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How do interception rates vary in the trees around our school site?


Trees intercept rain water, slowing it as it moves from the atmosphere to the ground surface and ultimately into the soil through infiltration. By studying the canopy cover of different trees, as well as how much rain reaches the ground surface during a rain event, students may be able to find a relationship between the two variables.

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Length of time

One lesson.


Suitable locations

School grounds.


Equipment needed

  • Rain gauges
  • Small mirror
  • Acetate grid


Objectives and Key Skills

  • Understanding the sequence of flows through the water cycle
  • Understanding how plants physiologically adapt to both rainfall and sunlight
  • Understanding the nature of correlations in two sets of data


Suggested delivery

Students select a tree in the school grounds for which they will measure canopy cover and interception rates.


Canopy cover

On a sheet of acetate the same size as a mirror, a grid is drawn using a permanent marker pen, ideally dividing the size of the mirror into a 10 x 10 grid. A mirror is placed as close to the base of a tree as possible with the reflective side facing the underside of the tree canopy. The acetate is then placed on top of this. Without blocking their view of the canopy, students should look into the mirror and count the number of squares in which there is a clear view of the sky. By subtracting this figure from 100, students can then express this new value as the percentage canopy cover.



When a rain event is predicted, students place their rain gauge at the base of their selected tree and collect it again once the rain event has finished. Students record the amount of rainfall that fell under the tree canopy.

At the same time a control rain gauge needs to be set up in an open space where there is no canopy cover. This will tell students the total amount of rainfall that fell during the rain event and therefore allow them to calculate how much rain was intercepted by their surveyed trees.


With data for a variety of trees collected across the school site, students may have enough data to plot canopy cover against interception rate in a scattergraph. This will allow students to conclude whether trees with the greatest canopy cover intercept the most water.


Potential risks to consider

  • Ensure that students do not attempt this enquiry if there is any chance of an electrical storm accompanying the rain event.
  • Make sure students do not look directly up at the tree canopy to judge its coverage as direct sunlight can damage their eyes.


Possible follow up activities

  • Students could also record the age or height of the tree and see if these factors also correlate with higher or lower levels of interception.
  • If students record the species of tree, they can form an afforestation plan for the school. This would include details of which species of tree should be planted where to prevent the school site flooding.


Useful links

The Rivers Trust – National Tree Week resources

Woodland Trust Tree identification app


This collection of Fieldwork activities were created by Paula Richardson and the Fieldwork and Outdoor Learning Special Interest Group (FOLSIG) for the National Festival of Fieldwork, the GA invites everyone to take part during the summer term.

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