The Mean Annual Potential Evapotranspiration (PET) digital data set was calculated using the Thornthwaite formula. This method for estimating PET requires only two variables, mean monthly temperature values, and the average monthly number of daylight hours. Also, the Thornthwaite empirical method was chosen in this case to estimate PET, because other more theoretical methods (e.g. Penman's formula) require greater in-situ measurements of meteorological parameters than are readily available.