Four years of daily weather observations from Central Tunisia were available for testing the application of the Penman Equation to estimate reference evapotranspiration in the arid climate of Central Tunisia. The data included minimum and maximum temperature, minimum and maximum relative humidity, 24-hour wind run, and lysimeter measurements of reference ET from Kasserine weather station. Solar radiation was extrapolated from a nearby weather station. Day/Night wind ratio had to be assumed. A computer program named PENMAN was developed to use these data with the Penman Equation including the Wright and Jensen, FAO and Wright wind functions to estimate reference ET. A diskette holding this program and data files accompanies this thesis. Statistical, graphical and relative error analyses were performed to compare measured and estimated values of ET. The results of this study indicated that all three versions of the Penman Equation overestimated low values and underestimated high values of ET in Central Tunisia. A five day average was found to considerably improve the relationship between measured and estimated ET. Calibration coefficients have been derived to adjust calculated ET in Central Tunisia. This calibration has been tested and showed unbiased results. Cumulative ET analysis showed that the Penman equation estimated ET fairly well. Of the three wind functions, the FAO wind function was selected "best" to estimate ET in Central Tunisia. The analysis of the validity of the wind function for high wind conditions showed that it did not significantly improve the model performance under high wind conditions, and limiting the wind speed at less than 200 Km/day may even reduce that accuracy.