Infiltration‐excess caused by Stemflow in a cyclone‐prone tropical rainforest

Abstract Tropical rainforest canopy trees that have large projected areas of upwardly inclined branches are capable of funnelling large volumes of rainwater down their trunks. During periods of prolonged heavy rainfall on Mount Bellenden Ker in northeast Queensland, Australia, stemflow volumes were found to be as much as two orders of magnitude greater than the volume of incident rainfall expected in a rain gauge occupying an area equal to the trunk basal area. Stemflow totals ranging from 6000 to 70000 litres were generated by individual trees from 7800 mm of rainfall over two successive wet seasons. The combination of high intensity rainfall and the funnelling effect results in significant quantities of infiltration‐excess at the ground surface. Stemflow fluxes as high as 31.4 cm 3 min −1 per cm 2 of basal area (i.e. the equivalent of 314 mm min −1 ) were recorded when rainfall intensity was only 2 mm min −1 . The mean infiltration capacity of the topsoil was determined to be 6.2 mm min −1 . The areas over which the stemflow would have had to spread in order to infiltrate were computed to be as much as 3 m 2 around the bases of individual canopy trees. Approximations of the distances that the infiltration‐excess would have travelled away from the tree bases were calculated by assuming that the infiltration area either expands radially outward in the form of an annulus or extends straight downslope from the tree base.