Throughfall drop size distribution in relation to leaf canopy state

  • Sean Hudson (right), a UD graduate student, and faculty adviser Delphis Levia work on a disdrometer, a scientific instrument that measures raindrop size and velocity.
  • The disdrometer is a scientific instrument that measures raindrop size and velocity.

Forests cover roughly one-third of the Earth’s surface.  Their canopies interact with falling precipitation, partitioning rainfall into interception, throughfall and stemflow.  Throughfall represents a key subcanopy water flux, affecting both the water and nutrient balance of the forested ecosystem.  Multiple factors influence how a tree canopy interacts with precipitation and routes it to the subcanopy, notably canopy shape and surface hydrophobicity, wind speed and direction, rainfall intensity, air temperature and relative humidity.  Temperate forests add an additional layer of complexity through annual leaf abscission and drop, which in effect changes canopy shape and surface characteristics.  In temperate forests little is known about how the throughfall drop generation process changes in response to canopy foliar state, and how this affects the development or disappearance of biogeochemical hot spots and hot moments. 

Dr. Delphis Levia and graduate student Sean Hudson of the University of Delaware Ecohydrology Group, and Dr. Kazuki Nanko of the Forestry and Forest Products Research Institute of Japan have been measuring throughfall drop size distributions at Fair Hill Natural Resources Management Area in Northeastern Maryland, using a laser disdrometer. Historically, the in situ measurements of drop size has been difficult with researchers using either pans of sifted flour or filter paper treated with dye, however the laser disdrometer allows for precise continuous measurement of drop size and velocity during a precipitation event.  A laser disdrometer is an advanced precipitation gage consisting of a laser transmitter emitting a laser beam of a known voltage across a given area to a receiver.  As a drop crosses the laser beam, the voltage drops from its initial value for a period of time. The voltage drop can be converted to the object’s diameter, and the period of time to its velocity.  

Currently > 900,000 throughfall drops have been measured and some interesting trends in the diameters of throughfall drops between the leaf on and leaf off periods have appeared.  This project has developed a robust dataset, which we hope to be able to answer many questions regarding the seasonality of canopy to forest floor flow paths and biogeochemical fluxes, and the effect of various meteorological variables on canopy storage and drainage. 


Department of Geography

Ecohydrology Focus Area

Project Team


CEOE School & Departments

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Discovering how geological processes have operated over various time scales to create and influence the planet’s surface environments.

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