The effect of intercepted rainfall on the water balance of a hardwood forest

Abstract

Evaporation from a large hardwood forest is estimated from measurements of the required meteorological variables and from measured stomatal resistances. A correction factor is derived to overcome the incorrect assumption that evaporative demand remains the same during wet and dry periods. The factor is based on the ratio of the isothermal resistances during the wet and subsequent dry periods. The stomatal measurements were converted to canopy resistances by dividing by the leaf area index and were used to obtain the water balance for the entire season. The results are analyzed to show that evaporation from a wet canopy is often 2–3 times greater than transpiration from the same surface. Rates of interceptional loss, calculated from the wet and dry evaporation rates, were verified by direct measurements of throughfall and stemflow. Net interceptional loss, equal to the excess evaporation from a wet canopy over a dry one, depended on rainfall duration and character and was on the average about 60–80% of total interception, In the overall summer water balance of 442 mm of precipitation and 52‐mm depletion of soil storage, transpiration via the trees accounted for 261 mm; evaporation from the wetted leaves and branches for 111 mm, and runoff for 131 mm, giving an error of closure in the water balance of only 9 mm. If transpiration only had been used instead of interception when the canopy was wet, the error in the water balance would have been 100 mm.