In urban areas, the impermeable materials used for roads and pavements mean that rain is not absorbed and remains on the surface (Pauleit & Duhme, 2000). During periods of heavy rainfall this water accumulates and when the drainage capacity of the area is exceeded, flooding will occur.
In contrast, vegetated surfaces are able to intercept (Asadian & Weiler, 2009) and store water (Sanders, 1986), reducing the volume of rainwater run-off. Benefits from individual trees are maximised if they are planted in tree pits containing permeable soils able to absorb additional water (Armson et al., 2013), or structural soils that facilitate the growth of tree roots beneath pavements and roads (Bartens et al., 2008).
A further consequence of high levels of surface water run-off is that rainwater washes pollutants away from the surfaces it falls onto, transporting them into water courses (Ellis, 1991). This can be detrimental to water quality in streams, rivers and lakes and lead to high pollutant loading at water treatment facilities (Characklis & Wiesner, 1997).
In the UK, climate change is likely to lead to wetter winters (Jenkins et al., 2009) which would exacerbate existing flooding and water quality issues. Including green spaces as part new urban developments, as well as integrating them within existing urban regions, could help to reduce these risks (Ellis et al., 2002; Villarreal et al., 2004; Gill et al., 2007) and offers an alternative to other hard engineering flood control that can be disruptive and expensive to install.