Three-dimensional modeling of canopy tree interception of wind-driven rainfall

Abstract

Field measurement of interception loss from rainforest vegetation in the cyclone-prone tropics is complicated by high wind speeds that cause incident rainfall to be inclined from vertical fall paths. Given the characteristic roughness of tropical rainforest canopies, we hypothesized that the more prominent canopy tree crowns create lateral rainshadows and intercept greater volumes of rainwater per unit projected crown area than less prominent neighboring canopy trees under inclined rainfall conditions. This hypothesis was tested by: (1) modeling the three-dimensional (3-D) geometry of a tropical rainforest canopy surface in northeast Queensland, Australia, using photogrammetrically derived crown elevation data; (2) computing the inclination angles and azimuths of wind-driven rainfall for raindays on which net rainfall was measured from selected canopy trees; (3) creating and applying a ray-tracing program to the 3-D canopy model to quantify lateral rainshadows and the effective rainfall-intercepting crown areas of the selected canopy trees; (4) calculating the C e C index (effective rainfall-intercepting crown area/projected crown area ratio) of each tree; (5) analyzing the relationship between the C e C index values and the measured net rainfall totals after correcting for differences in the trees' interception storage capacities. A significant correlation ( P < 0.05) was found for more than 80% of the raindays examined with rainfall inclination angles of more than 19° from a vertical fall path. These results suggest that the variation in net rainfall totals among neighboring canopy trees may be explained in part by the differential interception of inclined rainfall. Interception loss is generally measured as the difference between gross rainfall measured in the open and net rainfall reaching the forest floor. We conclude that more meaningful measures of interception loss could be obtained by accounting for differences in gross rainfall intercepted by canopy trees sampled for net rainfall.