Comparison of Methods for Estimating REF-ET

The Penman-Monteith, Makkink, Priestley-Taylor, Turc, Hargreaves-Samani, and Thornthwaite methods were used to estimate reference evapotranspiration (REF-ET) at three sites in eastern North Carolina. The Penman-Monteith method with grass as the reference crop was selected as the standard of comparison for evaluating the other five methods. Good correlation was found between the REF-ET values estimated by each of the five radiation- and temperature-based methods and the Penman-Monteith method, although there were some differences. Based on the statistical analyses, the methods that performed the best in estimating daily and seasonal Penman-Monteith REF-ET at each location were recommended. Reliability of the radiation methods was also evaluated when data from another site were used. Penman-Monteith estimates were used to develop correction factors for their potential use in the temperature-based Thornthwaite and Hargreaves methods at each location and in the study areas in general.