Christiansen's formula was used to compute evaporation and potential evapotranspiration using 1013 months of data from 19 stations in Venezuela. Thirteen probability levels of precipitation were determined for each station. Potential irrigation requirements for 16 stations were computed based on gamma distribution of 5 probability levels subtracted from the potential evapotranspiration. A formula for computing precipitation at any level of probability developed based on gamma distribution and average monthly values of record available.