Throughfall drop sizes suggest canopy flowpaths vary by phenophase
Kazuki Nanko, Richard F. Keim, Sean A. Hudson, Delphis F. Levia
Abstract
Water flowpaths caused by incident rainfall onto forest canopy surfaces have a notable effect on the water budgets and chemistry of wooded ecosystems. This study revealed varying canopy flowpaths and residence times at the intra-event scale and across the phenological transition from leafed to leafless states for a set of three American beech (Fagus grandifolia Ehrh.) trees in a multilayered canopy. Simultaneous measurements of raindrops and throughfall drops by fifteen laser disdrometers over five rain events were analyzed during the transition from leafed to leafless phenophases. Throughfall was partitioned into free throughfall, splash throughfall, and canopy drip with four drop size classes. Throughfall drop size distributions and volume of each throughfall type varied at both intra-event and inter-event scales. Smaller canopy drips, <5.5 mm in diameter, were initiated earlier in rain events, whereas more rainfall accumulation was necessary to generate larger canopy drips, >5.5 mm in diameter. Smaller canopy drips were more dominant in the leafed phenophase when some structurally-mediated woody surface drip points were more muted. These results suggested throughfall from foliar surfaces generated smaller-sized canopy drip with shorter residence time, whereas throughfall from structurally-mediated woody surface drip points generated larger-sized canopy drip with longer residence time. There was also an increase in both free throughfall and splash droplets from leafed to leafless states, consistent with increased canopy gaps and direct interaction with woody surfaces in the leafless state. Based on the results, a conceptualization of the genesis and development of leaf and branch flowpaths in canopies is proposed.