Advocating a physically-based hydrological model for green roofs:

Advocating a physically-based hydrological model for green roofs: Evapotranspiration during the drying cycle Simon Poë, Alumasc Exterior Building Products Ltd, (poes@alumasc‐exteriors.co.uk), U.K. Dr. Virginia Stovin, University of Sheffield (Civil & Structural Engineering), (v.stovin@sheffield.ac.uk), U.K.

Abstract Green roofs temporarily store rainwater during infiltration, resulting in delayed and attenuated runoff (detention). They also retain rainfall, which is subsequently released via evapotranspiration (ET) (retention). However, as with many infiltration‐based sustainable drainage solutions (SUDS), modelling the hydrological performance of green roofs is complicated by the dependence upon both configuration and climatic factors. A green roof’s finite hydrological capacity ‐ a function of substrates’ structure and texture and plant architecture and physiology ‐ is seldom fully available at the outset of a storm event. ET regenerates the available water capacity (AWC), affecting the roof’s response to a specific event. This paper presents data from an experimental study at the University of Sheffield, aimed at identifying the drying cycle behaviour of nine different green roof configurations (with combinations of three characterised substrates and three contrasting planting strategies) under different climatic conditions. After saturation and drainage to field capacity, the mass of each microcosm was continuously recorded within a controlled environment facility, where relative humidity, air temperature and lighting were programmed to replicate typical UK diurnal cycles during spring and, later, summer. Initial analyses highlight the effect of both climate and configuration. As expected, in summer test conditions, higher ET losses were observed, initially exceeding 3 mm/day before decaying below 0.5 mm/day over time. During spring condition tests, lower initial losses of between 1.5 and 2.5 mm/day later fell to below 1.0 mm/day. The response of each configuration varied with the climate, with high early evaporative losses from non‐vegetated configurations subsequently falling below planted configurations as transpiration of deeper water became a factor.

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Authors’ Biographies

Simon Poe is the Product Director at Alumasc Exterior Building Products Ltd – the UK partner to ZinCo – where he is responsible for the development of Alumasc’s extensive portfolio of roofing solutions. With a background in roofing and drainage, Simon is a member of the Technical Advisory Group of the UK’s green roof trade association, GRO, and has worked on some major national and international roofing projects, including Centre Court at Wimbledon. Simon is also a part‐time PhD candidate at the University of Sheffield, where he is researching the hydrological response of green roofs.

Dr Virginia Stovin is a Senior Lecturer in the Department of Civil and Structural Engineering at The University of Sheffield. Her work focuses on urban drainage structures and processes, most recently on the hydrological performance of SuDS. She is an enthusiastic proponent of SuDS retrofitting, and is a co‐author of the recently‐published CIRIA retrofitting guidance. As part of the Sheffield‐ based Green Roof Centre, she has constructed 11 green roof test beds. The long term records from these beds are being combined with laboratory studies to underpin the development of green roof hydrological performance modelling tools suitable for urban stormwater management planning.

Background Industrial Context The need for complementary, sustainable, drainage strategies has become increasingly apparent in the UK over recent years, as a higher frequency of flooding has further demonstrated that traditional below‐ground drainage networks cannot cope with runoff from the increased impervious surface areas in urban spaces during extreme storm events. Green roofs are often heralded as potential Sustainable Drainage Systems (SuDS) due to their capacity for the retention and detention of stormwater, without the need to extend beyond the building’s footprint (and occupy scarce and expensive ground space). By retaining stormwater in the green roof layers (for subsequent evapotranspiration (ET) back to atmosphere), the volumes discharging into watercourses are reduced. Detention affords attenuated peak rates of runoff by temporarily storing water as it permeates the green roof layers; discharging at a later time and/or over the longer period of time.

An increasing number of green roof hydrological research programmes have been conducted, reporting variable average annual retention levels ‐ typically between 30 and 100%. Indeed, retention of individual storm events can range between 0 and 100% (Berghage et al., 2007).

Without greater clarity regarding the quantifiable stormwater benefit of green roofs, the UK public bodies charged with ensuring that Low Impact Developments (LIDs) maintain the site’s pre‐development hydrology will be unable to unequivocally advocate the incorporation of green roofs into SuDS management trains, as acknowledged by the Environment Agency (CIBSE, 2007). It is therefore incumbent upon the UK green roof industry to provide the requisite substantiation of green roofs’ contribution to stormwater management.

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Problem Green roofs reduce rainfall runoff rates due to the plant cover (by interception), the substrate (by infiltration and/or retention for ET) and the additional storage capacity in the underlying drainage reservoir. However, as with all drainage systems, green roofs have a finite capacity to store moisture. Furthermore, the full extent of a green roof’s maximum capacity will seldom be fully available (Berghage et al., 2007) due to the presence of a level of residual moisture. The antecedent moisture content (AMC) is critical to the hydrologic response of the green roof (Koehler & Schmidt, 2008) because greater values of AMC will limit the roof’s available water capacity (AWC) and therefore restrict the volume of rainfall that can be retained.

Antecedent moisture is variably depleted between storm events via ET. The rate of ET loss is a function of (1) the climate – affecting heat energy (i.e. air temperature and solar radiation) and the air’s water‐holding capacity (i.e. relative humidity and wind); and (2) the green roof configuration – substrate texture and structure and plant architecture and physiology.

The influence of climatic factors is manifested in observations of different levels of ET occurring: 1. Throughout the diurnal cycle: Fassman & Simcock (2008) identified clear diurnal patterns of both evaporation from bare surfaces and evapotranspiration from planted configurations, whilst Koehler & Schmidt (2008) observed condensation on the plant cover at night times; 2. As a result of the climate and micro‐climate at a specific location; and 3. As a function of season: with significantly greater daily ET rates observed in summer compared to winter. According to Rezaei & Jarrett (2006), ET rates in Pennsylvania State’s high summer (3.23 mm/day) were approximately four times greater than the 0.79 mm/day observed on average during the winter. Similar patterns were observed in European conditions by Koehler & Schmidt (2008); albeit at lower winter losses of 0.1 ‐ 0.5 mm/day and a greater range of summer losses (1.5 ‐ 4.5 mm/day).

Voyde et al. (2010) concluded that plant transpiration is an important control on ET rates; accounting for between 20 and 48% of moisture lost to atmosphere. The influence of vegetation can be explained, firstly, by the fact that the increased surface area (afforded by plant foliage) results in a greater capacity to intercept rainfall, increasing surface evaporation back to atmosphere. Secondly, the plant’s root system absorbs pore water, trans‐locating it through the xylem to stomatal cavities in the leaf, where it is vapourised by solar energy. The deficit in the leaf cells creates a difference in potential between the leaves and roots, such that a suction force is transmitted back to the root (van den Honert, 1948). Generally, previous research has focused on Sedum (or other hardy, drought tolerant) species and hydrological differences are therefore not widely known. However, Fassman & Simcock (2008) reported that configurations planted with Sedum mexicanum tended to result in higher ET rates than with New Zealand Ice Plants and there is evidence that Sedum‐planted configurations reduced runoff to a significantly greater extent than equivalent configurations with a mix of ‘Meadow Flowers’ (Poë et al., 2011).

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A substrate’s physical characteristics are typically recognised as a key influence in the system’s capacity to actively store rainfall (Palla et al., 2010). The tenacity with which water is held in pores is a function of a soil’s structure (Miller, 2003). As moisture is retained with lower tension in larger pores, there is a greater tendency for these pores to empty more quickly than with smaller pores. Pores of different sizes serve specific functions (Rowell, 1994), with transmission pores (>50 μm) for drainage and aeration, storage pores (0.2‐50 μm) for plant water consumption, and residual pores (<0.2 μm) that largely dictate the soil’s mechanical strength.

Soil structure (i.e. Particle Size Distribution [PSD] and Void Size Distribution [VSD]) is an important control on the filling and emptying of voids (Manning, 1987) and is therefore a significant influence upon the rate at which retained water is released during the drying cycle.

ET rates are expected to decay exponentially with respect to time (Fassman & Simcock, 2011; Kasmin et al., 2010) as a greater amount of energy is required to remove the moisture from the smaller pores.

Therefore, whilst the Antecedent Dry Weather Period (ADWP) is an important factor – determining the length of the drying cycle – in isolation, it “fails to characterise the complex processes that account for the roof’s antecedent moisture content” (Stovin et al., 2012).

A hydrological model for green roofs will inevitably require a physical basis if it is to account for the numerous hydrological processes and heterogeneous climatic factors and soil characteristics that dictate AMC (Beven, 2001) and hydrological response of different green roof configirations. In order to facilitate development of such a model, data must be collected to inform the physical drivers behind water balance changes, both within different configurations and in response to diverse climatic conditions.

Learning Objectives: 

To clarify that green roofs’ hydrological responses should be expected to vary as a function of location (and therefore climate) and configuration



To highlight the importance of a green roof’s configuration to its performance



To advocate the need for physically‐based green roof hydrological models

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Approach Trials were designed and conducted to identify the influence of configuration and climate upon ET rates. To this end, saturated green roof samples were placed onto load cells within the confines of a temperature‐controlled facility at the University of Sheffield (see Figure 1).

Figure 1: Load cell arrangements in chamber

Mass readings were continuously logged. Nine different green roof configurations, containing a drainage layer (with zero storage capacity), filter fleece and combinations of three substrates (to a settled depth of 80 mm) and three planting strategies were established, in triplicate, in 250 x 250 mm trays. A non‐vegetated option facilitated isolation of the influence of two other plant strategies – ‘Sedum Carpet’ (a pre‐cultivated mat comprising a dense coverage of 4 to 7 different Sedum species) and ‘Meadow Flower’ (a mix of wildflowers, grasses and hardy succulents; originated from seed). A temperature‐controlled glasshouse was used to allow the establishment of the plant cover prior to commencing trials in April 2011.

Sedum Carpet – a typical extensive green roof cover in the UK – is considered to be ideally suited to the harsh microclimates prevailing on UK rooftops. The Crassulacean Acid Metabolism of Sedum species ensures excellent tolerance to drought conditions; regulating water consumption patterns in line with availability. Meadow Flower is a strategy that can contribute to the drive for increased biodiversity in the UK. However, their use as green roof plants can often require additional irrigation measures.

Three different substrates were trialled: (1) Alumasc ZinCo Sedum Substrate (SCS) – a commercial extensive substrate with few fine particles (1.8% < 0.063 mm) and relatively high permeability of 14.8 mm/minute; (2) Alumasc ZinCo Heather & Lavender Substrate (HLS) – a commercial semi‐intensive mix with a greater proportion of fines than SCS (3.6%) and a significantly reduced permeability of 2.41 mm/min; (3) A mix based on Lightweight Expanded Clay Aggregate (LECA) with a high proportion of large particles (25.7% in excess of 4 mm) and voids (such that air content at MWHC is 49.8%). As a result, permeability is extremely high: 33 mm/min.

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Each configuration was tested under two climatic regimes; being subjected to diurnal cycles that are akin to, first, a UK spring and then a UK summer. Each condition was replicated three times on a sequential basis. The final test trialled for 28 days; the previous two ran for 14 days.

The climatic settings of the chamber were derived through consideration of hourly data for temperature and relative humidity, as recorded by a Met Office weather station in Sheffield during 2009. For trials in spring conditions, the average diurnal temperature range from March 2009 (i.e. 5.01‐9.76 °C) was adopted – a range that is largely in line with the long‐term averages published for spring months in the period between 1971 and 2000 (March: 3.1 – 9.3 °C; April: 4.4 – 11.8 °C; May: 7.0 – 15.7 °C). The diurnal temperature range adopted for summer trials (13.76 – 19.84 °C) reflected the range measured in August 2009 – the warmest month of that year. This range compares well to long‐term averages published for summer months in the period between 1971 and 2000 (June: 10.0 – 18.3 °C; July: 12.4 – 20.8 °C; August: 12.1 – 20.6 °C). The lighting system was programmed to provide artificial sunlight to allow the replication of daylight and night time conditions. In spring, lights were programmed to be on for 12 hours per day (as sunlight hours published for Sheffield in March 2009); whilst, in summer trials, lights were on for 17 hours (as sunlight hours published for June 2009 – the longest daylight hours recorded in summer months).

Before the start of each trial, the nine configurations were submersed in water for 2 hours and then allowed to drain to field capacity over a period of 2 hours. Each tray was randomly placed on to a calibrated RLS010 single‐point compression load cell (with a safe working capacity of 10 kg and a maximum linearity error of 0.02% ‐ equivalent to 0.032 mm of ET). The signal (in mV) was amplified and recorded on an hourly basis for each microcosm by a Modular 600 multi‐channel signal conditioning and datalogging unit. The data was converted to mass (in kg) using calibration equations that were derived prior to the commencement of tests. Changes in mass over time were subsequently converted to moisture losses in mm/m2. The chamber’s climatic data was captured via a separate, central logging system.

Analysis Cumulative ET losses are seen to vary as a function of configuration and season, with a range of ET losses observed (see Figures 2 and 3). In spring, cumulative losses over the 28 day trial period ranged from 18.8 to 28.9 mm (mean: 23.9 mm). The corresponding losses during summer conditions ranged from 20.7 to 35.5 mm (mean: 28.3 mm). However, the lower‐than‐ anticipated mean daily ET rates of 0.85 mm and 1.0 mm for spring and summer respectively are clearly affected by the decay in ET rates over the 28 days – particularly in summer conditions, when virtually no losses are measured from many of the configurations during the final 14 days.

Indeed, after 10 days, mean cumulative losses of 13.8 mm (spring) and 21.2 mm (summer) indicate significantly higher mean daily ET losses of 1.4 mm and 2.2 mm respectively.

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40

35

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Sedum on HLS

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Sedum on SCS

ET Losses (mm)

ET Losses (mm)

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Sedum on LECA 20

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ADWP (Days)

ADWP (Days)

Figure 2: Cumulative ET (Spring)

Figure 3: Cumulative ET (Summer)

A closer investigation of the mean daily ET rates further highlights this trend and suggests that a further variable – moisture availability – is an important consideration. As Figure 4 highlights, during the initial 10 days following saturation, when moisture availability would not typically be considered limiting, daily ET rates are generally greater in summer conditions (initially >3 mm/day) than in spring (initially 2 mm/day).

5.00 4.00

ET Rates [mm/day]

3.00 2.00 1.00 0.00 ‐1.00 ‐2.00

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Thereafter, ET rates subsequently fall towards, and ultimately below, 1 mm/day in both spring and summer.

In spring, this decline is generally more gradual and linear. However, in summer, the significant depletion of moisture during the first 10 days appears to restrict further significant ET losses; in many cases reaching virtually zero loss towards the latter stages of the trials.

ADWP (Days) Ave_Summer

Ave_Spring

Figure 4: Mean Daily ET Rates

These mean trends highlight the importance of several physical influences that require further investigation; namely, configuration, climate and moisture availability over time.

Importance of the green roof configuration Plant Strategy

The addition of a plant layer into green roof configurations can, in some instances, have a detrimental impact on the systems’ propensity to regenerate available moisture capacity; particularly in the earlier stages of the trial periods. This period varies seasonally, with greater losses observed from non‐vegetated configurations in the initial 12 days of spring conditions and in the initial 4 days of summer conditions. This trend would be explained by the greater amount of evaporation that occurs from a bare surface of a dark and porous growing medium, relative to a green, planted coverage that contributes to the localised cooling of air.

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Incremental ET Loss due to Plant [mm]

12.0 However, after this initial effect, 10.0 plants appear to make a positive 8.0 contribution to losses, due to the 6.0 incremental moisture loss that is 4.0 attributable to transpiration and the 2.0 configurations’ greater overall 0.0 ‐2.0 moisture capacity. ‐4.0

‐6.0 The patterns of ET losses can be seen 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 ADWP [Days] to vary according to the plant species Sedum Carpet [Summer] Meadow Flower [Summer] in question. The different responses Sedum Carpet [Spring] Meadow Flower [Spring] of the Sedum and meadow flower plant strategies to spring and summer Figure 5: Net Effect of Vegetation on Cumulative ET conditions are highlighted in Figure 5.

It was expected that the Sedum vegetation would improve the hydrological response, due both to its greater coverage (relative to meadow flowers) and to its Crassulacean Acid Metabolism (CAM). In general, plants regulate water demands in line with supply, by closing stomata, but CAM plants are able to metabolise at night when water loss is lower, such that losses are controlled to a greater extent than would be the case with non‐CAM species. In spring conditions, these properties translated into lower losses being observed from Sedum‐planted configurations, when compared with meadow flower configurations. Indeed, ET losses from Sedum planted configurations were also lower than those that were observed from non‐ vegetated configurations for ADWPs of up to 20 days, such that the net effect of including Sedum planting is just 2 mm of additional ET at the end of the 28 day trial period. However, it is in summer conditions that Sedum’s properties can be seen to positively impact the hydrological response. Here, after an initial 6 days when the losses from non‐vegetated configurations are greater, the Sedum planted configurations contribute significantly greater ET losses than those observed from non‐vegetated configurations. Importantly, the pattern of ET losses from Sedum plants is such that net losses that are attributable to the plant layer exceed the equivalent losses observed from meadow flower after an ADWP of 20 days; as Sedum’s controlled losses gradually transpire soil‐water, such that the net effect of the planting is circa 8 mm of additional ET.

The coverage of the meadow flower also resulted in lower ET losses during the first 12 days of spring trials, relative to non‐vegetated configurations. Thereafter, however, the plants contribute towards ET losses that exceed those from non‐vegetated samples, such that the net effect of incorporating meadow flowers is to increase cumulative ET losses by 6 mm. In summer conditions, with the exception of the first 4 days, the meadow flower makes a positive net contribution to ET losses. This incremental ET loss increases rapidly, reaching circa 9 mm after 14 days. Thereafter, the net contribution starts to fall, as the plants start to wilt due to the moisture stress that is a result of the fast initial consumption of the plant‐available water. By the end of the 28 day trial period, the meadow flower had made a positive net contribution to cumulative ET losses of 6 mm. However, in light of the apparent permanent wilting of the plant layer by this time, it is unlikely that the planting would be able to make any positive contributions in future.

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It is apparent that meadow flower will typically regenerate the available water capacity at a faster rate than Sedum. This trend was evident both in the spring trials (where the net contributions to cumulative ET losses were 6 mm and 2 mm for meadow flower and Sedum respectively) and in the initial 14 days of the summer trials (where the net contribution of meadow flower was circa 9 mm, compared to 6 mm for Sedum). However, in order for this to be suitable for a real green roof installation, additional irrigation measures may be required to prevent the permanent wilting of the plants that led to the net effect of meadow flower falling below the level of Sedum (i.e. 6 mm, compared with 8 mm).

Substrate Type

The composition of the different tested substrates is seen to produce variable responses, often depending on the climate and any interaction with the plant layer. Losses from all substrates are typically greatest during the initial 14 day period. From a stormwater management perspective, it is this initial period that would be considered to be of greatest relevance. During this time, different ET loss patterns are observed from the three substrates: after an ADWP of 10 days, the differences in cumulative ET losses (and therefore in available moisture capacity) could differ by as much as 7 mm ‐ approximately 30% of the mean cumulative loss to that point in time.

In summer, ET losses from HLS are seen to be the highest of all three substrates. Here, the warm temperatures and greater plant water consumption appear to result in potential energy (ψ) that is sufficient to break the pore‐water tension in the smaller pores and therefore extract significant proportions of the substrate’s MWHC. Mean cumulative losses therefore exceed 25 mm within 10 days of the trial start date, before the rate of loss decays, such that losses are 29 mm after 14 days and 32 mm after 28 days. Conversely, in the cooler conditions of spring, the relatively high proportion of large particles (and voids) within SCS leads to a greater volume of moisture being released (compared to HLS); reaching 26 mm by the end of the 28 day trial period. The unique composition of LECA results in the lowest losses of all three substrates, reaching approximately 22.5 mm after 28 days of spring and 26 mm in summer.

Climate

The impact of climatic regime is apparent in virtually all of the presented analyses; not only affecting the rate of ET loss, but equally, variably impacting on the survival of the plant layer and on the retention and release of moisture from the substrate. As expected, greater overall losses are witnessed in summer compared to spring, with the influence of season apparently greatest in the initial 10 to 14 days that follow saturation; such that the differential between mean cumulative ET losses steadily increases to a peak of approximately 7 mm after 11 days.

Indeed, during the initial 14 day period of summer, cumulative losses from planted configurations are approximately double those observed during the tests under spring conditions. Thereafter, the differential starts to reduce; presumably as a result of restrictive moisture availability in the configurations that were subjected to summer conditions.

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Results and Business Impacts Key Findings With a focus on the period that is of most relevance to stormwater management – the initial 10 to 14 days following a storm event – it can be seen that daily ET rates over the course of the 10 day period average 1.4 mm in spring and 2.2 mm in summer; regenerating 14 mm and 22 mm of capacity respectively. After this time, moisture availability is a key constraint to further significant losses and ET rates tend to fall below 1 mm/day. This trend of decay is apparent in both summer – due to the limited moisture availability – and in spring, where there is greater moisture availability, but where the lower energy source in the cooler, spring climate yields a lesser capacity to draw moisture out of the smaller pores and induce further ET. Whilst mean trends are of value in communicating the SuDS potential of green roofs, there are clear differences apparent in the responses of the different configurations. The incorporation of planting into a green roof is important in delivering many of the key benefits of green roofs.

However, from a hydrological perspective, the additional moisture capacity of a plant layer is initially partly offset by a net reduction in ET, relative to non‐vegetated roofs. The length of this initial period is a function of the climate and plant layer, with positive hydrological contributions witnessed from planting after 14 to 21 days in spring and 4 to 6 days in summer.

Business Impacts The research findings presented herein form part of a wider research programme at the University of Sheffield, where the same nine configurations are being monitored at a field research site. The findings from this set‐up will allow the information from the drying cycle to be evaluated against actual rainfall and runoff records; thereby facilitating the development of a predictive model. With the stormwater management contribution heralded as a key driver behind green roof installations, such a model will be critical to the number of green roofs that are installed as part of SuDS management trains. In addition, the findings highlight the central importance of the green roof configuration to its performance and its reliance upon the climate prevailing at the installation site. Such factors must be considered by green roof manufacturers and specifiers in order to ensure that the roof fulfils its objectives during its life cycle. Indeed, further analysis is expected to provide information pertaining to the optimisation of plant and substrate combinations.

Conclusions Green roofs can provide valuable drainage capacity that complements traditional drainage measures and other SuDS methods. To put this capacity into perspective, an ability to retain 20 mm would equate to 91% of a rainfall event in Sheffield with a 10 year return period and of 1 hour in duration (Stovin et al., 2012). There are several factors that will be critical to the regeneration of the green roof’s capacity to retain this level of moisture; notably, the green roof configuration and the climate. Such factors have proven difficult to capture in statistics‐ based predictive models. However, with these influences having clear physical bases, more accurate predictions of a green roof’s hydrological response are to be expected if a physically‐ World Green Roof Congress, 19-20 September 2012, Copenhagen Page 10

based model is developed. An increased level of predictive accuracy will be necessary to justify the use of green roofs in providing this additional, sustainable drainage capacity in the UK.

Key Lessons Learned: 

A green roof’s hydrological response is expected to vary as a function of the configuration’s retention and release properties, the climate and the resultant moisture availability.



AWC is a more significant variable than MWHC when considering the hydrological response of a green roof.



A physically‐based model is required to capture the influences affecting a green roof’s hydrological response.

Acknowledgement

The authors would like to acknowledge the support and input of several colleagues working as part of the EU MCIAPP‐funded Green Roof Systems project, and in particular Zoë Dunsiger and Joerg Werdin.

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