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Update:July 28, 2017

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Throughfall drop size distributions: a look into the hydrology and biogeochemistry of forests


Article title

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

Author (affiliation)

Delphis F. Levia (a), Sean A. Hudson (a), Pilar Llorens (b), Kazuki Nanko (c)

 (a) University of Delaware, Newark, DE, USA.
 (b) Institute of Environmental Assessment and Water Research (IDAEA-CSIC), Barcelona, Spain.
 (c) Department of Disaster Prevention, Meteorology and Hydrology, FFPRI, Tsukuba, Ibaraki, Japan.

Publication Journal

Wiley Interdisciplinary Reviews: Water, 4(4): e1225, July/August 2017, DOI: 10.1002/wat2.1225( External link )

Content introduction

Rain that has fallen in a forest eventually reaches the surface after passing through a filter consisting of leaves and branches (i.e., the canopy). Droplets adhering to the canopy gradually coalesce into larger drops, which can cause or exacerbate soil erosion in forests where the ground is exposed. Conversely, when large raindrops first fall onto the canopy, they splash into smaller droplets. Such small droplets evaporate more readily than water accumulated on leaves and branches and according to some hypotheses, may well be a factor in increasing interception loss in forests. Changes in the size of such drops, collectively called throughfall drops, have always been believed to have a substantial impact on the water balance in forests as well as hydrological and elemental cycles; however, not much is known about the mechanism of throughfall drop formation and there remains much speculation.

In the present study, we reviewed the previous literature on throughfall drops, from both Japanese and international researches, and compiled data into a practical introduction for throughfall drop research. The paper first summarizes the significance of throughfall drop research, characteristics of throughfall drops compared with open rainfall, relevant technical terms, and measurement methods. It also examines the factors determining the size of throughfall drops from the following three perspectives: plant surface, canopy structure, and meteorological conditions. Based on the previous achievements, the paper enumerates future challenges.

To the best of our knowledge, this is the world’s first review paper on the history of throughfall drop research. We hope that further advances on this topic will contribute to a more accurate understanding of the cycles of water, soil, and materials in forests.



Figure 1. Water droplet coalescence on cedar needles

Photo. Water droplet coalescence on cedar needles.