| Abstract: | This paper consists of two parts. In the first part, the significance of five major factors, including solar radiation, vapour pressure deficit, relative humidity, wind speed and air temperature, that control evaporation were evaluated comparatively at different time-scales using the data from Changines station in Switzerland. The comparative evaluation was made at hourly, daily, 10-day and monthly time-scales. It was found that the role of controlling variables in evaporation varied with the time-scale. The vapour pressure deficit was best correlated with pan evaporation at all time-scales, while the wind speed was least correlated with pan evaporation, especially when the time period was longer than a day. In the second part, four equations for calculating evaporation, including temperature-based methods, humidity-based methods, mass transfer methods and radiation-based methods, were compared with pan evaporation. Of these four equations, the Penman equation, representing the mass transfer method, resulted in monthly evaporation values that agreed most closely with pan evaporation values. The Romanenko equation, representing the humidity method, also compared reasonably well with pan evaporation. The Turc equation, representing the radiation method, and the Thornthwaite equation, representing the temperature method, were found to underestimate evaporation significantly, especially for cold months. © 1998 John Wiley & Sons, Ltd. |
