Evaluation of Daily Reference Evapotranspiration Methods as Compared with the ASCE-EWRI Penman-Monteith Equation Using Limited Weather Data in Northeast Louisiana

The ASCE-EWRI Penman-Monteith (ASCE PM) combination equation was recommended by the American Society of Civil Engineers–Environmental and Water Resources Institute as the standard equation for estimating reference evapotranspiration (${\mathrm{ET}}_o$). The standardized model was intended to simplify and clarify the presentation and application of the method; however, it requires numerous inputs that frequently are not available at most weather stations. In this study, comparisons were made between daily ${\mathrm{ET}}_o$ estimates from five different ${\mathrm{ET}}_o$ approaches using limited data to those computed with the full-form ASCE PM equation during the growing season in northeast Louisiana. Procedures to estimate missing data were followed when limited weather inputs were available. The results revealed that the differences between ${\mathrm{ET}}_o$ obtained with complete and limited data were influenced by the underlying surface, the weather conditions during the study, and the application of empirical methods to determine missing inputs. The mean absolute error (MAE) and root mean square error (RMSE) were less than 0.83 and $0.99\mathrm{mm}·{\mathrm{d}}^{-1}$, respectively, whereras low correlation was found through the coefficient of determination $R^2$. Overall, the reduced data set ${\mathrm{ASCE}\mathrm{PM}}_{SP}$ using wind speed data from a nearby station provided better results ($\mathrm{MAE}:0.56\mathrm{mm}·{\mathrm{d}}^{-1}$, $R^2:0.64$, $\mathrm{Mean}\mathrm{ratio}:0.98$) when compared with the ${\mathrm{ASCE}\mathrm{PM}}_{W2}$, Hargreaves-Samani, Turc, and Priestley-Taylor methods. The radiation-based equations tended to underestimate ${\mathrm{ET}}_o$ by as much as 10%, whereas the temperature-based Hargreaves model overestimated ${\mathrm{ET}}_o$ by 8% during the growing season.