as'dust, films on the, water'surface, and matter dissolved or suspended in the water. This section will be restricted to discussion of methods for esti- mating evaporation from a lake or reservoir (free-water surfaces), and limitations of these methods. Knowledge of evaporation losses is highly important in design of ,reservoirs and in, planning their most'efficient operation for irrigation, hydro-electric power, water supply, or other purpose. In the arid parts of the West, where evaporation greatly exceeds precipitation, serious consideration must be given to the reduced water yield which will resultfrcm the construction of a reservoir. It should be emphasized that evaporation loss from the reservoir area is always greater than the evapotranspiration loss under pre..,reservoir conditicns. In humid areas, the increased loss may be of no consequence, but in arid areas the utility of a regulated water supply must more than balance the reduced water yield. Methods of determination Water budget The simplest method for determining lake evaporation would seem to be the water-budget method, which can be expressed by the volumetric equation:. E=P+I-O-.6,s (1) in which E = lake evaporation P = precipitation on lake surface I = inflow, by streams or effluent ground water o = outflow, by streams or influent ground water S = increase in stored water Reliability of the method depends on the accuracy to which each item of the budget can be measured or estimated, also on the magnitude of each item in relation to the evaporation. For each of the budget items, accept- able percentage error diminishes about in inverse proportion to relative magnitude of that particular item. Thus, unless evaporation is of about the same magnitude as the largest of the budget items, its determination by the water-budget method may not be reliable.