Do Urban Trees Count? The Role of Trees in Urban Hydrology
Urban greening has great potential to transform watersheds for improved water quality, more desirable hydrographs, enhanced ecological resilience and human health benefits. Studies have shown that tree canopies play an important role in precipitation interception and reducing runoff in urban areas, which help in achieving stormwater management goals. Under certain circumstances, regulators can provide stormwater credits to urban managers who use urban trees as a best management practice (BMP). However, hydrological benefits from trees depend on the characteristics of the urban forest, climate, and existing land cover. More research is needed to better quantify the hydrological benefits of trees in California for stormwater management purposes. What hydrological benefits can a tree really bring when most storm events occur during the non-growing season? If the hydrological benefits of trees can be quantified, is there an optimal solution for the integration of Green Stormwater Infrastructure (GSI) with other urban greening efforts to achieve the most cost-effective runoff reduction?
Trees reroute stormwater along three pathways: throughfall, stemflow, and interception loss. To better represent the hydrological processes at the canopy level and better quantify the interception loss from canopies across a temporal and spatial scale, we expanded the GreenPlan-IT toolkit to estimate the interception loss from tree canopy based on morphology of trees, and added trees as an additional type of GSI. Tree species and tree type (deciduous and evergreen) information were collected for the City of Sunnyvale. The hydrological benefits of existing trees were evaluated using the GreenPlan-IT toolkit. In order to identify key areas of opportunity where GSI and urban greening activities can act synergistically to achieve hydrological benefits, a multi-objective optimization was conducted to compare the potential effects of trees with other GSI types. Recommendations of optimal GSI and urban greening configurations can be given based on the cost and runoff reduction.
The results of this study will be used by the stormwater community to better quantify the stormwater management benefits of trees. The outputs of the analysis will help guide municipalities’ planning and implementation efforts in stormwater management and urban greening. The outputs will also inform the city’s stormwater management planning for stormwater permit compliance and support the development of urban greening strategies.