Urban Trees as Green Infrastructure for Stormwater Mitigation and Use

Abstract

Impervious surfaces in urban areas generate substantial volumes of polluted surface runoff, resulting in flooding and degradation of waterway ecosystems. Urban trees can help to mitigate the adverse effects of runoff by restoring key hydrological processes, including canopy interception, throughfall, stemflow, and transpiration. Understanding how trees contribute to these processes can guide tree species selection and the design of green infrastructure elements. Climate, specifically the distribution of precipitation and evaporative demand, will ultimately determine the extent to which trees contribute to each process. In general, canopy interception, throughfall, stemflow, and transpiration will be greater where the rainfall distribution is dominated by smaller events separated by longer inter-event periods with higher evaporative demand. However, in any given climate, different tree species, and more importantly the traits which define them, can significantly alter their role in the urban hydrological cycle. For example, species with large, dense canopies (high leaf area) are likely to show greater canopy interception loss, resulting in lower throughfall and stemflow and reduced surface runoff. Additionally, larger trees with high leaf area can potentially transpire a significant amount of captured runoff when combined with stormwater control measures. However, selecting species to maximise retention and detention of runoff must do so without compromising other highly valued ecosystem services provided by trees. This chapter reviews the studies which contribute to our current understanding of how different species contribute to hydrological processes in the built environment. We discuss how this understanding has been integrated into urban hydrological models as well as opportunities for future studies to continue their development.