Watershed-scale impacts of stormwater green infrastructure on hydrology, nutrient fluxes, and combined sewer overflows in the mid-Atlantic region

Abstract

Stormwater green infrastructure (SGI), including rain gardens, detention ponds, bioswales, and green roofs, is being implemented in cities across the globe to reduce flooding, combined sewer overflows, and pollutant transport to streams and rivers. Despite the increasing use of urban SGI, few studies have quantified the cumulative effects of multiple SGI projects on hydrology and water quality at the watershed scale. To assess the effects of SGI, Washington, DC, Montgomery County, MD, and Baltimore County, MD, were selected based on the availability of data on SGI, water quality, and stream flow. The cumulative impact of SGI was evaluated over space and time by comparing watersheds with and without SGI, and by assessing how long-term changes in SGI impact hydrologic and water quality metrics over time. Most Mid-Atlantic municipalities have a goal of achieving 10-20% of the landscape drain runoff through SGI by 2030. Of these areas, Washington, DC currently has the greatest amount of SGI (12.7% of the landscape drained through SGI), while Baltimore County has the lowest (7.9%). When controlling for watersheds size and percent impervious surface cover, watersheds with greater amounts of SGI have less flashy hydrology, with 44% lower peak runoff, 26% less frequent runoff events, and 26% less variable runoff. Watersheds with more SGI also show 44% less NO3(-) and 48% less total nitrogen exports compared to watersheds with minimal SGI. There was no significant reduction in phosphorus exports or combined sewer overflows in watersheds with greater SGI. When comparing individual watersheds over time, increases in SGI corresponded to non-significant reductions in hydrologic flashiness compared to watersheds with no change in SGI. While the implementation of SGI is somewhat in its infancy in some regions, cities are beginning to have a scale of SGI where there are statistically significant differences in hydrologic patterns and water quality.

Green infrastructure and its catchment‐scale effects: an emerging science

“…The LID design resulted in runoff volumes that were lower compared to non‐LID management and demonstrated LID's capacity to improve the catchment's water quality, specifically by decreasing catchment export of nitrate, ammonia, and total phsophorus . Pennino and others used measured hydrological and water quality data in multiple catchments (0.5–34.3 km 2 ) in the mid‐Atlantic coastal plain of the US and corroborated that stormwater LID—including rain gardens, detention ponds, bioswales, and green roofs—lowers the magnitude, frequency, and variability of stormwater runoff and decreases nitrate and TN export compared to catchments with limited LID implementations. In fact, when controlling for catchment size and percent impervious cover, catchments with higher percentages of LID implementation were found to have less flashy hydrological responses to storm events .…”

“…The goal of LID is to use plants, soils, and landscape design to control nonpoint sources of water and materials in built environments (Figure ; Table ), an approach that has become increasingly popular across communities worldwide as a cost‐effective way of managing stormwater pollution (e.g., runoff volumes and nutrient pollution) in urbanizing landscapes. For example, many municipalities across the mid‐Atlantic region of the United States (US) have a goal that 10–20% of the landscape drains through LID by 2030 . The increased interest in LID practices has led to a corresponding ascent in LID‐related research.…”

Cumulative Effects of Low Impact Development on Watershed Hydrology in a Mixed Land-Cover System

“…PP was 28% more effective at increasing shallow subsurface runoff and infiltration than RG. Recent studies point to a similar effectiveness of RG on water balance components at watershed scales [16,18]. Studies also have shown that PP can effectively mitigate surface runoff [57,58]; however, the degree of RG and PP functionality depends on the extent of the application area of LID within the watershed [59].…”

“…The results from the subwatershed with smaller LID lots and underdrain connections showed a substantial reduction in peak discharge (up to 33%) and total storm runoff (up to 40%). Additionally, recent field-based research provides evidence of the cumulative watershed scale effects of LID on hydrologic responses, such as peak flows and pollutant loads [17][18][19][20]. Such experimental studies can be resource intensive (e.g., financial, personnel, time) [21]; however, process-based models provide a means to go beyond measured data and explore the projected "what if" LID scenarios using potentially less resources.…”

Modeling Watershed‐Scale Impacts of Stormwater Management with Traditional versus Low Impact Development Design

“…The no treatment zone/riparian zone in the LID watershed is exclusively riparian with no impervious cover and can be considered a nonstructural BMP in itself (Figure ). Although the LID watershed in our study has a relatively high amount of impervious surface cover as compared with LID design in other locations (Bedan and Clausen, ; Koch et al ., ; Duan et al ., ; Pennino et al ., ), BMPs in this LID watershed aimed to replicate natural hydrologic functioning. This replication was accomplished, in part, by infiltrating stormwater close to impervious surfaces that generate stormwater runoff (Bhaskar et al ., ) and included site‐specific LID goals as specified for Montgomery County Special Protection Areas (Montgomery County DEP, ).…”