Sustain Our Nation's Forests and Grasslands

Study quantifies urban trees’ ability to intercept rainfall

DAVIS, Calif. - In urban settings, a tree’s ability to intercept or slow the rate that rainfall reaches the ground can reduce the amount spilling onto paved surfaces and lost as stormwater runoff. Researchers with the Forest Service’s Pacific Southwest Research Station and the University of California, Davis measured the crown storage capacity of 20 common California urban tree species, and then calculated each species’ potential interception amount over 40 years.

Researchers represented storage capacity as the depth of the water film stored on the leaf, needle or branch. They calculated it by measuring the volume of water retained by the particular piece of vegetation in relation to its surface area.

Some of the key findings included:

  • Blue spruce (Picea pungens) held the most rainwater (1.81 mm) with its many narrow spaces formed by rigid needles, buds and stems.
  • Canary island pine (Pinus canariensis) had the lowest storage capacity for conifers (0.99 mm) because water flowed readily down its long, flexible needles.
  • Overall, Bradford pear (Pyrus calleryana ‘Bradford’) had the lowest storage value (0.51 mm) because of its smooth leaf and stem surfaces.
  • Chinese pistache (Pistacia chinensis) had the highest storage values for deciduous trees (1.17 mm) because its rigid leaflets and rough stems slowed water flow across its surfaces.

Deciduous trees also were tested without leaves to simulate their storage capacity during autumn and winter. Potential accumulated interception after 40 years was calculated to range from 6 to 17 percent of the amounts that would have been stored during the leaf-on period.

The study stressed the importance of matching foliation periods with local rain patterns so that foliage is present when needed the most. For example, in regions with winter rainfall, evergreens are more effective than deciduous trees for optimal rainfall interception.

Bradford pear (Pyrus calleryana). Forest Service photo.

Canary island pine (Pinus canariensis). Forest Service photo.