Greening up stormwater infrastructure: Measuring vegetation to establish context and promote cobenefits in a diverse set of US cities

Abstract

Urban greening practices are often adopted to mitigate the negative impacts of increasing impervious surfaces in urban areas. In the United States, green approaches are prevalent in the field of stormwater management as some cities are required to install green stormwater infrastructure (GSI) to meet regulatory requirements. While the primary function of GSI is to address stormwater quality and quantity issues, stormwater managers often tout the ancillary social and environmental benefits, or co-benefits, when promoting their green approach. Co-benefits are difficult to quantify because they span a diverse set of categories and cannot be easily measured using any single metric. Drawing from existing techniques in the field of ecosystem services, this study uses the normalized difference vegetation index (NDVI) to establish trends in ecosystem services in ten US cities with GSI programs and to evaluate the impacts of GSI interventions on urban greenness. Results show that only two of the ten study cities (Seattle, WA and Milwaukee, WI) are getting greener, likely due to maturing vegetation. A case study for one of the 10 cities, Philadelphia, utilizes a stormwater control measure (SCM) inventory of GSI installations, and shows decreasing greenness at the city-wide scale. This case study demonstrates that 62% of GSI project area is composed of non-vegetated SCMs. High-resolution imagery and spatial GSI data identifies densification trends in Philadelphia where non-vegetated SCMs are installed to control post-development stormwater, resulting in a decrease in NDVI. Smaller, vegetated SCMs contribute to greenness and related co-benefits when installed in series, especially when near larger vegetated vacant lots. Moving forward, decision makers are encouraged to incorporate NDVI into their planning processes to move beyond water quality and quantity control measures and directly incorporate and incentivize co-benefits into GSI goals.