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Throughfall drop size distributions: a review and prospectus for future research

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Throughfall is the dominant input of water to forests. Throughfall drop size and the distribution thereof are important because of their influence on the forest water balance, soil erosion, and, possibly, biogeochemical cycling. However, our inadequate understanding of throughfall drop size distributions has hampered progress in the identification of direct and indirect linkages between throughfall inputs and the biogeochemistry and physiological ecology of forests. This review provides a snapshot of our current understanding of throughfall drop size distributions by tracing the historical development of throughfall drop size studies and examining the determinants of throughfall drop size. The theory and methods of drop size studies also are reviewed to consolidate our collective knowledge of throughfall drop size distributions to date. Some of the gaps in our current knowledge, among many, include: (1) the effects of snowmelt on throughfall drop size; (2) the role and extent to which different canopy phenophases affect throughfall drop size; and (3) the extent to which throughfall drop size affects the chemistry of and biogeochemical cycling within forest soils. Closing these knowledge gaps will likely lead to the better conceptualization of rainfall partitioning processes and more definitive linkages between the cause‐and‐effect relationships between throughfall and soil erosion, forest biogeochemistry, and plant physiological ecology, for example. WIREs Water 2017, 4:e1225. doi: 10.1002/wat2.1225 This article is categorized under: Science of Water > Hydrological Processes
Schematic diagram of throughfall drop component separation and drop size distribution of the different throughfall components. (a) GR, gross rainfall; I, interception; SF, stemflow; TF, throughfall; TFF, free throughfall, TFR, release throughfall; TFD, canopy drip; TFS, splash throughfall. The sum of TFD and TFS equals TFR. TF equals the sum of TFF and TFR. The box details the interaction of different throughfall types with the canopy. Please note that the dashed line for TFF denotes that it passes through the canopy without ever contacting any vegetative surface. (b) The drop size of the different throughfall types and GR in relation to drop volume.
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Historical evolution of throughfall drop size distribution (DSD) studies related to the development of raindrop measurement methods. Open and filled circles indicate manual and automated sampling methods, respectively. Please note that this figure highlights certain key studies and does not attempt to identify every study by name. The numbers in the top right of each box correspond with its number in the reference list. The clustering of circles is intended to demonstrate the increase of throughfall DSD studies from 1980 onwards.
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Example of drop size distribution of open rainfall and throughfall based on drop number density, N(D) and drop volume ratio, V(D) observed in 1‐h with 14.7 mm h−1 rainfall intensity. The dashed blue and bold green lines indicate open rainfall and throughfall, respectively. (Data source: K. Nanko, Ref 10.)
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