Concentrative drop impacts by a bunch of canopy drips: hotspots of soil erosion in forest

Soil erosion induced by rainwater in forest ecosystems is mainly determined by throughfall kinetic energy (TKE) and ground vegetation cover. TKE is determined by raindrop size and velocity as well as precipitation amounts. Lateral canopy water flow paths can create localized concentrations of throughfall as impact points with considerable high TKE. At structurally mediated woody surface drip points notably bigger canopy drips can thus be formed under forest canopy. It is also assumed that TKE per 1mm rainfall amount (i.e., unit TKE) at impact locations is considerably higher than that at general locations due to increased rain drop sizes, resulting in a higher risk of soil erosion. However, the TKE and subsequent splash erosion potential at these impact locations have rarely been described in the previous literature and have not been quantified yet. The objectives of this study are (1) to evaluate the intensity of TKE and unit TKE at an impact location and (2) to compare those with general locations and freefall kinetic energy. We measured TKE using splash cups at seven points under a beech tree in a cool temperate forest, Japan, during five rainfall events in each leafed and leafless season. Five splash cups were further installed at an open area outside the forest as a reference. A rainfall collector was installed next to each splash cup, and throughfall at each point was quantified. TKE at the impact location (9142 ± 5522 J m^-2) was 15.2 times higher than that at general locations under beech (601 ± 495 J m^-2) and 49.7 times higher than at the open area (184 ± 195 J m^-2). The ratio of TKE at the impact location to those at general locations was higher in the leafless season. Unit kinetic energy at the impact location (39.2 ± 23.7 J m^-2 mm^-1) was higher than those at general locations (22.0 ± 12.7 J m^-2 mm^-1) and at the open area (4.5 ± 3.5 J m^-2 mm^-1). The branch height at the impact location was lower than most areas at general locations, suggesting that higher unit TKE was induced by a bigger drop size. Our results imply that big-sized canopy drips in addition to intense throughfall amount generated at specific structurally-mediated points of the branch surface contribute far above the average to the erosion potential under the forest.