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 of crops 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 between daily ETo estimated from eight different ETo equations and ETo measured by lysimeter to provide information helpful in selecting an appropriate ETo equation for the Cumberland Plateau located in the humid Southeast United States. Based on the standard error of the estimate (S-yx), the relationship between the estimated and measured ETo was the best using the FAO-56 Penman-Monteith equation (coefficient of determination (r(2)) = 0.91, S-yx = 0.31 mm d(-1), and a coefficient of efficiency (E) = 0.87), followed by the Penman (1948) equation (r(2) = 0.91, S-yx = 0.34 mm d(-1), and E = 0.88), and Turc's equation (r(2) = 0.90, S-yx = 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 correlation with lysimeter ETo data (r(2) = 0.51, S-yx = 0.68 mm d(-1), and E = 0.20), while those for the Kimberly Penman were reasonable (r(2) = 0.87, S-yx = 0.40 mm d(-1), and E = 0.87). These results support the adoption of the FAO-56 Penman-Monteith equation for the climatological conditions occurring in the humid Southeast. However, Turc's equation may be an attractive alternative to the more complex Penman-Monteith method. The Turc method requires fewer input parameters, i.e., mean air temperature and solar irradiance data only.