Mapping Average Daily Pan Evaporation

Voluminous data on monthly averages of Class A pan evaporation can be described concisely by mapping the parameters of Fourier series describing the seasonal variations. Average monthly Class A pan evaporation data were obtained for the 146 U.S. stations east of the Rocky Mountains. The average monthly pan evaporation, converted to daily rates, was assumed to occur at mid-month. A Fourier series with a single harmonic explained more than 95 percent of the variance of monthly mean evaporation for most of the stations. Maps of the means and amplitudes and phase angles of the first harmonic are presented. The magnitudes of the errors introduced by the Fourier series fit, and interpolation of parameters from the maps, are demonstrated for selected stations. These parameter maps provide a quick and easy method of estimating input for simple water balance models.