Estimating evapotranspiration in Puerto Rico

The United Nations Food and Agriculture Organization (FAO) has recommended the Penman-Monteith method as the single method for estimating reference evapotranspiration throughout the, world. A disadvantage of the method, however, is its relatively high data requirement. Measurements of wind speed, humidity (or dew point temperature) and radiation tend to be the least available of the required parameters; therefore, the FAO has presented estimation procedures for these parameters. The purpose of this study was to evaluate estimation procedures for climate data to be used in the Penman-Monteith method for estimating long-term daily reference evapotranspiration, and to verify the accuracy of the procedures at four locations in Puerto Rico. Comparison of reference evapotranspiration determined by using the estimated and measured climate data shows reasonably good agreement. The methods presented in this paper are potentially valuable for calculating the long-term average daily reference evapotranspiration at any location in Puerto Rico. An example is provided to illustrate the use of the proposed estimation procedures for climate parameters. This study presents a comparison of reference evapotranspiration calculated by the Penman-Monteith method, with estimates previously made by using the Hargreaves-Samani method, for thirty-four locations in Puerto Rico. In addition, estimated peak evapotranspiration from the Soil Conservation Service (SCS) (now Natural Resources Conservation Service) Irrigation Guide for the Caribbean Area, the SCS Blaney-Criddle method and the Penman-Monteith method were compared for six vegetable crops at three locations in Puerto Rico. The results suggest that some irrigation systems may have been under-designed in terms of flow capacity in Puerto Rico.