Evaluation of Methods for Estimating Daily Reference Crop Evapotranspiration at a Site in the Humid Southeast United States

Estimated daily reference crop evapotranspiration (ETo) is normally used to determine the water requirement ofcrops using the crop factor method. Many ETo estimation methods have been developed for different types of climatic data,and the accuracy of these methods varies with climatic conditions. In this study, pair-wise comparisons were made betweendaily ETo estimated from eight different ETo equations and ETo measured by lysimeter to provide information helpful inselecting an appropriate ETo equation for the Cumberland Plateau located in the humid Southeast United States. Based onthe standard error of the estimate (Syx), the relationship between the estimated and measured ETo was the best using theFAO-56 Penman-Monteith equation (coefficient of determination (r2) = 0.91, Syx = 0.31 mm d-1, and a coefficient ofefficiency (E) = 0.87), followed by the Penman (1948) equation (r2 = 0.91, Syx = 0.34 mm d-1, and E = 0.88), and Turcsequation (r2 = 0.90, Syx = 0.36 mm d-1, and E = 0.88). The FAO-24 Penman and Priestly-Taylor methods overestimated ETo,while the Makkink equation underestimated ETo. The results for the Hargreaves-Samani equation showed low correlationwith lysimeter ETo data (r2 = 0.51, Syx = 0.68 mm d-1, and E = 0.20), while those for the Kimberly Penman were reasonable(r2 = 0.87, Syx = 0.40 mm d-1, and E = 0.87). These results support the adoption of the FAO-56 Penman-Monteith equationfor the climatological conditions occurring in the humid Southeast. However, Turcs equation may be an attractive alternativeto the more complex Penman-Monteith method. The Turc method requires fewer input parameters, i.e., mean air temperatureand solar irradiance data only.