Green roof vegetation management alters potential for water quality and temperature mitigation

Abstract

The global increase of urban impervious land cover poses a significant threat to the integrity of river ecosystems. Hence, it is critical to assess the efficiency of green roofs (GR) to mitigate the negative impacts of urbanization on river ecosystems, such as thermal surges and pollutants. In this study, we evaluated the ecohydrological behaviour of two fully established GR under differing management regimes at the Chicago Botanical Gardens from July to September 2019. The drainage outflow from a non‐vegetated roof, a managed GR (perennial native and non‐native plants) and an unmanaged GR (perennial natural prairie vegetation) were monitored, and thermal dynamics, dissolved organic matter (DOM) composition and nitrate concentration assessed. The managed GR runoff had a lower DOC concentration and less humic‐like DOM signal (SUVA254) compared to the unmanaged GR. In contrast, lower concentrations of nitrate and more recalcitrant DOM (less protein‐like compounds relative to humic‐like compounds) were associated with the unmanaged GR. The unmanaged GR also displayed a greater capacity to reduce thermal surges associated with storm events. Our study provides new information on the implications of GR management for water quality with particular relevance to the urban stream syndrome. Further, the impacts of GR management on the mitigation of thermal surges and DOM composition can help to improve future GR design, as these ecohydrological responses have been largely overlooked to date. Our findings can support future urban planning, particularly for scenarios where green infrastructures are used to mitigate the impacts of climate change on urban river ecosystems.

Instrumentation and Monitoring of Nature-Based Solutions for Urban Stormwater Control

“…Soil moisture is normally measured using sensors that detect the volumetric water content of the surrounding soil. Soil moisture sensors are frequently used in green roofs, [19][20] rain gardens, 21 and natural areas. 22 Volumetric water content data can be used to estimate the total stormwater storage provided by NBS, determine the impact of antecedent soil moisture on the response to rain events, and monitor in situ conditions for plant growth.…”

“…In engineered NBS designed with a discharge or overflow pipe, outflow is frequently measured using a flow meter 19,20,30 or a weir and pressure-transducer water level sensor. 28 In addition to outflow from the NBS systems, discharge is often measured within the stormwater drainage system that receives water from the NBS to determine performance in terms of the reduction of stormwater flow.…”

The Autobot-WQ: A portable, low-cost autosampler to provide new insight into urban spatio-temporal water quality dynamics

“…Indeed, the Autobot-WQ were drawing water from roof drainage pipes, where the space was highly restricted, and the flow depths were small at the beginning and end of runoff, and where conventional autosamplers would not have worked for this application due to these constraints. Ouellet et al (2021) provide the full detail of the green roof experiment realized in 2019, and here, we used unpublished results for the same study design. The study was conducted on two semi-intensive GR, adjacent to a non-vegetated roof from July to September 2019 and located at The Daniel F. and Ada L. Rice Plant Conservation Science Center at the Chicago Botanic Garden, Illinois, United States (42.14 °N, −87.78 °E).…”

“…River pollution is a remarkably complex issue with multiple sources contributing across variable time scales, meaning that a high spatial density of sampling is required to facilitate understanding. Urban areas typically display a higher number of pollution sources and can have strong negative water quality impacts (Walsh et al, 2005;Booth et al, 2016;Ouellet et al, 2021). Urbanization is typically associated with increased pressure on urban streams as physical habitats are modified (e.g., channelization) and water quality impaired (e.g., by road runoff and effluent inputs) (Walsh et al, 2005;Palmer et al, 2010;Booth et al, 2016).…”

Unmanaged Vegetated Roofs Hydrological Performance in Subtropical Areas:An Investigation in São Paulo, Brazil

“…In the two VRs observed by Thuring and Dunnett (2019), less than half the original planted species persisted over time, but the entirety of the green cover was maintained by colonizing species. (Ouellet et al, 2021) compared managed and unmanaged VR and observed that the unmanaged had a greater capacity in reducing thermal surges associated with storm events. Not only that, but Schrieke and Farrell (2021) demonstrated spontaneous VRs can reduce construction costs, applying less pressure on property value.…”