A formula for estimating the evapotranspiration rate from temperature and relative humidity data is given. Results of some tests of the accuracy of the equation indicate that it is better than other equations based on a similar number of variables. The accuracy was not as good as equations requiring more variables for their utilization, however. It was found from an investigation of the amount of water required for crop production that average yields of corn, soybeans and wheat were obtained when the total precipitation during the growing season averaged 72% of the maximum evapotranspiration. This average actual evapotranspiration rate may be more appropriate than potential evapotranspiration for some general purposes such as assessing the water resources of undeveloped countries, or determining the aridity indices in climatic studies. Abstract A formula for estimating the evapotranspiration rate from temperature and relative humidity data is given. Results of some tests of the accuracy of the equation indicate that it is better than other equations based on a similar number of variables. The accuracy was not as good as equations requiring more variables for their utilization, however. It was found from an investigation of the amount of water required for crop production that average yields of corn, soybeans and wheat were obtained when the total precipitation during the growing season averaged 72% of the maximum evapotranspiration. This average actual evapotranspiration rate may be more appropriate than potential evapotranspiration for some general purposes such as assessing the water resources of undeveloped countries, or determining the aridity indices in climatic studies.