Robust Vegetation Parameterization for Green Roofs in the EPA Stormwater Management Model (SWMM)

Abstract

In increasingly expanding cities, roofs are still largely unused areas to counteract the negative impacts of urbanization on the water balance and to reduce flooding. To estimate the effect of green roofs as a sustainable low impact development (LID) technique on the building scale, different approaches to predict the runoff are carried out. In hydrological modelling, representing vegetation feedback on evapotranspiration (ET) is still considered challenging. In this research article, the focus is on improving the representation of the coupled soil–vegetation system of green roofs. Relevant data to calibrate and validate model representations were obtained from an existing field campaign comprising several green roof test plots with different characteristics. A coupled model, utilizing both the Penman–Monteith equation to estimate ET and the software EPA stormwater management model (SWMM) to calculate the runoff, was set up. Through the application of an automatic calibration procedure, we demonstrate that this coupled modelling approach (Kling–Gupta efficiency KGE = 0.88) outperforms the standard ET representation in EPA SWMM (KGE = −0.35), whilst providing a consistent and robust parameter set across all green roof configurations. Moreover, through a global sensitivity analysis, the impact of changes in model parameters was quantified in order to aid modelers in simplifying their parameterization of EPA SWMM. Finally, an improved model using the Penman–Monteith equation and various recommendations are presented.

Unprecedented Retention Capabilities of Extensive Green Roofs—New Design Approaches and an Open-Source Model

“…For instance, green roofs reduce runoff and hence urban flooding due to their storage capacity. Evapotranspiration is also increased at the expense of runoff (Iffland et al, 2021), which better matches the characteristics of the natural water cycle. Moreover, green infrastructure (GI) in general, and green roofs in particular, provide important ecosystem services (Bolliger and Silbernagel, 2020), which demonstrates their value beyond storm water management.…”

“…Since the runoff coefficient C s becomes smaller for longer flow lengths, the measurement design is safe even for longer flow lengths, since it tends to be oversized. In contrast, the measurements of annual water retention of natural precipitation are taken over a period of at least 4 years (Iffland et al, 2021). The tests for this purpose are carried out on test plots of 2 m × 2 m with a 2% slope, located outdoors and exposed to natural weather conditions.…”

“…SWMM is a conceptual rainfall-runoff model with hydrodynamic flow routing in sewer networks, designed for urban hydrology, which was complemented by GI/LID modeling capabilities. Several studies addressed the model component to model GI and more specifically green roofs (Burszta-Adamiak and Mrowiec, 2013;Cipolla et al, 2016;Krebs et al, 2016;Carson et al, 2017;Niazi et al, 2017;Peng and Stovin, 2017;Leimgruber et al, 2018;Limos et al, 2018;Liu and Chui, 2019;Iffland et al, 2021). For single green roofs, detailed physically-based models have been set up, utilizing models of the Hydrus family with different dimensions, ranging from 1D vertical modeling of the unsaturated zone (Hilten et al, 2008;Palla et al, 2008;Qin et al, 2016;Palermo et al, 2019;Mobilia and Longobardi, 2020) and two-dimensional cross-sections (Li and Babcock, 2015) up to three-dimensional representations of the green roof (Brunetti et al, 2016).…”

Quality Assessment of Small Urban Catchments Stormwater Models: A New Approach Using Old Metrics

“…In urban catchments, important rainfall events last a few hours or even minutes, and therefore the time step of measurement records is usually only of a few minutes. The urban drainage literature reports NSE values from the calibration of hydrologic or hydraulic models, normally between 0.5 and 0.9 [2], most of them being greater than 0.7 [7,10,20,22,[46][47][48][50][51][52][53]. However, in some cases, NSE values above 0.95 are reported [1,53,55], while in other specific cases, usually for water quality parameters, very low values, close to zero, are considered as acceptable [9].…”

“…Obviously, in urban drainage, graphical and statistical techniques have always been applied to model calibration and evaluation, although the generalized use of aggregated efficiency metrics, such as the Nash-Sutcliffe coefficient and the Kling-Gupta coefficient [37], is more recent (e.g., [46][47][48][49][50][51][52][53][54][55]).…”