Evaluation of Reference Evapotranspiration Equations Under Humid Conditions

Five reference evapotranspiration (ET₀) equations are evaluated using data from seven humid locations. The equations evaluated include Hargreaves, Thornthwaite, Turc, Priestley–Taylor, and Jensen–Haise. The objective of this study is to evaluate ET₀ estimated by these equations against the corresponding values estimated using the standardized FAO-56 Penman–Monteith (PM) equation. For each location, ET₀ estimates by the all equations were statistically compared with FAO-56 PM ET₀ estimates. The Turc equation yielded the smallest root-mean-square-difference (RMSD) values at the all locations except Novi Sad, Serbia. The final ranking of equations was based on the weighted RMSD. The Turc equation has the lowest weighted RMSD and ranking first, and other equations ranked in decreasing order are: Priestley–Taylor, Jensen–Haise, Thornthwaite, and Hargreaves. The Turc equation gives the reliable calculation at all humid locations and it has proven to be the most adjustable to the local climatic conditions. The results obtained from this study, indicate very clearly that the Turc equation is most suitable for estimating reference evapotranspiration at humid locations when weather data are insufficient to apply the FAO-56 PM equation.