Evaluation and generalization of radiation-based methods for calculating evaporation

Eight radiation-based equations for determining evaporation were evaluated and expressed in five generalized forms. Five evaporation equations (Abtew, Hargreaves, Makkink, Priestley and Taylor and Turc), where each represents one generalized form, were then compared with pan evaporation measured at Changins station in Switzerland. The comparison was first made using the or iginal constant values involved in each equation, and then using the recali brated constant values. Evaluation of the Priestley and Taylor equation req uires net radiation data as input, in this study, net radiation was estimat ed using Equation (16) owing to the lack of observation data. The results s howed that when the original constant values were used, large errors result ed for most of the equations. When recalibrated constant values were substi tuted for the original constant values, four of the five equations improved greatly, and all five equations performed well for determining mean annual evaporation. For seasonal and monthly evaporation, the Hargreaves and Turc equations showed a significant bias, especially for cold months. With prop erly determined constant values, the Makkink and modified Priestley and Tay lor equations resulted in monthly evaporation values that agreed most close ly with pan evaporation in the study region. The simple Abtew equation can also be used when other meteorological data except radiation are not availa ble. Copyright (C) 2000 John Wiley & Sons, Ltd.