Estimating actual evapotranspiration for forested sites: modifications to the Thornthwaite Model

A previously coded version of the Thornthwaite water balance model was used to estimate annual actual evapotranspiration (AET) for 29 forested sites between 1900 and 1993 in the Upper Great Lakes area. Approximately 8 percent of the data sets calculated BET in error. Errors were detected in months when estimated AETT was greater than potential evapotranspiration. Annual climate variability led to errors in accrued soil water storage, which led to errors in the calculation of AET. Two hydrologically justifiable modifications were made to correct errors resulting from the use of the original coded Thornthwaite model. The first modification allows for soil water accumulation throughout the year, whereas the second allows for oversaturated soil conditions in the spring. The modified program appears to be robust for any temperate climate condition.