Actual, potential and equilibrium evaporation for a beanfield in southern Ontario

Abstract

Evaporation from a row crop of beans in southern Ontario is examined within the combination model framework using micrometeorological measurements of temperature, humidity, wind speed, net radiation and soil heat flux collected during ten days in 1970. Actual evaporation values were higher than anticipated from the 30–40% vegetation ground cover but in keeping with the high ratio (80–95%) of transpiring surface to ground plan area. Equilibrium evaporation compared closely (to within 10%) with actual evaporation on six days when the atmosphere was very humid and the ground was moderately dry. Poorer agreement was obtained on the other days which were cooler and less humid. Wind profile and silhouette analyses showed that the bean crop was aerodynamically smoother than the crop height would suggest. Values of surface resistance were calculated from actual and potential evaporation, air temperature and aerodynamic resistance. These showed a response to changes in soil moisture and were related to the ratio of actual evaporation to available energy in a similar manner to previously published values for agricultural crops. Potential evaporation was significantly larger than actual evaporation and close to the net radiation.