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UD researchers study raindrops to determine their connection to local water resources

Evan Krape


Many of us can recall being fascinated with raindrops as kids. As it turns out, so are scientists. At University of Delaware, researchers have been watching raindrops fall from a tree canopy at a research site in Fair Hill, Maryland.

When raindrops fall, they might stop at the leaves of the tree canopy, drip down the stem of the tree, or fall through the canopy. The researchers focused on the latter scenario, which is known as “throughfall” to hydrologists.

“When you’re driving under a tree covered street and all of a sudden really big rain drops hit your windshield. So they’re throughfall drops and they can either be much bigger or much smaller than the raindrops in your area," said Sean Hudson, a UD graduate student who has been monitoring seasonal changes in raindrops.

Hudson, along with Delphis Levia at University of Delaware and Kazuki Nanko at the Forestry and Forest Products Research Institute in Japan, authored a paper that analyzes the size of raindrops as they fall through a tree canopy. Under scientific parameters, a small raindrop is under a millimeter in diameter, while a large raindrop is at least a millimeter and a half. To measure the raindrop’s diameter, velocity and kinetic energy, he uses a tool called a laser disdrometer, which basically shoots a laser through each rain drop.

Knowing the size of the raindrops that drop through the trees helps scientists characterize what throughfall is. Hudson and his colleagues want to use this information to determine how trees might help or prevent the passage of rain flow into the local reservoir.

"If you have a bunch of trees around a reservoir, are those trees going to be helpful, as in, are they going to guide water into the streams in your reservoir?" said Hudson. "Or are they going to block and act as an umbrella and either evaporate the rain or redistribute it in the canopy somehow so you lose that water?"

Asking questions like these could potentially help understand how trees might influence water resources in arid regions, such as the western U.S. The researchers have applied for a grant to collect more data on raindrops at Fair Hill to learn more about how tree canopies and the size of raindrops that fall from them could impact the local water supply.

“Having a better understanding of how throughfall is generated is going to give us a better understanding of how much water is going to be available for people to use," Hudson said.

Hudson’s research will be published in the Hydrological Sciences Journal.