The effect of street trees and amenity grass on urban surface water runoff in Manchester, UK

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 9 m2 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.