In this study a simple simulation model for the design of microcatchment water harvesting systems for rain-fed tree cultures in the Bajgah area, Fars province, I. R. of Iran, was developed. The actual daily evapotranspiration was calculated during the years 1984 to 1987. This model has been calibrated based on 34 measured volumetric soil water contents at depths of 120 cm, and soil water at depths of 0-120 cm in the study area. To calculate the daily actual evapotranspiration during these years, daily crop coefficient, the coefficient of evaporation from the soil surface, mean daily soil water stress coefficient for four years (1984-1987) and the Penman-FAO method for the calculation of reference crop potential evapotranspiration (ETo) were used. The results indicated that the total amounts of actual evapotranspiration (ETa) for growing seasons were 407, 346, 376 and 362 mm (mean value of 373 mm). Also, the values of yearly ETa were 537, 472, 488 and 485 mm (mean value of 496 mm). Furthermore, a simple method based on Type III Pearson distribution and Penman-FAO ETo was used to estimate the amount of actual evapotranspiration for different probabilities of occurrence in each month of the year. So, the total amount of actual evapotranspiration during the growing season can be obtained for different probabilities of occurrence. Moreover, by using the yearly actual evapotranspiration and yearly rainfall with different combinations of probabilities, the microcatchment areas were estimated for different probabilities of occurrence. These data were used in a simple equation for determining the microcatchment area based on the probability of annual rainfall and yearly ETa. The results indicated that with 50 % probability of annual rainfall and yearly ETa, the microcatchment area (sum of cropping and runoff areas) was estimated to be about 8.7 m2 (cropping area=1.8 m2 and runoff area=6.9 m2). According to the indigenous knowledge of the local farmers, the microcatchment size in the study area is also 9 m2,which is similar to the microcatchments size with a 50 % probability.