Environment - Food and Water

Throughfall Drop Size Distributions: A Review and Prospectus for Future Research

Most people recognize that trees are affected by the environment and by people. Trees, however, also affect the surrounding environment in many different ways. For example, trees affect the water cycle, concentration of carbon dioxide in the atmosphere, and the cycling of elements. So, what happens under a tree, or in the forest, when it rains? The answer to this fundamental question has occupied generations of forest hydrologists, forest ecologists, and biogeochemists. As one might have guessed, the answer is complex and is dependent upon a myriad of factors, including tree species and the characteristics of the rain event itself. Suffice it to say, trees affect the size of the water droplets that pass through them (the technical term is throughfall) when rainfall occurs. So, why is throughfall important and why should people care?

Schematic diagram of throughfall drop component separation and drop size distribution of the different throughfall components. More information here.

Think about the water you drink. Think about the water droplet’s marvellous journey from the sky, through the land, and into a reservoir or the ground that people tap for water. People live in a water scarce world so it very much matters whether that single rain drop passes through the forest untouched, hits a leaf and splashes, or drips from a branch or leaf. Yes, that journey through the forest can make a difference as to whether that water is ultimately available to satisfy our thirst. This is all the more important since forests are the best protectors of water quality. They dominate the land around reservoirs that supply people with drinking water.

A recently published Review in WIREs Water, broadly speaking,  seeks to better understand how the trees in a forest affect the type of water droplet that is formed and how it is impacted by differences in plant surfaces and weather conditions. To date, it has been found that both tree species, the presence and absence of foliage, and weather conditions greatly affect the size distributions of water droplets that penetrates the forest canopy.  As seen in Figure 1, the distributions of drop size are markedly different for open rainfall and for throughfall. Scientists now must delve further into the physical processes of throughfall drop formation and couple it with both the cycling of water and elements to optimize our stewardship of forest ecosystems.


Contributed by D.F. Levia, S.A. Hudson, P. Llorens, K. Nanko.

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