Green Roof Test Research Articles

Evapotranspiration Measurement and Estimation of Crop Coefficient for Native Plant Species of Green Roof in the Tropics

Extensive green roof is one of the sustainable urban stormwater management alternatives to manage and mitigate the urban surface runoff. In order to implement green roofs more effectively, suitable plant species and substrate components for tropical climate must be identified. The aim of this study is to investigate the evapotranspiration (ET) behaviors in extensive green roofs based on different substrate types and local native plant species. Four green roof test beds containing pro-mixing pot and burn soils were each vegetated with Axonopus Compressus (grass) and Portulaca Grandiflora (sedum). A weather station with soil moisture sensors was installed to measure the weather and soil moisture data. The results showed that the mean ET rates for grass-pot soil, sedum-pot soil, grass-burn soil and sedum-burn soil were 1.32 ± 0.41 mm/day, 2.31 ± 0.72 mm/day, 1.47 ± 0.39 mm/day and 2.31 ± 0.43 mm/day, respectively. It is noted that environmental parameters such as ambient temperature, solar radiation and wind speed showed significantly positive relationship (p value < 0.01) with ET rates of green roofs except relative humidity. The crop coefficients (Ks) for the studied green roof plant species are estimated based on actual and reference evapotranspiration rates. The sedum planted in burn soil showed the highest crop coefficient (0.64), followed by sedum in pot soil (0.62), grass in burn soil (0.39) and grass in pot soils (0.37), respectively. The findings in this study also showed that substrate with better water retention capacity generally improved the Ks values.

Influence of Temperature and Moisture Content on Thermal Performance of Green Roof Media

The influence of moisture content on substrate thermal conductivity at different temperatures was investigated for four different commercially available substrates for green roofs. In the unfrozen state, as moisture content increased, thermal conductivity increased linearly. In the phase transition zone between +5 and −10 °C, as temperature decreased, thermal conductivity increased sharply during the transition from water to ice. When the substrate was frozen, thermal conductivity varied exponentially with substrate moisture content prior to freezing. Power functions were found between thermal conductivity and temperature. Two equally sized, green roof test cells were constructed and tested to compare various roof configurations including a bare roof, varying media thickness for a green roof, and vegetation. The results show that compared with the bare roof, there is a 75% reduction in the interior temperature’s amplitude for the green roof with 150 mm thick substrate. When a sedum mat was added, there was a 20% reduction in the amplitude of the inner temperature as compared with the cell without a sedum mat.

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

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.

Water and thermal regime of extensive green roof test beds planted with sedum cuttings and sedum carpets

Implementation of green roofs could help to reduce rapid runoff and help cities to mitigate heat islands. The aim of the study is to assess the water and temperature regimes of four experimental green roof test beds having different growing media and plant coverage during the vegetation season 2018. Experiments were conducted in four test beds (1 × 1 m) established on a flat roof. Two types of growing media were used. The first (A) was a substrate composed of crushed spongolite, crushed expanded clay, and peat. The second (B) was a coarser substrate, composed of crushed expanded clay, crushed bricks, peat, and compost. Two test beds, hereafter designated ACu and BCu, were filled with substrates A and B respectively and planted with a mixture of Sedum spp. cuttings with approximately 10% coverage. The substrate thickness was 6 cm. Two other test beds, designated ACa and BCa, were filled to a depth of 4 cm with A and B growing media, respectively, and planted with a carpet of Sedum spp. with approximate coverage of 100%. The experiment was conducted over one growing season. Continuous monitoring of substrate temperature, water content, and outflow was conducted on each test bed. The lowest runoff coefficient was observed in test bed ACu, while the highest runoff occurred in test bed BCu, with twice the amount of outflow as ACu. The total runoff coefficient of ACa was more than one-third higher than that of ACu. The lowest maximum substrate temperature on the hottest day of the season was observed in bed ACa with a temperature of 40.6 °C, while the highest temperature was seen in bed BCu, 7.9 °C higher. The analysis of the rainfall-runoff relationship calculated for individual rainfall events demonstrated that runoff coefficients depended on initial water content, rainfall intensity, rainfall depth, substrate type, and vegetation cover. Beds planted with sedum carpets and having more extensive vegetation coverage were superior at moderating extremes of temperature.

Hydrological and thermal regime of a thin green roof system evaluated by physically-based model

Green roofs, as an element of the green infrastructure, contribute to the urban heat island effect mitigation and the urban drainage outflow reduction. To achieve the desired functions, it is essential to understand the role of the individual roof layers and ensure their proper design.A physically-based model was used to assess the hydrological and thermal regime of two experimental green roof test beds containing distinct soil substrates (a local Technosol and a more permeable commercial substrate “Optigreen”). The test beds together with a meteorological station were built on the building green roof. Each test bed has an effective area of one square meter and is equipped with a soil temperature sensor and an outflow gauge; one of the test beds is continuously weighed. The observed conditions were simulated using one-dimensional numerical model describing the water flow in variably saturated porous medium by Richards’ equation and the heat transport by the advection-conduction equation.The model was able to satisfactorily reproduce the measured outflow and soil temperature. The water-potential-gradient based root water uptake module effectively captured the water storage depletion between the rainfall events. The difference between the two soil substrates tested is demonstrated by the contrasting ability of the soil layers to retain water. Model representation of the thermal conditions within the green roof soils was achieved using independently evaluated thermal properties of the soils and drainage board. The model was also used to analyze the effects of the substrate depth and type of vegetation cover on the transpiration and soil water regime of the green roofs. Increasing the substrate depth causes a rise of root water uptake and induces a significant reduction of the maximal temperature. The thinner soil profiles are more sensitive to the plant species selection.

The use of simple hydrological models to assess outflow of two green roofs systems

Hydrological response of anthropogenic soil systems, including green roofs, has crucial importance in many fields of water engineering and management. As a consequence, there is an increasing need for modelling of the anthropogenic soil systems behaviour. To obtain empirical data, two green roof test beds were established on a green roof of University Centre for Energy Efficient Buildings, Czech Technical University in Prague. Each test bed is 1 m<sup>2</sup> in area and is instrumented for the runoff monitoring. One test bed was filled with less permeable local soil, the other with highly permeable commercial soil substrate, both were planted with stonecrops. Two simple deterministic lumped models – a nonlinear reservoir model and a linear reservoir cascade model – were used to assess the hydrological response of these green roof systems. The nonlinear reservoir model seems more appropriate for extensive green roof systems than the linear reservoir cascade model because of better description of rapid system reaction typical for thin soil systems. Linear reservoir cascade model frequently failed to mimic internal variability of observed hydrographs. In systems with high potential retention (represented by the test bed with local soil), episodically applied models that consider the same initial retention capacity for all episodes do not allow plausible evaluation of the actual episode-related retention. In such case, simulation model accounting for evapotranspiration between the rainfall events is needed.

Internal fluctuations in green roof substrate moisture content during storm events: Monitored data and model simulations

Understanding how the moisture content in a green roof substrate varies during a storm event is essential for accurately modelling runoff detention. In this paper, a green roof test bed installed with moisture probes at three depths was used to understand how moisture content varies during storms. Detailed studies were conducted on five selected storm events. Physical characterisation tests and field-data based calibrations were performed to acquire the model parameters. Two alternative detention models, based on Reservoir Routing and Richard’s Equation, were validated against the measured green roof runoff and temporary moisture storage data. Once the moisture content exceeds local field capacity, its response at different depths occurs simultaneously during storms, although the recorded data indicate a vertical gradient in the absolute values of local field capacity. Both Reservoir Routing and Richard’s Equation can provide reasonable estimations of the runoff and the vertical moisture content profiles, although Richard’s Equation exhibited stronger vertical water content gradients than were observed in practise. The vertical water content profile is not sensitive to the soil water release curve, although the hydraulic conductivity function influences both the vertical water content profile and runoff rate. The modelled results are highly sensitive to the bottom boundary condition, with a constant suction head boundary condition providing a more suitable option than a free drainage boundary condition or a seepage boundary condition.

Avaliação do comportamento térmico de cobertura e fachada verde em células de testes

Com a busca de alternativas para melhorar o conforto ambiental em ambientes construídos, os sistemas de fachadas e tetos verdes aparecem como uma opção para melhoria, em especial, do conforto térmico. Utilizando-se de um episódio climático representativo no ano de 2016, analisou-se comparativamente o comportamento térmico de uma célula de teste com fachada e teto verde e uma célula de controle. O experimento foi realizado no Centro de Recursos Hídricos e Estudos Ambientais (CRHEA) da Universidade de São Paulo (USP), em Itirapina-SP. Os resultados indicaram que a célula de teste com fachada e teto verde possui um melhor desempenho térmico em comparação à célula de controle.

Effects of slopes on the stormwater attenuation performance in extensive green roof

This study is aimed to determine the effects of slope on stormwater retention and peak flow attenuation performances in extensive green roof in Malaysia. One green roof test bed with dimensions of 2.0 m (length) x 1 m (width) x 0.2 m (height) was constructed in Universiti Tenaga Nasional. The substrate layer was fixed as 130 mm depth and placed on top of the filter fabric and drainage layers. The green roof test bed was planted with Portulaca Grandiflora (sedum species) as the native plant in Malaysia. Different slopes (0 º, 2 º, 5 º and 7 º) of green roof test bed were tested in this study for its stormwater retention and peak flow attenuation performances. The results show that stormwater retention percentages are ranged from 56.9% to 52.3%. The stormwater retention percentages are decreased as slope increased in this study. Flat slope showed the longest observed time of peak flow while the 7 º slope has the shortest observed time of peak flow. This finding suggests that slope has a significant impact to the stormwater retention and peak flow attenuation performances in extensive green roof.

Performance of Blue-Green Roofs in Cold Climates: A Scoping Review

Green and blue-green roofs are emerging as an increasingly popular feature of rooftops, particularly in urban areas. Particular problematic conditions render their usage complex in the Nordic countries. In order to ensure that green roofs are built durable and with the service life expected of them, it is important to know all the relevant factors surrounding their construction and operation. A scoping study was conducted in order to gain an overview on green roof research and available scientific literature. One hundred articles of particular interest for Nordic climates were retrieved and their findings summarized. It is found that the vast majority of green roof research has been conducted on a theoretical basis, or with practical measurements on green roof test beds or isolated components. There is scarcely any literature on the operation of full-scale, building-implemented green roofs, and no articles were found on the building technical performance of aged green roofs. These knowledge gaps indicate a major risk factor in green roof operation, as their performance and integrity over time has not been documented. This despite the fact that green roofs have been implemented and in operation worldwide for decades.

Temporal variations in the potential hydrological performance of extensive green roof systems

Existing literature provides contradictory information about variation in potential green roof hydrological performance over time. This study has evaluated a long-term hydrological monitoring record from a series of extensive green roof test beds to identify long-term evolutions and sub-annual (seasonal) variations in potential hydrological performance. Monitoring of nine differently-configured extensive green roof test beds took place over a period of 6 years in Sheffield, UK.Long-term evolutions and sub-annual trends in maximum potential retention performance were identified through physical monitoring of substrate field capacity over time. An independent evaluation of temporal variations in detention performance was undertaken through the fitting of reservoir-routing model parameters. Aggregation of the resulting retention and detention variations permitted the prediction of extensive green roof hydrological performance in response to a 1-in-30-year 1-h summer design storm for Sheffield, UK, which facilitated the comparison of multi and sub-annual hydrological performance variations.Sub-annual (seasonal) variation was found to be significantly greater than long-term evolution. Potential retention performance increased by up to 12% after 5-years, whilst the maximum sub-annual variation in potential retention was 27%. For vegetated roof configurations, a 4% long-term improvement was observed for detention performance, compared to a maximum 63% sub-annual variation. Consistent long-term reductions in detention performance were observed in unvegetated roof configurations, with a non-standard expanded-clay substrate experiencing a 45% reduction in peak attenuation over 5-years. Conventional roof configurations exhibit stable long-term hydrological performance, but are nonetheless subject to sub-annual variation.

Independent Validation of the SWMM Green Roof Module

Green roofs are a popular Sustainable Drainage Systems (SuDS) technology. They provide multiple benefits, amongst which the retention of rainfall and detention of runoff are of particular interest to stormwater engineers. The hydrological performance of green roofs has been represented in various models, including the Storm Water Management Model (SWMM). The latest version of SWMM includes a new LID green roof module, which makes it possible to model the hydrological performance of a green roof by directly defining the physical parameters of a green roof’s three layers. However, to date, no study has validated the capability of this module for representing the hydrological performance of an extensive green roof in response to actual rainfall events. In this study, data from a previously-monitored extensive green roof test bed has been utilised to validate the SWMM green roof module for both long-term (173 events over a year) and short-term (per-event) simulations. With only 0.357% difference between measured and modelled annual retention, the uncalibrated model provided good estimates of total annual retention, but the modelled runoff depths deviated significantly from the measured data at certain times (particularly during summer) in the year. Retention results improved (with the difference between modelled and measured annual retention decreasing to 0.169% and the Nash-Sutcliffe Model Efficiency (NSME) coefficient for per-event rainfall depth reaching 0.948) when reductions in actual evapotranspiration due to reduced substrate moisture availability during prolonged dry conditions were used to provide revised estimates of monthly ET. However, this aspect of the model’s performance is ultimately limited by the failure to account for the influence of substrate moisture on actual ET rates. With significant differences existing between measured and simulated runoff and NSME coefficients of below 0.5, the uncalibrated model failed to provide reasonable predictions of the green roof’s detention performance, although this was significantly improved through calibration. To precisely model the hydrological behaviour of an extensive green roof with a plastic board drainage layer, some of the modelling structures in SWMM green roof module require further refinement.

A hygrothermal green roof model to simulate moisture and energy performance of building components

Green roofs enjoy a good reputation concerning comfort in summertime, energy savings, rain water retention and durability. However, applied on moisture sensitive substrates like timber constructions, their high thermal inertia and limited drying potential may lead to failure, especially in case of insufficient design or workmanship. As the conditions below the greenery cannot be evaluated by dew point calculations, only a more detailed simulation allows for a realistic prediction of the hygrothermal performance of the components. This contribution introduces a new hygrothermal green roof model based on field and laboratory tests. In difference to previous models it focuses on the moisture and temperature conditions beneath the greenery using more simple approaches for the plant cover but more detailed approaches for the moisture balance in the drainage and growth media. Therefore material data for moisture storage, vapor diffusion and liquid transport were determined in the laboratory and adapted to the built in situation by the help of additional field tests. The model is validated by the help of measured temperature conditions beneath the green roof test fields, as well as concerning the moisture conditions in the critical layers of sensitive timber roofs! The validation shows that the performance of the green roof can be well reproduced throughout the whole year and under all climate conditions including rain water absorption and storage, evaporation and freezing influences. Thus it can be used for both, moisture and energy performance simulations of roofs with greenery under different climatic conditions.

The impact of green roof ageing on substrate characteristics and hydrological performance

Green roofs contribute to stormwater management through the retention of rainfall and the detention of runoff. However, there is very limited knowledge concerning the evolution of green roof hydrological performance with system age. This study presents a non-invasive technique which allows for repeatable determination of key substrate characteristics over time, and evaluates the impact of observed substrate changes on hydrological performance.The physical properties of 12 green roof substrate cores have been evaluated using non-invasive X-ray microtomography (XMT) imaging. The cores comprised three replicates of two contrasting substrate types at two different ages: unused virgin samples; and 5-year-old samples from existing green roof test beds. Whilst significant structural differences (density, pore and particle sizes, tortuosity) between virgin and aged samples of a crushed brick substrate were observed, these differences did not significantly affect hydrological characteristics (maximum water holding capacity and saturated hydraulic conductivity). A contrasting substrate based upon a light expanded clay aggregate experienced increases in the number of fine particles and pores over time, which led to increases in maximum water holding capacity of 7%. In both substrates, the saturated hydraulic conductivity estimated from the XMT images was lower in aged compared with virgin samples. Comparisons between physically-derived and XMT-derived substrate hydrological properties showed that similar values and trends in the data were identified, confirming the suitability of the non-invasive XMT technique for monitoring changes in engineered substrates over time.The observed effects of ageing on hydrological performance were modelled as two distinct hydrological processes, retention and detention. Retention performance was determined via a moisture-flux model using physically-derived values of virgin and aged maximum water holding capacity. Increased water holding capacity with age increases the potential for retention performance. However, seasonal variations in retention performance greatly exceed those associated with the observed age-related increases in water holding capacity (+72% vs +7% respectively). Detention performance was determined via an unsaturated-flow finite element model, using van Genuchten parameters and XMT-derived values of saturated hydraulic conductivity. Reduced saturated hydraulic conductivity increases detention performance. For a 1-hour 30-year design storm, the peak runoff was found to be 33% lower for the aged brick-based substrate compared with its virgin counterpart.

A green roof segment for monitoring the hydrological and thermal behaviour of anthropogenic soil systems

Jelinkova V., Dohnal M., Picek T. (2015): A green roof segment for monitoring the hydrological and thermal behav iour of anthropogenic soil systems. Soil & Water Res., 10: 262–270. Green roofs and similar anthropogenic soil-plant systems in conurbations have a high relevance for society, especially in a changing climate. Understanding the hydrological performance of green roof substrates is a significant task in the framework of sustainable urban planning and water/energy management in urban areas. Potential retention and detention capabilities of anthropogenic, light weight, highly permeable soil systems and their continued performance over time are of major importance. A green roof test segment was designed to investigate the benefits of such anthropogenic systems. This adaptable low-cost system allows for long-term monitoring of preferred characteristics. Temperature and water balance measurements complemented with me teorological observations and studies of physical properties of substrates provide a basis for a detailed analysis of thermal and hydrological regime in green roof systems. The very first results obtained from the test segment have confirmed the green roof systems benefits. Reduced temperature fluctuations as well as rainfall runoff were attained compared to the traditional roof systems. Depending on numerous factors including the substrate material or vegetation cover, in the green roof tested the temperature amplitude for a selected period of nonfreezing days (with minimum ambient air temperature of 2.8°C) was suppressed by about 6.5 °C o n average. The

The influence of substrate and vegetation configuration on green roof hydrological performance

A four-year record of rainfall and runoff data from nine different extensive (80mm substrate) green roof test beds has been analysed to establish the extent to which the substrate composition and vegetation treatment affect hydrological performance. The test beds incorporated three different substrate components with different porosity and moisture retention characteristics, and three different vegetation treatments (Sedum, Meadow Flower and unvegetated).Consistent differences were observed, with the vegetated beds showing higher levels of rainfall retention and better detention compared with unvegetated beds. The seasonal Meadow Flower beds had similar hydrological performance to Sedum-vegetated beds. There was a 27% performance reduction in annual volumetric retention attributable to differences in substrate and vegetation. The beds with the most porous/permeable substrates showed the lowest levels of both retention and detention.As with previous studies, retention efficiency in all nine beds showed a strong dependency on rainfall depth (P), with retention typically >80% for events where P<10mm, but significantly lower when P>10mm. The effects of vegetation and substrate were most evident for rainfall events where P>10mm, with the mean per-event retention varying between beds from 26.8% to 61.8%. On average, the test beds were able to retain the first 5mm of rainfall in 65% of events where P>5mm, although this ranged from 29.4% to 70.6% of events depending on configuration. In terms of detention, all but one of the test beds could achieve runoff control to a green field runoff equivalent of 2l/s/ha for more than 75% of events.Detention was also characterised via the calibration of a reservoir-routing model that linked net rainfall to the measured runoff response. The parameter values identified here – when combined with a suitable evapotranspiration/retention model – provide a generic mechanism for predicting the runoff response to a time-series or design rainfall for any unmonitored system with comparable components, permitting comparison against local regulatory requirements.

Simulation of green roof test bed runoff

Low Impact Development (LID) aims to mitigate the hydrological impacts of urbanization by replication of processes in natural catchments. Green roofs covered with vegetation and pervious substrate are one alternative among a wide range of LID tools. Water retention of green roofs depends on many factors (e.g. local climate), and measurements remain crucial in evaluating their performance. The simulation of green roof retention by a hydrological model is one option to evaluate their potential benefits before implementation. In this paper, we evaluated the ability of the recently introduced LID green roof module of the stormwater management model to replicate runoff from monitored green roof test beds under Nordic climate conditions. A parameter sensitivity analysis was conducted to identify calibration parameters. The model showed an overall acceptable performance, and the results indicated the importance of accurately estimating potential evapotranspiration rates for inter-event periods, which is essential in representing the retention capacity regeneration. Copyright © 2015 John Wiley & Sons, Ltd.

Moisture content behaviour in extensive green roofs during dry periods: The influence of vegetation and substrate characteristics

Evapotranspiration (ET) is a key parameter that influences the stormwater retention capacity, and thus the hydrological performance, of green roofs. This paper investigates how the moisture content in extensive green roofs varies during dry periods due to evapotranspiration. The study is supported by 29 months continuous field monitoring of the moisture content within four green roof test beds. The beds incorporated three different substrates, with three being vegetated with sedum and one left unvegetated. Water content reflectometers were located at three different soil depths to measure the soil moisture profile and to record temporal changes in moisture content at a five-minute resolution. The moisture content vertical profiles varied consistently, with slightly elevated moisture content levels being recorded at the deepest substrate layer in the vegetated systems. Daily moisture loss rates were influenced by both temperature and moisture content, with reduced moisture loss/evapotranspiration when the soil moisture was restricted. The presence of vegetation resulted in higher daily moisture loss. Finally, it is demonstrated that the observed moisture content data can be accurately simulated using a hydrologic model based on water balance and two conventional Potential ET models (Hargreaves and FAO56 Penman–Monteith) combined with a soil moisture extraction function. Configuration-specific correction factors have been proposed to account for differences between green roof systems and standard reference crops.

A Green Roof Test Bed for Stormwater Management and Reduction of Urban Heat Island Effect in Singapore

A Green Roof Test Bed for Stormwater Management and Reduction of Urban Heat Island Effect in Singapore

A Green Roof Test Bed for Stormwater Management and Reduction of Urban Heat Island Effect in Singapore

A green roof test bed, established at the Nanyang Technological Universityin Singapore, was used to investigate its benefit for storm water management and urban heat island effect mitigation. The system comprised 3 units, 2 in the form of vegetated roofs and the other a bare roof.The system was equipped with automatic monitoring devicesfor measuring the hydrological data. Continuous data monitoring on the roofs was conducted to evaluate the thermal andhydrological effects. The study shows that the green roof test bed can significantly reduce the roof surface temperature (by an average of 7.3 ∫C) and lower the ambient air temperature (by an average of 0.5 ∫C) when compared with a bare roof during day timehours (from 10:00 am to 4:00 pm).The ability to reduce and delay the peak runoff was demonstrated by using a typical rainfall event with 18 mm depth. The designed system is useful in evaluating both thermal and hydrological benefits of a green roof systemin tropical areas and can offer guidance to local managers in mitigating the urban heat island effect and designing storm water management strategies.