It is well known that the process of urbanization alters the hydrological performance of an area, reducing
the ability of urban areas to cope with heavy rainfall events. Previous investigations into the role that
trees can play in reducing surface runoff have suggested they have low impact at a city wide scale, though
these studies have often only considered the interception value of trees.
This study assessed the impact of trees upon urban surface water runoff by measuring the runoff from
9m2 plots covered by grass, asphalt, and asphalt with a tree planted in the centre. It was found that, while
grass almost totally eliminated surface runoff, trees and their associated tree pits, reduced runoff from
asphalt by as much as 62%. The reduction was more than interception alone could have produced, and
relative to the canopy area was much more than estimated by many previous studies. This was probably
because of infiltration into the tree pit, which would considerably increase the value of urban trees in
reducing surface water runoff.