Tapping the urban forest: Integrating tree canopy runoff (stemflow) into blue-green infrastructure

As urbanization accelerates and climate change intensifies, cities increasingly turn to green infrastructure to manage stormwater and enhance resilience. Urban trees not only sequester carbon and reduce heat island effects but also partition rainfall into interception, throughfall, and stemflow. Stemflow (the water channeled down tree trunks) is often voluminous and nutrient-rich in urban settings, yet remains largely overlooked in conventional stormwater management. In this paper, we propose that repurposing stemflow can complement blue-green infrastructure by reducing runoff and providing supplemental irrigation for urban agriculture. We offer a brief review of stemflow's contribution to the city surface, its nutrient composition (which often features elevated concentrations of nitrogen and potassium relative to open rainfall), and compare these values to runoff water and nutrient reduction targets. Using 1‑m resolution land cover data from EPA EnviroAtlas and a GIS-based methodology, we pair theory with a first-order feasibility assessment to identify “prime” canopy areas in four metropolitan regions: Philadelphia, Cleveland, Tampa, and Phoenix. Results indicate that prime canopy areas for stemflow capture, or diversion to other low impact development structures, range from 16.63 % in Philadelphia to 8.4 % in Phoenix, reflecting each city's unique urban forestry and stormwater challenges. Ultimately, with minimal retrofits, stemflow harvesting appears to offer a low‑cost, scalable strategy that not only diverts runoff but also supplies valuable, nutrient‑rich water to urban gardens and infiltration systems. By integrating natural tree-canopy hydrology with established stormwater practices, cities might meaningfully reduce infrastructure burdens and advance sustainable water management.