Modeling the hydrologic effects of watershed-scale green roof implementation in the Pacific Northwest, United States

Brad Barnhart; Paul Pettus; Jonathan Halama; Robert McKane; Paul Mayer; Kevin Djang; Allen Brookes; L Monika Moskal

doi:10.1016/j.jenvman.2020.111418

Modeling the hydrologic effects of watershed-scale green roof implementation in the Pacific Northwest, United States

J Environ Manage. 2021 Jan 1:277:111418. doi: 10.1016/j.jenvman.2020.111418. Epub 2020 Oct 17.

Authors

Brad Barnhart¹, Paul Pettus², Jonathan Halama², Robert McKane², Paul Mayer², Kevin Djang³, Allen Brookes², L Monika Moskal⁴

Affiliations

¹ U.S. Environmental Protection Agency, Office of Research and Development, Center for Public Health and Environmental Assessment, Pacific Ecological Systems Division, 200 SW 35th Street, Corvallis, OR, 97333, USA. Electronic address: bradleybarnhart@gmail.com.
² U.S. Environmental Protection Agency, Office of Research and Development, Center for Public Health and Environmental Assessment, Pacific Ecological Systems Division, 200 SW 35th Street, Corvallis, OR, 97333, USA.
³ Inoventures, LLC, Center for Public Health and Environmental Assessment, Pacific Ecological Systems Division, c/o U.S. Environmental Protection Agency, Corvallis, OR, 97330, USA.
⁴ University of Washington, College of the Environment, Precision Forestry Cooperative and Remote Sensing and Geospatial Analysis Laboratory, School of Environmental and Forest Sciences, Seattle, WA, 98195-2100, USA.

Abstract

Green roofs are among the most popular type of green infrastructure implemented in highly urbanized watersheds due to their low cost and efficient utilization of unused or under-used space. In this study, we examined the effectiveness of green roofs to attenuate stormwater runoff across a large metropolitan area in the Pacific Northwest, United States. We utilized a spatially explicit ecohydrological watershed model called Visualizing Ecosystem Land Management Assessments (VELMA) to simulate the resulting stormwater hydrology of implementing green roofs over 25%, 50%, 75%, and 100% of existing buildings within four urban watersheds in Seattle, Washington, United States. We simulated the effects of two types of green roofs: extensive green roofs, which are characterized by shallow soil profiles and short vegetative cover, and intensive green roofs, which are characterized by deeper soil profiles and can support larger vegetation. While buildings only comprise approximately 10% of the total area within each of the four watersheds, our simulations showed that 100% implementation of green roofs on these buildings can achieve approximately 10-15% and 20-25% mean annual runoff reductions for extensive and intensive green roofs, respectively, over a 28-year simulation. These results provide an upper limit for volume reductions achievable by green roofs in these urban watersheds. We also showed that stormwater runoff reductions are proportionately smaller during higher flow regimes caused by increased precipitation, likely due to the limited storage capacity of saturated green roofs. In general, green roofs can be effective at reducing stormwater runoff, and their effectiveness is limited by both their areal extent and storage capacity. Our results showed that green roof implementation can be an effective stormwater management tool in highly urban areas, and we demonstrated that our modeling approach can be used to assess the watershed-scale hydrologic impacts of the widespread adoption of green roofs across large metropolitan areas.

Keywords: Green roofs; Stormwater; Urban watershed modeling; VELMA.

MeSH terms

Conservation of Natural Resources
Ecosystem
Hydrology*
Rain
Washington
Water Movements*

Grants and funding

EPA999999/ImEPA/Intramural EPA/United States