Low Impact Development Controls Research Articles

Urban Roadside Flooding Analysis Using SWMM: A Case Study of a Road Section in Pokhara Metropolitan City, Nepal

Pokhara City, in Nepal, experiences heavy rainfall during four months of monsoon, causing stress on drainage infrastructures. This situation is exacerbated by poor drainage systems, ineffective urban planning, and rapid urbanization. This study aims to develop mitigation strategies for urban roadside flooding in the Barahi Chowk region in Pokhara City. The focus is to identify the causes and potential solutions for the recurring issue of urban roadside flooding. For this purpose, the study utilizes Storm Water Management Model (SWMM) computer modeling to analyze the hydraulic capacity of existing drainage systems during peak flows. The SWMM modeling reveals that the current drainage usage is between 80% and 100% during peak times, leading to flooding. Consequently, resizing or expanding various drainage sections using SWMM modeling is suggested. The study introduces Low Impact Development (LID) controls, such as rain gardens and permeable pavements, to manage surface runoff. Implementing rain gardens on just 3% of the impervious area in each sub-catchment showed an average 21.5% reduction in peak runoff and an average 6.68% reduction in the total runoff while implementing 5% in each sub-catchment, permeable pavements reduced peak runoff by 22-26% and total runoff by 8-11%. The research explores the impact of land use and land cover change and unplanned urban growth on roadside flooding, resulting in impermeable urban surfaces that disturb the natural drainage system and infiltration pattern. Thus, the outcomes of this study should be carefully considered by policymakers and stakeholders to address issues through sustainable urban planning and infrastructure development. Also, implementing measures like resizing drains and LID controls appears to be effective in controlling flooding during the study period and should be considered for future studies as well.

Improving water quality modelling for green roof runoff using Storm Water Management Model

Modelling of green roof is important for assessing its effectiveness as a low impact development (LID) practice. This study evaluated the performance of Storm Water Management Model (SWMM)’s LID Controls for modelling runoff water quality load from a green roof in Portland, Oregon, USA, for possible improvement. Such an attempt has been made rarely, although plenty of studies have evaluated SWMM's LID Controls for predicting green roof runoff. From the evaluation, the applicability of the SWMM for evaluating water quality such as total phosphorous (TP) and total nitrogen (TN) was “not satisfactory”. Nash-Sutcliffe Efficiency (NSE) value was 0.27 for TN and −1.45 for TP when using “rain concentration”, the only parameter available for water quality simulation of green roof runoff. The applicability was improved to “very good” level for TP with an NSE value of 0.66 when buildup and washoff were made possible by creating an imaginary twin subcatchment of the same size as the original one. However, NSE value for TN was not improved. The value of the work is that it could lead to a more accurate green roof design by minimizing pollutant release with improved prediction technology for TP.

Low-impact development (LID) control feasibility in a small-scale urban catchment for altered climate change scenarios

ABSTRACT Rainfall is considered a major input in designing stormwater management measures, especially for any low-impact development (LID) control design. With the impact of climate change, rainfall frequency and its patterns are changing continuously. Quantification of these changes and their impact on the performance of LID design becomes crucial. This paper presents a methodology to quantify the change in rainfall patterns using the Coupled Model Intercomparison Project 5 (CMIP5) climate model and to select the most feasible LID for a catchment with haphazard development. Interconnected decentralization-based LID controls are evaluated with the objective of emulating a pre-urbanized scenario. The overall analyses indicated that green roof (GR) followed by infiltration trenches (IT), rooftop disconnection (RTD), and permeable pavement (PP) showed better performance. Furthermore, a combination of IT, PP, and RTD accomplishes better efficiency for extreme rainfall events. Implementation of the most feasible combination will provide the additional benefit of water recycle and reuse.

Adaptation of SWAT Watershed Model for Stormwater Management in Urban Catchments: Case Study in Austin, Texas

Computer simulation models are a useful tool in planning, enabling reliable yet affordable what-if scenario analysis. Many simulation models have been proposed and used for urban planning and management. Still, there are a few modeling options available for the purpose of evaluating the effects of various stormwater control measures (SCM), including LID (low-impact development) controls (green roof, rain garden, porous pavement, rainwater harvesting), upland off-line controls (sedimentation, filtration, retention–irrigation) and online controls (detention, wet pond). We explored the utility and potential of the Soil and Water Assessment Tool (SWAT) as a modeling tool for urban stormwater planning and management. This study demonstrates how the hydrologic modeling strategies of SWAT and recent enhancements could help to develop efficient measures for solving urban stormwater issues. The case studies presented in this paper focus on urban watersheds in the City of Austin (COA), TX, where rapid urbanization and population growth have put pressure on the urban stormwater system. Using the enhanced SWAT, COA developed a framework to assess the impacts on erosion, flooding, and aquatic life due to changes in runoff characteristics associated with land use changes. Five catchments in Austin were modeled to test the validity of the SWAT enhancements and the analytical framework. These case studies demonstrate the efficacy of using SWAT and the COA framework to evaluate the impacts of changes in hydrology and the effects of different regulatory schemes.

Biovalley analysis as a LID control to delay surface runoff in the city of Recife/PE

Urban growth brought with it the formation of large impermeable areas in cities, causing a reduction in the infiltration of rainwater into the soil and an increase in the volume of surface runoff, causing setbacks on days of extreme precipitation events. In this context, the present research aimed to analyze the use of a biotaver as a Low Impact Development (LID) for delaying surface runoff in areas prone to flooding in the city of Recife/PE. As a methodology, the QGIS model was used to extract data from raster images, and the Storm Water Management Model (SWMM) for scenario simulation. Three scenarios were simulated with rainfall of 20.00mm, 39.00mm and 60.00mm. The results showed that the implementation of the bioditches provided a reduction in the total flow of the study area by 0.50% and in the maximum flow of the SB1 sub-basin for scenarios 2 and 3 by 77.96% and 41.23% respectively. In addition, there was a reduction in surface runoff of 11.15% on average and an increase in green area of 4.00%. The bioretention system showed infiltration ranging from 29.83mm to 89.35mm between the three scenarios. Thus, through the SWMM model, it was possible to analyze that the implantation of bioditches as a control for delaying surface runoff and increasing infiltration into the soil is valid, and the literature recommended that various types of sustainable devices be implemented and integrated into the existing drainage network and urban space so that more expressive results are possible.

Analysis of the effects of low impact development practices on hydrological components using HSPF

The purposes of this study are to propose the new approach for modeling the effectiveness of low impact development (LID) practices using Hydrological Simulation Program-Fortran (HSPF)'s Surface-FTABLE (function table) and to evaluate the impacts of LID application on hydrological components and water balance. LID was simulated using Surface-FTABLE, and changes in hydrological components and water balance were analyzed. These results were compared with results simulating LID using the HSPF LID Controls Tool built in the HSPF model. Embedded within the HSPF model, the HSPF LID Controls Tool is used to design and simulate infiltration-based best management practices. Surface runoff decreased similarly for both methods using Surface-FTABLE and LID Controls Tool. For Surface-FTABLE, the infiltration in the facility was reflected in the model, so interflow, outflow and baseflow outflow increased. As a result of the water balance analysis, the results of Surface-FTABLE showed a similar bias to those of the model without LID. In contrast, the results of the LID Controls Tool showed a large bias due to uninvolved infiltration. This study showed that HSPF Surface-FTABLE is applicable to LID simulation and that it is possible to simulate the change of each element of hydrologic components reasonably.

Assessing the Flow Reduction of Low Impact Development Controls in a Residential Setting

The widespread development in recent years has resulted in the expansion of impervious surfaces, allowing runoff to accumulate and cause flooding issues. To reduce the impacts of runoff accumulation, the concept of low impact development (LID) has been adopted to restore the hydrological balance of urbanized areas. The objective of this study was to assess the flow reduction of the LID combinations in a residential park in Cavite, Philippines using Stormwater Management Model (SWMM). The 90<sup>th</sup> percentile of historical rainfall from the years 1975-2019 was used in the assessment. The bioretention (BR), infiltration trench (IT), and permeable pavement (PP) LID controls, which captured 43%, 42%, and 14% of the park area respectively, were selected for implementation alongside their different combinations. Results of this study have shown that most LID scenarios reached their maximum reduction capability once it takes up around 20-40% of the total available area. The highest reduction, at around 90%, was attained by the BR+IT+PP scenario. The use of the PP scenario, however, only yielded a 20% flow reduction due to its small capture area. Implementing LID controls in urbanized locations can help in reducing runoff buildup, thereby alleviating the potential impacts of flooding.

Comparative performance and cost-integrated life cycle assessment of low impact development controls for sustainable stormwater management

Climate change and urbanization have led to an increase in the frequency and intensity of floods in urban regions. To cope with such extreme events, low impact development (LID) has been adopted as a sustainable alternative to conventional stormwater management systems. LID controls mitigate runoff and manage flooding at the source by providing additional storage, filtration, and infiltration of stormwater. This study aims to investigate the techno-economic and environmental performance of multiple LID controls: bioretention cells (BC), infiltration trenches (IT), and permeable interlocking concrete pavement (PICP) compared to a conventional stormwater pipe drainage system. A cost-integrated life cycle analysis was conducted to assess the environmental impacts and evaluate the financial feasibility of the examined stormwater management systems. An eco-efficiency analysis (EEA) was carried out to quantify the environmental costs of LID controls by incorporating the results of a life cycle assessment (LCA) and life cycle cost analysis (LCCA). The technical analysis showed that LID controls significantly reduced runoff by 67%, 53%, and 36% for IT, BC, and PICP, respectively compared to the conventional system. Moreover, the LCA results revealed that the pipe drainage system was the least favorable with a single score of 191 mPt, followed by PICP (188 mPt), BC (40 mPt), and IT (30 mPt). Over a 30-year assessment period, the LCCA findings indicated that the IT and BC controls had the lowest net present costs of $11.6 and $14.5 million, respectively, compared to the PICP ($17.0 million) and conventional ($19.5 million) systems. The EEA revealed that IT was the most eco-efficient (EI = 0.34 Pt/$) among the LID systems, whereas PICP was the least cost-efficient (EI = 0.97 Pt/$). It was also found that applying hybrid LID models further reduced the runoff, environmental impacts, and financial burdens by up to 79%, 84%, and 48%, respectively. Overall, these findings emphasized the potential environmental and financial benefits of implementing integrated LID controls as sustainable stormwater management systems.

Comparison of the Engineering Strategies for Low Impact Development in a Densely Populated Old Urban Area

Most old urban areas of China have a dense population, severely indurated underlying surface, and highly developed underground space. Those increase the waterlogging risk and obstruct the stormwater management in old urban areas. To propose an appropriate engineering strategy for low impact development (LID) transformation in an urban area, a simulation was carried out by storm water management model (SWMM) in this project. Bioretention cells, permeable pavements, and green roofs were selected according to the study area surface. Runoff control performance of single LID control and combined schemes were compared. Results illustrate that only 50.21% of roofs can build green roofs in urban areas with dense populations, and the runoff control performance of green roofs is unsatisfactory, while bioretention cells and permeable pavements can effectively mitigate runoff caused by storms with a recurrence period less than 10 years, and combined LID controls can obtain better runoff control performance with less construction area. Those outcomes screened out the LID controls suitable for application in densely populated old urban areas and put forward reasonable engineering practice strategies. This study provides guidance and reference for the LID transformation in the densely populated old urban area.

Application of soil-based low-impact development system for Flash Flood management of Jeddah, Saudi Arabia

Jeddah is the most important port city in the Kingdom of Saudi Arabia. The climate of Jeddah is mostly dry; however, events of intensive rainfall followed by urban flooding have been reported in recent history. These floods interrupt the logistics and lifelines of the city. In this study runoff analysis and Flash Flood management by the green roof low impact development (LID) control system, was performed using United States Environmental Protection Agency (EPA)’s Stormwater Management Model (SWMM). Six middle districts of Jeddah city had been selected and modeled in five sub-catchment areas. The local soil and geographic conditions were incorporated into the model. The effect of the soil-based, rooftop, LID system has been emphasized in the analysis. In flood simulations, 30–70% of the impervious area of each sub-catchment has been treated with LID. It has been observed that the LID application can significantly reduce the overall flood flow. Thus, simple but well-planned LID control systems can be effectively utilized for the flood management of heavily urbanized regions.

An efficient simulation-optimization approach based on genetic algorithms and hydrologic modeling to assist in identifying optimal low impact development designs

High rates of soil imperviousness, intensified by urbanization, have been contributing strongly to the occurrence of floods all over the world. To mitigate these impacts, Low Impact Development (LID) techniques seek to preserve the hydrology of urban catchments closer to pre-development conditions by using distributed stormwater control systems. Nevertheless, the application of these techniques is associated with a variety of challenges, including the design of the LID controls, due to the significant number of variables involved and the need to attend to multiple objectives simultaneously. In this context, the application of hydrologic simulation models integrated with optimization techniques has been recently explored as an alternative to assist in planning LID scenarios. This work aims to verify the applicability of an adaptation of the Genetic Algorithm NSGA-II, together with the hydrologic model SWMM, to assist the optimal design of a LID scenario seeking to reduce the stormwater runoff and the total costs on different return periods. This scenario has considered the combined implementation of permeable pavements, green roofs and bioretention cells. The results showed that the model was capable of finding a great variety of optimal solutions on various levels of runoff reduction, at different costs, for all return periods considered. Regarding the applicability of the optimization model as a LID design method, some limitations were found related to practical applications and possible oversizing of the subjacent layers of the LIDs. Therefore, suggestions on how to improve the model have been made to solve the identified problems.

Sensitivity analysis of permeable pavement hydrological modelling in the Storm Water Management Model

The Storm Water Management Model (SWMM), widely used by engineers to design or analyse stormwater networks, allows to model the so-called Low Impact Development (LID) controls, which reduce the flow conveyed to traditional networks. But, values for LID control parameters are often unknown. Furthermore, it is not always easy to link the cross-section materials to those provided by the model, particularly in the soil layer. This article provides a global sensitivity analysis for the PP type of LID control, in order to support practitioners in calibration tasks. The analysis explores what factors are the most influential and which can be fixed while calibrating a model. In particular, flow volume and peak are studied but the analysis also explores the influence of storm length and drain layer, which is optional. At the end, the most influential parameters, and those that can be neglected are presented, showing that we can focus on quite less parameters than initially given when calibrating a PP model in SWMM.

Modeling the impact of development policies and climate on suburban watershed hydrology near Portland, Oregon

We simulated watershed hydrologic response to land use scenarios representing different sets of policies for suburban development and watershed management in an urbanizing watershed near Portland, Oregon. We asked (1) Can watershed-scale planning and development policies reduce the hydrologic impacts of urbanization? (2) How effective are different policies (including “green” neighborhood design, riparian protection, and representative Low Impact Development (LID) stormwater controls) in maintaining the pre-development hydrologic regime? and (3) Does climate change influence these results? We co-developed 50-year land and water management scenarios with a stakeholder advisory committee and used a spatially-explicit natural and human systems model to evaluate changes in water balance and streamflow characteristics under each scenario. We also used a factorial analysis to explore the influence of individual scenario elements on watershed hydrology. Results indicate that the biggest driver of change for this watershed was the spatial extent of development, followed by the nature of the development pattern, climate, forest harvest, and use of LID controls. Stormwater runoff increased as developed area increased, as did stream discharge for both periods of high and low flow. Increases in evapotranspiration (ET) with climate warming were offset by reductions in ET with development. Designs that condensed development and preserved upland and riparian forest showed the greatest potential for maintaining pre-development water balance and streamflow regimes, thereby reducing impacts of urbanization. These results bridge spatial and temporal scales important to decision-makers, highlighting the importance of considering impacts over time of neighborhood-scale development decisions on watershed-scale hydrology.

Optimal implementation of low impact development for urban stormwater quantity and quality control using multi-objective optimization.

Stormwater runoff is a major concern in urban areas which is mostly the result of vast urbanization. To reduce urban stormwater runoff and improve water quality, low impact development (LID) is used in urban areas. Therefore, it is vital to find the optimal combination of LID controls to achieve maximum reduction in both stormwater runoff and pollutants with optimal cost. In this study, a simulation-optimization model was developed by linking the EPA Storm Water Management Model (SWMM) to the Multi-Objective Particle Swarm Optimization (MOPSO) using MATLAB. The coupled model could carry out multi-objective optimization (MOO) and find potential solutions to the optimization objectives using the SWMM simulation model outputs. The SWMM model was developed using data from the BUNUS catchment in Kuala Lumpur, Malaysia. The total suspended solids (TSS) and total nitrogen (TN) were selected as pollutants to be used in the simulation model. Vegetated swale and rain garden were selected as LID controls for the study area. The LID controls were assigned to the model using the catchment characteristics. The target objectives were to minimize peak stormwater runoff, TSS, and TN with the minimum number of LID controls applications. The LID combination scenarios were also tested in SWMM to identify the best LID types and combination to achieve maximum reduction in both peak runoff and pollutants. This study found that the peak runoff, TSS, and TN were reduced by 13%, 38%, and 24%, respectively. The optimal number of LID controls that could be used at the BUNUS catchment area was also found to be 25.

Modeling a Bioretention Basin and Vegetated Swale with a Trapezoidal Cross Section using SWMM LID Controls

The Low Impact Development (LID) Control module is utilized in the United States Environmental Protection Agency’s Stormwater Management Model (USEPA SWMM) to predict the hydraulic performance of a variety of sustainable stormwater technologies. Data collected in 2019 from the monitoring of a pilot project in Montreal was used to verify the ability of the Bioretention LID Control (which assumes a rectangular cross-section) to accurately simulate outflow from a structure with a trapezoidal cross-section. Two types of LID facility were modeled: one releases captured inflow through a perforated underdrain below the soil layer (bioretention basin; BB); and the other is drained at the surface of the soil layer (vegetated swale; VS). Initially, the modeled LID structures were sized identically to the field surface areas. However, it was necessary to change their model representation to account for the non-rectangular shape of the soil layer. In addition, a sensitivity analysis was completed, and the most influential parameters were identified as the conductivity slope and seepage rate. Both the alteration of the LID structure representation and the parametric calibration greatly improved the simulated outflows from the vegetated swale resulting in an increase of the Nash–Sutcliffe efficiency (NSE) coefficient from −0.6 to 0.64 (NSE >0.5 is acceptable for hydrologic models according to the literature). The bioretention basin calibration did not prove as successful. The evaluated LID Control module presented better predictive capabilities for the basin with a simpler overall design (VS).

The Effect of Sponge City Construction for Reducing Directly Connected Impervious Areas on Hydrological Responses at the Urban Catchment Scale

Low-impact development (LID) has been widely used at both site-specific and local scales to try and mitigate the impact of urban stormwater runoff caused by increasing impervious urban areas. Recently, the concept of a “sponge city” was proposed by the Chinese government, which includes LID controls at the source, a pipe drainage system midway, and a drainage system for excess stormwater at the terminal. There is a need to evaluate the effectiveness of sponge city construction at the large urban catchment scale, particularly with different spatial distributions of LIDs that reduce directly connected impervious areas (DCIAs). In this paper, the performances of five design scenarios with different spatial distributions but same sizes of LID controls at the urban catchment scale were analyzed using a geographic information system (GIS) of the United States Environmental Systems Research Institute (ESRI)—based Storm Water Management Model (SWMM) of the United States Environmental Protection Agency (USEPA) and MIKE 11 of Danish Hydraulic Institute (DHI) in Xining City, China. Results confirmed the effectiveness of sponge city construction in reducing the urban stormwater runoff. The hydrological performance reduction was positively correlated and linearly dependent on DCIA reduction. Peak flow reduction was most sensitive to DCIA reduction, followed by runoff volume and peak time. As rainfall intensity increased, the hydrological performance was more sensitive to rainfall intensity than DCIA reduction. Results of this study provide new insights for stormwater managers to implement LID more effectively at the urban catchment scale.

Hydrological modelling of green and grey roofs in cold climate with the SWMM model

Rooftop retrofitting targets the largest land-use type available for reduction in impervious surfaces area in urban areas. Extensive green and grey roofs offer solution for retention and detention of stormwater in densely developed urban areas. Among the available green roof types, the extensive green roof has become a popular selection and commonly adopted choice. These solutions provide multiple benefits for stormwater and environmental management due to stormwater retention and detention capacities. The Storm Water Management Model (SWMM) 5.1.012 with Low Impact Development (LID) Controls was used to model the hydrological performance of a green and a grey (non-vegetated detention roof based on extruded lightweight aggregates) roof (located in the coastal area of Trondheim, Norway) by defining the physical parameters of individual layers in LID Control editor. High-resolution 1-min data from a previously monitored green and grey roof were used for calibration. Six parameters within the individual LID layers: soil (four parameters) and drainage mat (two parameters) were selected for calibration. After calibration, the SWMM model simulated runoff with a Nash-Sutcliffe model efficiency (NSME) of 0.94 (green roof) and 0.78 (grey roof) and a volume error of 3% for the green roof, and 10% for the grey roof. Validation of the calibrated model indicates good fit between observed and simulated runoff with a NSME of 0.88 (green roof) and 0.81 (grey roof) and with volume errors of 29% (green roof) and 11% (grey roof). Concerning the snowmelt modelling, the calibrated model showed a NSME of 0.56 (green roof) and 0.37 (grey roof) through the winter period. However, regarding volume errors, additional model development for winter conditions is needed; 30% (green roof) and 11% (grey roof). Optimal parameter sets were proposed within both the green and grey configurations. The results from calibration and especially validation indicated that SWMM could be used to simulate the performance of different rooftop solutions. The study provides insight for urban planners of how to target and focus the implementation of rooftop solutions as stormwater measures.

Considering the effect of groundwater on bioretention using the Storm Water Management Model

The Storm Water Management Model (SWMM), with its recently released low impact development (LID) module, is among several models used for the performance evaluation of LID facilities in reducing runoff and pollutants. Modeling is often difficult because of the variety of factors affecting the LID system. Among these factors, the effect of groundwater can be important in the LID modeling results due to the possibility of its interaction with LID. In this study, the performance of the SWMM-LID controls in predicting runoff from bioretention cells was evaluated for a site under groundwater influence. In addition, for considering the groundwater effect in the model, this study explores the utility of the SWMM groundwater model in predicting runoff under groundwater influence. Runoff from the considered watershed draining into the bioretention cells was well-simulated with very favorable performance statistic values (r2 = 0.96, NSE = 0.94, % difference = 2.76). However, comparison of simulated with observed runoff from bioretention cells produced weaker statistical values (r2 = 0.69, NSE = 0.65, % difference = 18.22), which is thought to be due to the presence of events affected by groundwater interference. Removal of these events and recalibration were able to improve the overall results, suggesting that the influence of groundwater should be taken into account for better LID modeling of the study site. In order to consider the groundwater influence, the SWMM groundwater model was used in tandem with LID controls to provide an additional influent source to bioretention cells. This resulted in a good fit for two events which were thought to be impacted by groundwater (events in which outflow exceeded inflow) and overall better performance (r2 = 0.95, NSE = 0.95, % difference = 3.49) compared to the results obtained by using only LID controls. In conclusion, the SWMM groundwater model can help deal with groundwater-impacted events. However, for better representation of the phenomenon, the LID module itself needs to be improved to account for direct interaction with groundwater.

Multiobjective optimization of low impact development stormwater controls

Green infrastructure such as Low Impact Development (LID) controls are being employed to manage the urban stormwater and restore the predevelopment hydrological conditions besides improving the stormwater runoff water quality. Since runoff generation and infiltration processes are nonlinear, there is a need for identifying optimal combination of LID controls. A coupled optimization-simulation model was developed by linking the U.S. EPA Stormwater Management Model (SWMM) to the Borg Multiobjective Evolutionary Algorithm (Borg MOEA). The coupled model is capable of performing multiobjective optimization which uses SWMM simulations as a tool to evaluate potential solutions to the optimization problem. The optimization-simulation tool was used to evaluate low impact development (LID) stormwater controls. A SWMM model was developed, calibrated, and validated for a sewershed in Windsor, Ontario and LID stormwater controls were tested for three different return periods. LID implementation strategies were optimized using the optimization-simulation model for five different implementation scenarios for each of the three storm events with the objectives of minimizing peak flow in the stormsewers, reducing total runoff, and minimizing cost. For the sewershed in Windsor, Ontario, the peak run off and total volume of the runoff were found to reduce by 13% and 29%, respectively.

Assessing the significance of evapotranspiration in green roof modeling by SWMM

Abstract Green roof is a low impact development (LID) practice used to mitigate imperviousness in urban areas and to reduce flood risks. In order to have sufficient designs and accurate runoff predictions, computer models should be utilized with full understanding of green roofs' hydrologic processes. Evapotranspiration is usually considered important by researchers in the water balance modeling of a green roof. The Storm Water Management Model (SWMM) version 5.1 is widely utilized rainfall-runoff modeling software which has LID controls capable of modeling green roofs. A previous study has evaluated the performance of this model in green roof simulations for single events without considering evapotranspiration in its application, but attained negative outcomes. Thus, the objective of this study is to determine the significance of considering evapotranspiration in producing accurate runoff simulations specifically using SWMM 5.1. The results of this study have shown that when evapotranspiration was not considered, simulations failed to agree with observed values, whereas when evapotranspiration was considered, simulated runoff volumes attained a very good fit with the observed runoff volumes proving the significance of evapotranspiration as an important parameter in green roof modeling.