Rainwater canopy flowpath estimated by raindrop measurements

Water flowpaths caused by incident rainfall onto forest canopy surfaces have a notable effect on the water budgets and chemistry of wooded ecosystems. The objective of this work was to use drop-size distributions in throughfall to identify canopy flowpaths 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 and konara oak (Quercus serrata Murray) in a multilayered canopy.

Simultaneous measurements of raindrops and throughfall drops by laser disdrometers 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. The partitioning was based on the difference of drop size distributions between open rainfall and throughfall.

Throughfall drop size distributions and volume of each throughfall type varied at both intra-event and inter-event scales. As for American beech, 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.

Similar analysis was conducted for konara oak. More rainfall accumulation was necessary to generate larger canopy drips as with the American beech, but the amount of the larger canopy drips was stable after generation during rain events compared with smaller canopy drips. Thus, the fluctuation of throughfall amount was correlated with that of the amount of smaller canopy drips.

Based on the results, a conceptualization of the genesis and development of leaf and branch flowpaths in canopies is proposed.

This research was supported by JSPS KAKENHI (Grant numbers JP21K05837, JP17KK0159, JP15H05626). A part of the study is published in Nanko et al. (2022) in Journal of Hydrology (doi: 10.1016/j.jhydrol.2022.128144).