April showers
UD researchers, Japanese colleague work to correlate raindrop size to forest ecology
9:59 a.m., April 29, 2015--For many people, the well-known childhood rhyme “April showers bring May flowers” heralds the arrival of spring and conjures up images of warm weather, soaking rains and flowers bursting into bloom.
But for Sean Hudson, a University of Delaware graduate student studying forest hydrology, April showers lead to science — specifically the science of raindrops.
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For over a year, Hudson has been studying seasonal changes in the size of raindrops dripping from a tree canopy at Fair Hill in northeastern Maryland to understand how these changes ultimately can influence water resources.
The water cycle plays a key role in maintaining healthy ecosystem functions, which support trees, as well as plants, soil, animals, insects and microorganisms that live there.
Hudson is studying throughfall — rain that falls through or from the tree canopy during a rainstorm — and how the size of the raindrops that reach the forest floor change depending on two variables: meteorological conditions including precipitation, wind speed and temperature, and whether or not the tree canopy has leaves.
“There is an interesting interplay between the amount of water coming out of the canopy and the chemistry that it brings to the forest ecosystem,” Hudson says. “From a water resource standpoint, we are trying to determine whether larger drops have a better chance of actually reaching the soil, stream channel or ultimately recharging groundwater.”
Last year, Hudson and faculty adviser Delphis Levia, professor of ecohydrology in the College of Earth, Ocean, and Environment’s Department of Geography, placed an instrument called a disdrometer under a tree canopy in Fair Hill to measure the rainfall coming through the tree canopy above.
The disdrometer emits a 5V laser beam between a transmitter and receiver. As a raindrop passes through the beam, the voltage drops. In concert with their Japanese collaborator Kazuki Nanko from the Forestry and Forest Products Research Institute in Tsukuba, located in the Ibaraki district of Japan, the research team converts the recorded voltage drop to determine the diameter, velocity and kinetic energy of each raindrop.
In the span of 12 months, the research team had amassed measurements on over 907,000 individual drops.
The scientists looked at how the drop size changed when leaves were present in the summer and absent in the winter. The tree canopy includes the leaves, trunk, branches, even the bark, all of which create multiple paths for water to take when it drops from the sky. As the leave patterns and density change from spring to summer to fall and winter, water flow patterns change too.
Hudson and Levia theorize that larger drop sizes spend more time in the tree, which means they are likely to pick up more atmospherically deposited chemical species from leaf and bark surfaces. They’ve already learned that larger raindrops and higher volumes of throughfall occur in winter, not the other seasons.
“Most people equate summer with larger raindrop size, but the leaves actually store less water than branch surfaces. As such, the water dripping from leaf surfaces is generally smaller than that from the woody surfaces,” says Hudson.
Understanding the rain’s flow path fills a gap in available hydrology research and may help answer fundamental questions about how water flow patterns develop, whether drop sizes influence or drive the canopy’s chemistry, and what role this plays in sustaining forest ecology, according to Hudson.
His research was recently accepted to Hydrological Sciences Journal, a publication of the International Association of Hydrological Sciences.
The research team hopes to expand their study in the future to include multiple disdrometers, which Levia says will enable the researchers to tie water droplet size to biogeochemical hotspots in the soil, leaf turnover, root turnover and an overall improved understanding of forest hydrology and ecology.
Article by Karen B. Roberts
Video courtesy of the College of Earth, Ocean, and Environment
Photos by Evan Krape