An Evaluation of Four Reference Evapotranspiration Models in a Subtropical Climate

This paper describes a detailed evaluation of the performance and characteristic behaviour of four equations for estimating reference evapotranspiration (ET₀) at eight meteorological sites in a subtropical climate. The equations assessed were: Makkink (MK), Turc (TC), Priestley–Taylor (PT) and Hargreaves (HG). The sites were distributed throughout the north of Iran and represent an intermediate humidity regime. The Penman–Monteith (PM) method was chosen as the standard for comparison and calibration of the above—mentioned four equations. Good correlation was found between the ET₀ values estimated by each of the four empirical equations and the PM method for all the locations; however MK and TC equations produced considerable underestimations. The performance of the PT and HG equations improved slightly after region-specific coefficients were developed for each equation, and the TC and MK equations were improved greatly. The modified PT equation turned out to be the most precise method, demonstrating superiority over the other methods evaluated (0.48 mm d⁻¹ of root mean square error (RMSE)). Good performance from the modified HG equation (0.53 mm d⁻¹ of RMSE) must be emphasized, given the simplicity of that method, which only requires maximum and minimum air temperature data.