Evaluation Of Methods For Estimating Daily Reference Crop Evapotranspiration At A Site In The Humid Southeast United States

Authors

R. E. Yoder, University of Nebraska-LincolnFollow
Lameck O. Odhiambo, University of Nebraska-LincolnFollow
Wesley C. Wright, University of Tennessee, Knoxville

Document Type

Article

Date of this Version

2005

Citation

Applied Engineering in Agriculture, Vol. 21(2): 197−202

Comments

Copyright 2005 American Society of Agricultural Engineers

Abstract

Estimated daily reference crop evapotranspiration (ET_o) is normally used to determine the water requirement of crops using the crop factor method. Many ET_o 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 ET_o estimated from eight different ET_o equations and ET_o measured by lysimeter to provide information helpful in selecting an appropriate ET_o 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 ET_o was the best using the FAO−56 Penman−Monteith equation (coefficient of determination (r²) = 0.91, Syx = 0.31 mm d⁻¹, and a coefficient of efficiency (E) = 0.87), followed by the Penman (1948) equation (r² = 0.91, Syx = 0.34 mm d⁻¹, and E = 0.88), and Turc’s equation (r² = 0.90, S_yx = 0.36 mm d⁻¹, and E = 0.88). The FAO−24 Penman and Priestly−Taylor methods overestimated ET_o, while the Makkink equation underestimated ET_o. The results for the Hargreaves−Samani equation showed low correlation with 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 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.