ABSTRACT
Instruments for the measurement of evaporation are of two basic designs: open water evaporation pans and porous surface-type atmometers. Evaporimeters have been used for estimating the potential evapotranspiration of various crops. The process of evaporation is somewhat similar but not identical to that of transpiration. For even with fully opened stomata, there is still a diffusion resistance that is lacking in the evaporation from an open water surface. The energy budget and the aerodynamic roughness of the water surface may also differ from their counterparts on a vegetated surface. Furthermore, evaporation from the pan lags behind evapotranspiration for several hours because of the greater capacity for heat storage in the former. However, these differences are small, and a relationship— good enough for many agricultural purposes—usually exists between evaporation and evapotranspiration for a period of a day or longer.
