Earlier studies (Singh and Xu, 1997; Xu and Singh, 2000, 2001) have evaluated and compared various popular empirical evapotranspiration equations that belonged to three categories: (1) mass-transfer based methods, (2) radiation based methods, and (3) temperature-based methods; and the best and worst equations of each category were determined for the study regions. In this study a cross comparison of the best or representative equation forms selected from each category was made. Five representative empirical potential evapotranspiration equations selected from the three categories, namely: Hargreaves and Blaney-Criddle (temperature-based), Makkink and Priestley-Taylor (radiation-based) and Rohwer (mass-transfer-based) were evaluated and compared with the Penman-Monteith equation using daily meteorological data from the Changins station in Switzerland. The calculations of the Penman-Monteith equation followed the procedure recommended by FAO (Allen et al., 1998). The comparison was first made using the original constant values involved in each empirical equation and then made using the recalibrated constant values. The study showed that: (1) the original constant values involved in each empirical equation worked quite well for the study region, except that the value of alpha = 1.26 in Priestley-Taylor was found to be too high and the recalibration gave a value of alpha = 0.90 for the region. (2) Improvement was achieved for the Blaney-Criddle method by adding a transition period in determining the parameter k. (3) The differences of performance between the best equation forms selected from each category are smaller than the differences between different equations within each category as reported in earlier studies (Xu and Singh, 2000, 2001). Further examination of the performance resulted in the following rank of accuracy as compared with the Penman-Monteith estimates: Priestley-Taylor and Makkink (Radiation-based), Hargreaves and Blaney-Criddle (temperature-based) and Rohwer (Mass-transfer).
