Estimation of Standard Deviation of Potential Evapotranspiration

Abstract

Some weather simulation procedures require mean values of potential evapotranspiration (ETP) and the standard deviations in ETP. For purposes of irrigation planning and design it may be desirable to know the probable deviations from mean values of ETP. Various crop production or crop yield models have been developed that require input of daily measurements of climate. A weather simulation procedure utilizing a monthly climatic data base can be substituted for the daily climatic data to produce very comparable results. The weather simulation procedure recommended requires the standard deviation of ETP. A series of monthly mean values of maximum and minimum temperatures provides the required data for estimating mean ETP and the standard deviation. If only long‐term mean maximum and minimum temperatures and the mean temperatures for a series of years are available, the standard deviation of mean temperature provides a means for making an estimate of the standard deviation in ETP. The relationship is influenced by latitude and elevation. Relationships are presented for use in estimating the standard deviation of ETP.