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.

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A study was conducted to give some perspective to the importance of accurate evapotranspiration (ET) input data to hydrologic models. For the analysis, computed output from three hydrologic models was considered as the true watershed response. Variations in the form of random fluctuations and fixed biases were introduced into the potential evapotranspiration (Pet) input data of the test models. By leaving the other inputs and parameters unchanged, such an analysis shows the effect of evapotranspiration on streamflow under the regulation of the other components in the model. It is shown that a constant bias of 20% in the Pet input data has a cumulative effect and results in considerable error in the computed hydrograph peaks and recession characteristics, whereas the influence of the random error on estimated streamflow was generally not measurable for the watersheds and models studied.

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