Estimating U.S. Class A Pan Evaporation from Few Climate Data

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Abstract
In view of the shortage of good measurements of lake evaporation for testing a new estimation formula, a modification of the formula has been devised that can be compared with U.S. Class A pan measurements. which are available widely. Both formulae, for lake and pan evaporation respectively, are based on Penman's evaporation equation and are for use when the only reliable data that are available are daily extreme temperatures. for instance. The pan evaporation formula allows for the geometry of a U.S. Class A pan evaporimeter, and for the wetness and albedo of its surroundings, without requiring any empirical calibration based on evaporation measurements. The resulting equation for the monthly mean pan evaporation rate (the so-called ‘Penpan’ formula) includes an estimate of the solar irradiance, which may be obtained from the extra-terrestrial radiation and the cloudiness, derived from rainfall figures. Estimates of evaporation differ from measurements with pans at nine places around the world by about 0.6 mm/d.

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