Who will stop the rain?

by Joel Betts

Green solutions bring improved water quality at regional scales

Curb-cut Rain Garden, Photo by Plaster Creek Stewards

Curb-cut Rain Garden, Photo by Plaster Creek Stewards

Polluted run-off from residential and urban areas has become a major problem for stream health and water quality. Green infrastructure projects, like bioswales and detention ponds, are becoming increasingly implemented by municipalities trying to address this issue. A recently published study is the first to show that that these strategies are effectively improving stream health on a regional scale.



Stormwater green infrastructure (SGI) is designed to use planted areas to catch run-off of rain and snowmelt from impermeable surfaces like driveways, roofs, and roads, and partially permeable surfaces like lawns. SGI allows runoff to soak in through the ground before reaching the stormdrain system. This helps filter out fertilizers, pesticides, car effluent, sediments, and unnaturally warm water before the water enters streams. SGI also slows the time it takes for water to get from the landscape to streams, which helps to prevent erosion and habitat degradation in streams during large storm events.

SGI has been shown to be very effective at treating pollution at the local-site scale (See Stiffler’s helpful 2013 overview). Yet because SGI has only been more recently implemented, there have been few studies that have been able to demonstrate its effectiveness at regional scales.


To address this, Pennino, Jaffe, and McDonald (2016) compared hydrologic flashiness—the quickness of movement of water from the landscape into and through streams—and nutrient pollution between similar watersheds with low and higher percentages of SGI, in Baltimore and Washington DC. They found that hydrologic flashiness and nutrient pollution were lower in watersheds with higher SGI. These findings show a positive impact of SGI on stream health, and provide evidence for the effectiveness of SGI to address urban stream pollution and degradation. Although broad cross-watershed benefits are the goal of SGI, this comparison is the first of its kind to science, and is invaluable as it shows that SGI can have its intended impacts at a regional scale. The authors paved the way for similar future research in regions across the U.S.

What they did Pennino, McDonald, and Jaffe’s article, titled “Watershed-scale impacts of stormwater green infrastructure on hydrology, nutrient fluxes, and combined sewer outflows in the mid-Atlantic region”, was published in May 2016 in the Journal Science of the Total Environment. The authors chose to work in Baltimore and Washington D.C. because of the prevalence of SGI in these areas, the presence of streams with comparable hydrology and level of impact from urbanization, and the availability of hydrologic and nutrient data on these streams.

Chart - Stormwater Green Infrastructure correlates with reduced flashy hydrology and nitrogen loads at watershed scale


*The % values represent the percent change between low and high SGI.

Figure reprinted from Pennino et al. 2016 (see references below) under a Creative Commons open access license: https://creativecommons.org/licenses/by-nc-nd/4.0/

They used data from USGS stream gages and EPA and local monitoring programs. The types of SGI evaluated in this study are detention ponds, shallow marshes, wet ponds, sand filters, infiltration trench/basins, bioretention (such as rain gardens), bioswales, and catch basin filters. All of these have in common that they slow and filter runoff from the urban landscape, and most of them do so using native plants. See the EPA’s website https://www.epa.gov/green-infrastructure/what-green-infrastructure for a description of some of these types of SGI.



The authors hypothesis was that stream health would increase with more SGI present in a watershed. They compared watersheds where more than 5% of the land area drains through SGI with watersheds where less than 5% of the land area drains through SGI. They measured hydrologic flashiness—which is the quickness of movement of water from the landscape into and through streams, and is a symptom of unhealthy urbanized streams. Flashiness degrades stream habitat. It causes stream-bank and channel erosion and sedimentation, which degrades the stream ecosystem by covering fish spawning habitat and filling in the crevasses where stream invertebrates live.

They used six metrics of flashiness. Average annual peak runoff is the amount of water above normal baseflow coming through a stream during a storm event, averaged throughout the year. Peak frequency is number of times in a year a rain event causes a flow higher than three times the median flow. The coefficient of variation of runoff, or CV is the amount of variation in flow above and below the average yearly flow. Increases in these metrics are symptoms of flashy streams. Hydrograph duration is the amount of time it takes for water to move through a stream after a rain event. Volume to peak ratio is the total amount of water that flows through a stream after a storm event divided by amount of water going through the stream at peak flow. Decreases of these two metrics are symptoms of flashy streams.

The authors found that watersheds with more SGI had significantly lower average peak runoff, peak frequency, and CV, longer hydrograph duration, and bigger volume to peak ratios that than watersheds with less SGI. All of these metrics provide evidence that prevalence of SGI in a watershed lowers hydrologic flashiness. This led them conclude that SGI improves stream health by reducing flashiness! They also found that different types of green infrastructure have different levels of influence on these metrics.



The authors also hypothesized that the amount of nitrogen and phosphorous would be lower in watersheds with more SGI. In general, higher nitrogen and phosphorous levels in streams is known to lead to lower stream health. The combination of these nutrients leads to increased growth of algae, which can drive down oxygen levels in streams, thereby reducing the ability of fish and invertebrates to thrive. Nitrogen and phosphorous surpluses are common in urban streams and come from lawn fertilizers and car emissions caught up by rain. This study found that nitrogen pollution tended to be lower in watersheds with higher levels of SGI. Phosphorus trends were similar to nitrogen, but were not statistically significant. Although these results showed less strong of a pattern than the hydrology results, they showed the same trend, that SGI improves stream health.


What this means  

So what? Didn’t we already know that green infrastructure improves water quality? The problem of stream degradation from urban stormwater runoff has been known for decades (Leopold 1968), and green infrastructure emerged as a solution to this decades ago as well. Many municipalities have incorporated SGI into their city planning. Implementation has increased over the last decade as well, as hundreds of millions of dollars are being spent on SGI (Benedict & McMahon 2012). As investment in SGI programs has grown, monitoring of its impacts is increasingly important.

Monitoring has focused mostly on site level impacts of SGI (Bratieres 2008). But information about impacts on overall stream health is limited to a few two-watershed comparisons and modeling efforts (Smucker and Detenbeck 2014, Liu et al. 2015). Even though region-scale stream health improvements are the end goal of SGI, there had been no studies assessing the impact of SGI at a regional scale, despite calls for this sort of research (Hale et al. 2015). Pennino et al. (2016) broke the ground by being the first to collect original data comparing streams across watersheds with different amounts of SGI in a broader region.


Evaluation and Future Needs

Not only were the authors the first to do this, but the first to show positive impacts of SGI on a regional scale. Their article has significant implications for stream ecologists, drain commissioners, urban planners, and watershed managers, among others.  It was well designed, and the authors’ conclusions sound. They covered their bases by looking into potentially confounding factors. They normalized nutrient and hydrology data between watersheds for percent impermeable surface and differences in discharge, and determined no significant impacts of differences in precipitation or techniques of data collection between watersheds.

Like in any study, there were some limitations. The study did not control for differences in soil type between watersheds, which could influence differences in nutrients or flashiness. The study also was not able to show significant differences in phosphorus or combined sewer outflows related to SGI, and made no attempt to assess impacts of SGI on stream biota, sedimentation, and erosion levels in streams. Although the authors mentioned the extent to which a few different types of SGI impacted stream health, the discussion of this was limited. This is a shortfall, as information on the effectiveness of different SGI strategies would be highly useful for watershed managers and urban planners trying to prioritize limited SGI funds.

Given that many municipalities have goals of 10-20% drainage through SGI (NYC DEP, 2013), and this study evaluated watersheds with 5-10%, more studies of this sort should be carried out as more SGI is implemented, to document to what percent SGI positive impacts could be seen. These studies should also further evaluate the effects of different types of SGI. Future studies ought to assess impacts of SGI on stream biota, phosphorous, combined sewer outflows and sedimentation in streams, as these parameters are also impacted by urbanization, and have yet to be shown to be influenced by SGI on a regional scale. Now that Pennino, McDonald, and Jaffe have paved the way, more studies of this scale are needed in regions with different climate and geography, to continue to assess if, in different environments, green solutions really do “stop the rain”.


About the Author: Joel Betts

Joel Betts is a current Master’s Student in Fisheries and Wildlife at Michigan State University. He was formerly Green Infrastructure Associate at Plaster Creek Stewards (2016) where he installed stormwater green infrastructure projects. He has worked on a variety of stream restoration and monitoring projects.



  1. Benedict, Mark A., and Edward T. McMahon (2012). Green infrastructure: linking landscapes and communities. Island Press.
  2. Bratieres, K., Fletcher, T. D., Deletic, A., & Zinger, Y. (2008). Nutrient and sediment removal by stormwater biofilters: A large-scale design optimisation study. Water research42(14), 3930-3940.
  3. EPA (9 Sept. 2016). “What Is Green Infrastructure?” https://www.epa.gov/green-infrastructure/what-green-infrastructure.
  4. Leopold, L.B. (1968). Hydrology for urban land planning: A guidebook on the hydrologic effects of urban land use. U.S. Geological Survey Circular 554. US Geological Survey, Washington, DC.
  5. Liu, Yaoze, Vincent F. Bralts, and Bernard A. Engel (2015). “Evaluating the effectiveness of management practices on hydrology and water quality at watershed scale with a rainfall-runoff model.” Science of The Total Environment511: 298-308.
  6. NYC DEP (2013). NYC Green Infrastructure Annual Report. NYC Department of Environmental Protection, New York, NY. http://www.nyc.gov/html/dep/html/stormwater/nyc_green_infrastructure_plan.shtml
  7. Pennino, Michael J., Rob I. McDonald, and Peter R. Jaffe (2016). “Watershed-scale impacts of stormwater green infrastructure on hydrology, nutrient fluxes, and combined sewer overflows in the mid-Atlantic region.” Science of The Total Environment 565: 1044–1053.


  1. Stiffler, L. (2013). Are rain gardens mini toxic cleanup sites? Here’s what happens to the pollutants swept up in stormwater runoff. Sightline Institute, Seattle, WA. http://www.sightline.org/2013/01/22/are-rain-gardens-mini-toxic-cleanup-sites/