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

Abstract

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.

Throughfall partitioning by trees

“…Throughfall, the water that penetrates a plant canopy, can be subdivided into two broad categories—free throughfall and release throughfall (Dunkerley, )—with the latter being further categorized into splash throughfall and canopy drip (Levia, Hudson, Llorens, & Nanko, ; Moss & Green, ; Nanko, Hotta, & Suzuki, ). The diameter, velocity, and kinetic energy of the throughfall drops differ as a function of the three throughfall types.…”

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

“…His measurements made intuitive sense because larger raindrops provide more water for crops, and the drop size distribution (DSD) of rain drops is needed to calculate the kinetic energy for soil erosion processes (e.g., Fernández‐Raga et al, ; Nanko, Hotta, & Suzuki, ). In fact, the 1900s were a fertile time for the development of both theoretical advancements and new techniques and instruments to quantify raindrop size and DSD for both rainfall and throughfall (TF) (see Levia, Hudson, Llorens, & Nanko, ). Such examples for open rainfall include the Marshall and Palmer () distribution, which is a highly utilized exponential relationship between raindrop size and drop number density related to rainfall intensity, and the first automated DSD instrument (Gunn & Kinzer, ).…”

Influence of Raindrop Size Distribution on Throughfall Dynamics under Pine and Birch Trees at the Rainfall Event Level

“…Free throughfall falls directly through the gaps in the tree canopy, has no contact with leaves or branches, and maintains the drop size distribution of an open rainfall [1]. Dripping describes rainfall drops captured in the canopy that fall to the floor mainly due to saturation of the canopy, while splash describes drops falling from the canopy due to external influences such as wind or rainfall [5]. Throughfall splash droplets are usually smaller than drops induced by dripping.…”