Generalized estimates of free‐water evaporation

A modified method for deriving free‐water evaporation estimates from network observations of air temperature, dew point, wind movement, and incoming minus reflected radiation is presented. Taking into account the difference between air and water temperature in computing emitted radiation from the water surface, the expression is an improvement over the original Penman type equation where observation of net radiation over the actual water surface is lacking. The accuracy of the method depends on the applicable mass transfer wind function. Techniques are derived to adjust for the effects of advected energy and heat storage when applying the free‐water evaporation estimate to actual water bodies. Computations of lake evaporation made with the modified method for a number of locations where verification data are available indicate that the relation provides a suitable basis for estimating actual evaporation without the expense of continuous over‐water observations.