Throughfall partitioning by trees

Abstract

Although we know that rainfall interception (the rain caught, stored, and evaporated from aboveground vegetative surfaces and ground litter) is affected by rain and throughfall drop size, what was unknown until now is the relative proportion of each throughfall type (free throughfall, splash throughfall, canopy drip) beneath coniferous and broadleaved trees. Based on a multinational data set of >120 million throughfall drops, we found that the type, number, and volume of throughfall drops are different between coniferous and broadleaved tree species, leaf states, and timing within rain events. Compared with leafed broadleaved trees, conifers had a lower percentage of canopy drip (51% vs. 69% with respect to total throughfall volume) and slightly smaller diameter splash throughfall and canopy drip. Canopy drip from leafless broadleaved trees consisted of fewer and smaller diameter drops (D50_DR, 50th cumulative drop volume percentile for canopy drip, of 2.24 mm) than leafed broadleaved trees (D50_DR of 4.32 mm). Canopy drip was much larger in diameter under woody drip points (D50_DR of 5.92 mm) than leafed broadleaved trees. Based on throughfall volume, the percentage of canopy drip was significantly different between conifers, leafed broadleaved trees, leafless broadleaved trees, and woody surface drip points (p ranged from <0.001 to 0.005). These findings are partly attributable to differences in canopy structure and plant surface characteristics between plant functional types and canopy state (leaf, leafless), among other factors. Hence, our results demonstrating the importance of drop‐size‐dependent partitioning between coniferous and broadleaved tree species could be useful to those requiring more detailed information on throughfall fluxes to the forest floor.

Characterization of differential throughfall drop size distributions beneath European beech and Norway spruce

“…The median volume diameter D50 is often used to represent the entire DSD (Sempere‐Torres, Porrà, & Creutin, ), and it is used to compare DSD under different canopy species (Brandt, ; Hall & Calder, ; Levia et al, ; Nanko et al, ). In order to get a robust D50, only high intensity or long events with high‐drop numbers would be suitable (Salles, Poesen, & Borselli, ), but the natural rain contain a high variability compared with controlled artificial rain.…”

“…Due to the spectral change below the canopy, parts of the DSD can be separated into different TF fractions: TF with splash droplets (TF s ), TF composed of canopy drip (TF d ), mixed fraction TF mix , containing free TF as well as release TF, not distinguishable from TF d or TF s . With the TF mix component inherent to this study, it employed a different TF partitioning approach than Levia et al ().…”

“…Past studies have shown that TF DSD may be categorized into (a) drip (TF d ), which is generated by the accumulation of intercepted raindrops on leaves (Mosley, ) and branches (Nanko et al, ) and is characterized by larger drops. These drops can have a higher kinetic energy but are smaller in number (Nanko et al, ) and (b) splash or impact droplets (TF s ), which result from the impact of drops on the canopy surface (Herwitz, ) and (c) free TF (Levia et al, ; Moss & Green, ). However, kinetic energy depends highly on the released height of the droplet, therefore, under tree canopies, drops can accelerate to terminal velocity and have the full kinetic impact (Gunn & Kinzer, ), whereas under lower canopies such as maize, the full terminal velocity is not reached (Frasson & Krajewski, ).…”

“…These TF s have smaller diameters than TF d but are much larger in number (Levia et al, ; Nanko et al, ). The unaltered drops that pass through the canopy are termed free TF.…”

Advancing ecohydrology in the 21st century: A convergence of opportunities

“…Sensor technologies, as discussed above, offer great promise in propelling our understanding of interception loss and understory precipitation dynamics. For example, laser disdrometers, such as those developed by Nanko, Hotta, and Suzuki (2006), allow for distinctions to be made between different throughfall types (free‐throughfall, canopy‐drip, and canopy‐splash) and their relative quantitative importance (e.g., Levia et al, 2019;Levia, Hudson, Llorens, & Nanko, 2017; Nanko, Hudson, & Levia, 2016). By comparing the temporal characteristics of throughfall type and depth relative to rainfall, disdrometer technology can provide important insight into the wetting of the canopy during a rain event.…”