Rainfall interception near a forest edge

Abstract

The process of interception is studied by comparing observations of net rainfall near a wind exposed forest edge with simulations of evaporation from a wet canopy. The simulations show a strong enhancement of the evaporation rate from the wet forest canopy near the upwind edge. The increased evaporation rate should result in an increased interception loss and a decreased throughfall near the edge. Observed throughfall appears hardly dependent on fetch from the forest edge, in agreement with previously published results. The seeming discrepancy between model and observations is explained by effects of humidity and wind velocity. The model simulates interaction between the surface and the lower atmosphere. It is argued that this interaction is suppressed for humidity variations, as the atmospheric humidity is influenced by evaporation of falling rain. Even by prescribing a constant atmospheric humidity, the model simulates a higher evaporation rate near the edge as wind velocity is increased near the edge. However, observations show that throughfall is statistically independent of wind velocity during rain, which is explained by a decrease in water storage capacity of the forest. It is concluded that the concept of a constant water storage capacity is questionable. Direct observations of water storage are recommended to quantify possible sensitivities of water storage capacity to environmental factors. For aggregation studies it is concluded that rainfall interception is basically independent of patch size. However, it is argued that the forest edge dries more quickly and transpiration may start sooner after the rain has stopped. This would result in an aggregation problem for transpiration.