The role of urban canopy architecture in precipitation redistribution

Abstract:

Due to increased imperviousness, urban environments are often vulnerable to excess stormwater runoff and flash flooding during intense rainfall events. In the face of climate-induced storm intensification, green infrastructure has emerged as a tool to increase the resilience of urban communities. Urban street trees represent a dominant form of vegetation in many urban systems and play an important role in urban eco-hydrology. As such, urban trees can often serve as a form of “informal green infrastructure”. This work seeks to understand how the physical structure of urban street trees interacts with incoming precipitation through interception and redistribution of throughfall. We hypothesize that precipitation re-distribution within the tree canopy occurs during storm events and that the structural patterns of branch and limb location control throughfall patterns. The typical architecture of trees, combined with the regular placement of urban trees, suggests the possibility that tree canopies preferentially re-direct precipitation to or from the permeable soil of the terrace rather than the impermeable street. To measure throughfall, a grid of plastic buckets (~1m spacing) were set out beneath three sample street trees located in Fond du Lac, WI and Madison, WI. Precipitation and throughfall were measured in each bucket after rainfall cessation. Tree canopy architecture in the form of Leaf Area Index (LAI) was captured using digital images of the canopy taken at zenith above each throughfall gage. Additional imagery was captured of each study tree using a UAV within a week of each storm. These images and structure-from-motion software were used to produce a three-dimensional model of the canopy. Results suggest that urban canopies can act as “funnels” that preferentially route water to specific locations within the canopy footprint and “umbrellas” that redirect water to the edge of the tree canopy. These results have implications for stormwater management because urban forestry practices directly impact runoff generation in residential neighbourhoods.