Revegetation strategies helping urban areas combat the effects of heat waves

04 November 2015Erwan Cordeau

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Erwan Cordeau

Plant life, or vegetation, occupies a number of functions within the urban ecosystem, playing a significant role in the sustainable development measures that enable towns and cities to adapt to climate change and the threat of heat waves. Here we examine the results of urban revegetation simulations.

© Sophie Laurent, IAU îdF

Recent climate studies have shown a clear increase in the number, duration and intensity of heat waves in the Île-de-France region; using the most realistic scenario for future greenhouse gas emissions, it is estimated that by the end of the century the region will experience 11 days of heat waves per year on average.

Cities are also subject to the effects of the Urban Heat Island (UHI) phenomenon, characterised by hotter air temperatures in central districts than around the urban periphery, especially during the night; primarily due to the use of artificial surface materials (see  below for a glossary of terms), this phenomenon is exacerbated by certain meteorological conditions. This was the case during the 2003 heat wave; in the Île-de-France region, the UHI reached a peak intensity of 8°C during the night, or double its usual intensity during the summer months (Source: Météo France), worsening the overall impact on health and sanitary conditions.

Revegetation: an asset in the fight against climate change

Cooling effects of vegetation

The challenges associated with adapting to and limiting the dangers of climate change are intensified in cities(1). As densely populated areas requiring urgent solutions, only a global school of thought integrating both factors will enable authorities to bring about strategies to reduce the vulnerability of city populations.  Amongst the measures being considered, revegetation (cf. "Glossary") of the urban landscape constitutes a potentially effective course of action.

Using adaptation strategies, we are currently designing individual and collective measures that aim to reduce the vulnerability of both natural and human systems to the predicted effects of climate change. These measures will complement "alleviation" strategies (cf. "Glossary"), which aim to directly reduce the amounts of greenhouse gases being emitted into the atmosphere, and to protect and develop systems that act as carbon sinks (cf. "Glossary", ), of which natural ecosystems are a prime example.

Urban revegetation works on two fronts (cf. "Cooling effects of vegetation"): the first is evapotranspiration, referring to plant transpiration (which occurrs during the day as part of the process of photosynthesis) and the subsequent evaporation of water caught by the soil and leaves. The transformation of liquid water into vapour consumes energy, and cools the surrounding environment. The second advantage of (primarily arboreal) revegetation is that the plant life intercepts a portion of the solar rays descending on the area by shading the ground and the surfaces of buildings.  During the summer, these two mechanisms contribute to the improvement of the urban microclimate. Revegetation of urban surfaces is often termed a "no-regret" strategy. In effect, apart from the primary objective of providing a solution to a current problem (in this case, the harmful effects of Urban Heat Islands), vegetation is also valued for its secondary benefits: maintaining biodiversity, or the improvement of an area’s aesthetic appearance, for example.

Data modelling using a global approach to urban systems

Various options for revegetation exist: trees lining streets, grass or bush beds in roads and roundabouts, green belts around buildings, parks, undeveloped land, grassy ditches, agriculture and peri-urban forests, etc. Each presents distinct characteristics and therefore does not induce the same effects.

The evaluation of different revegetation measures for the urban ecosystem and their potential interactions is achieved via a global approach to the territory at hand - a so-called ecosystemic approach. This involves taking into account the synergetic and potential nefarious effects of the same adaptation measure for one or several urban issues, or indeed several measures aimed at tackling one specific problem. Digital modelling is a tool that is particularly well-adapted for the evaluation of interactions between urban issues and adaptation measures, for cities and diverse climates.

When considering a prospective approach, data modelling acts as a virtual laboratory. Prior to implementation, the strategies having been proposed by architects, town planners and developers can be tested, their respective and combined performances quantified against a given problem (in this case, the degradation of the urban microclimate and secondary effects). However, any harmful side effects that the strategy may have upon the urban ecosystem will also be observed.

In response to the question of adaptation to climate change at the wider city level, the Météo France National Meterologcal Centre is equipped with a sophisticated urban climate model. This enables researchers to simulate different revegetation strategies and to quantify their microclimatological, energetic and hydrological impacts, on scales ranging from individual districts to the wider to urban area.

This type of simulation has been undertaken using this model for the Île-de-France urban area as part of the Muscade(2) project; the findings of the subsequent report are exhibited in this Note rapide.

Revegetation Scenarios

Though having been simplified due to the constraints of the data modelling procedure, the study zone (100kmx100km) is represented in a fairly realistic way.

The urban "fabric" is categorised into five types of buildings: individual, collective, Haussmanian, office towers, depot/warehouse. There are four types of building use: residental, office, industrial/agricultural, and commercial.

It is therefore possible to simulate the various characteristics of the buildings’ relevant facilities: air conditioning, heating, solar protection, ventilation, etc.

It must be noted that data for these air conditioning and heating systems is based on respective ambient temperatures of between 26°C and 19°C, in accordance with the new thermal regulations published in 2012 (RT2012) which assume reasonable practices on the part of residents. The choice of revegetation strategy - outlined across 9 scenarios (cf. "Revegetation scenarios")- is established based on previous results and in keeping with the following principles:

  • The increase in vegetation density is spread across the entire city, as this is both more efficient and more equitable.
  • The increase in vegetation density is achieved via plantation in open ground by partially planting "available" ground surfaces (i.e. those which have not been built on and do not form part of any roadway) with low vegetation or mixed arboreal plants.
  • Installation of extensive roof gardens (French: toitures végétalisées extensives, or TVE), which provide an opportunity to increase vegetation density in areas where buildings already occupy the ground space; they may be laid out on individual or multi-dwelling buildings, as well as depots and warehouses.
  • During the summer, vegetation in open ground is systematically watered (except in cases where this would be inefficient(3)) using sprinklers, between 9pm and 5am, at a rate of 25L/sq m per week. Watering of TVE roof gardens using drip irrigation is also an option.
UTCI scale for levels of thermal stress (heat stroke)

Relevant indicators for the evaluation of performance scenarios

Performances were evaluated using the context of a heat wave, by applying the meteorological conditions observed during the 2003 heat wave, but also those of other heat waves from the 1999-2008 period, in order to account for the effects of seasonality. Three types of indicators were selected:

  • Thermal indicators: based on the air temperature at street level and the "felt air" or "apparent" temperature, which is often associated with heat stroke (cf. "Glossary") in humans (Universal Thermal Climate Index(4)) (cf. UTCI scale for levels of thermal stress (heat stroke)).
  • Energetic indicators: based on energy consumption required for heating and air conditioning in buildings. This is expressed in terms of Total Final Consumption, which is to say the amount of energy actually consumed.
  • Hydrological indicators: based on consumption of water used to irrigate urban vegetation both in open ground and on terraces, as well as the surface runoff.

Lessons drawn from simulations using heat wave context

Urban shape and revegetation levels are indissociable in the context of a heat wave period.
The effect of arboreal strategies varies depending on the urban typology because of two main factors: the amount of ground surface available for revegetation and the morphological characteristics of the city district.

Improved temperature comfort at street level

As the nocturnal temperature anomaly maps illustrate, the increase in vegetation density does indeed produce a cooling effect on streets in the Paris region.
Mixed arboreal vegetation is significantly more effective in this regard than low vegetation (shrubs, bushes etc.) (cf. "Variations in nocturnal temperature during various revegetation strategies"). Moreover, the cooling effect is all the more notable as the rate of revegetation rises, going from 0.25°C to 2°C depending on the rate and localisation of the vegetation within the urban area. Finally, TVE planted rooftops appear to only produce any effect when they are irrigated. Even then, the impact remains minimal, ranging from -0.25 to -0.5°C.

Variations in nocturnal temperature during various revegetation strategies

Variations in minimum nocturnal temperatures along city streets, 2m above surface level, obtained by revegetation at 25, 50 and 75% of available ground surfaces, using low vegetation or mixed arboreal vegetation.
Sources: Cécile de Munck, CNRS © IAU îdF

Increasing revegetated open ground surfaces in cities also limits the effects of thermal stress (heatstroke) felt by individuals at street level. Here also, this effect increases with the rate of revegetation and the proportion of arboreal vegetation (cf. "Thermal comfort/stress during a heat wave according to UTCI scale"). For example, increasing the rate of revegetation of urban surfaces from 25% to 75% using low vegetation allows the amount of time spent experiencing extreme heatstroke under the sun, or highly elevated heatstroke in the shade, to be diminished by 30 minutes.

However, arboreal vegetation strategies are more effective, especially in districts with multi-dwelling units (cf. "Thermal comfort/stress during a heat wave according to UTCI scale"): revegetation of 75% of available surfaces in these kinds of urban areas allows a reduction in the amount of time a person spends experiencing highly elevated heatstroke of more than 1 hour, compared to only 40 minutes in individual dwellings.

It must also be noted that the impact of arboreal strategies have been underestimated in this study, because the effects of tree shade are not currently taken into account in the data model and calculation of "felt air temperatures".

Reduction in the use of air conditioning

The cooling of the exterior microclimate induced by revegetation leads to a reduction in the use of air conditioning, and therefore of associated energy consumption. Usage rates also vary depending on the type of revegetation measure and the proportion of ground surface having been planted (cf. "Water and energy consumption during the 6 days of a heat wave according to revegetation scenarios").

Without specific adaption (REF), the consumption of energy used for air conditioning counted cumulatively across the study area amounts to 759 GWh (gigawatt-hours). Non-irrigated planted rooftops function primarily as insulators, creating energy savings of around 4%. Irrigation increases their evapotranspiration effect, leading to an even more significant reduction in final energy consumption (12%).

By regulating the exterior microclimate, revegetation strategies indirectly cause a reduction in the demand for air conditioning in buildings: the maximum effect is observed with a 75% revegetation rate of available ground surfaces using mixed arboreal vegetation. This results in energy savings of 13%, directly comparable to the performance levels expected from the installation of TVE irrigated rooftop gardens.

Thermal comfort/stress during a heat wave
according to UTCI scale

Example of the number of hours of the day (between 6 and 22) spent in thermal comfort or thermal stress (heatstroke) during the heat wave, for an individual in the shade in a multi-dwelling urban area according to the UTCI scale.
Example of the number of hours of the day (between 6 and 22) spent in thermal comfort or thermal stress (heatstroke) during the heat wave, for an individual in the shade in a multi-dwelling urban area according to the UTCI scale.

Water and energy consumption during the 6 days of a heat wave according to revegetation scenarios

Sources: Cécile de Munck, CNRS Infographics: Laurie Gobled, IAU îdF

Finally, the 25% reduction in energy consumption brought about by the Maximum revegetation scenario (Vmax) clearly demonstrates the synergetic effect, and therefore the practical interest, of combining two revegetation strategies employing different physical mechanisms: cooling of the air at street level via arboreal revegetation in open ground in tandem with the insulating effects of roof gardens.

Availability of water resources questioned

To put energy-saving performances into perspective, it is necessary to consider the water consumption necessary to irrigate types of vegetation. Shown cumulatively for the 6 days of the heat wave and integrated across the study area, water resources are here expressed in tens of millions of cubic metres (cf. "Water and energy consumption during the 6 days of a heat wave according to revegetation scenarios"). Compared to the flow rate of the river Seine at its low water mark (i.e. during a dry period, 66m3 per second), the irrigation of all urban vegetation in the study area before revegetation would consume 53% of the volume of the Seine. This figure reaches 71% for the densest arboreal scenario, compared to 69% for irrigated roof gardens.

The energy performances of various revegetation strategies are strongly linked to their watering protocol (volume, frequency, timing, etc.). It is therefore essential to undertake a review of irrigation practices, both current and future, in order to simulate the most realistic or innovative management methods, with the objective of optimising our use of redirected water for the irrigation of urban vegetation.

One important result of this study that must be kept in mind is the observation that trees are a more effective means of managing ground water than herbaceous vegetation. In effect, owing to their highly developed root system, trees are able to mobilise greater amounts of soil water, and in doing so produce an advantageous cooling effect on surrounding air temperatures thanks to their having much more ample foliage than herbaceous species. Consequently, with equal water consumption trees induce greater energy savings in terms of climate control.

Densities of urban vegetation simulated by sq km

Simulations carried out using current city parameters (REF) and with revegetation of 25%, 50% and 75% of available ground surfaces, excluding habitats and roadways (pavements, car parks, squares, roundabouts, etc.) or with revegetation of building rooftops (TVE).

Energy and hydrological consumption: simulations by season

Across the Paris urban area over the 1999-2008 decade, which served as a reference base for seasonal impacts on vegetation strategies, the final energy consumption per year is primarily linked to heating.

Variable impacts on energy consumption

Planted rooftops demonstrate building insulation properties throughout the year (cf. "Simulated evolution of energy consumption levels by season (GWh/gigawatt-hour)"); their energy efficiency is significant in all seasons, as they can influence demand for air conditioning as well as heating.

The effects of arboreal strategies on energy consumption are more complex. The reduction in the use of air conditioning due to the cooling of air at street level has been proven during the summer. Nevertheless, outside this period trees also cause a slight cooling effect (caused by the direct evaporation of water intercepted by tree leaves), which eventually leads to increased use of indoor heating. Still, this effect should be lessened in the future as the use of indoor heating is expected to decline (due to improved building insulation and milder winters caused by global warming).

Simulated evolution of energy consumption levels by season (GWh/gigawatt-hour)

Differences in final energy consumption are represented for each revegetation scenario according to the reference situation obtained from data averages for 1999-2008 period. During the summer, in response to the need for cooler air, evapotranspiration of irrigated vegetation the substrate humidity level mean the TVEirr scenario performs better in energy terms. Source: Cécile de Munck, CNRS

Innovative systems for managing water resources at local levels

The urban ecosystem is currently facing two significant issues with differing timeframes: management of rainwater throughout the year and the use of redirected water for irrigation of green spaces during the summer. To evaluate the impact of different revegetation strategies on these two issues, the volume of runoff water from rooftops and artificial ground surfaces has been quantified and compared to the volume of water required for the irrigation of all vegetation in the urban area.

Annual water runoff levels are notably impacted by the revegetation of urban round surfaces. Regardless of the season, revegetation in open ground reduces runoff largely due to the increase in vegetated, permeable ground cover (cf. "Comparison of impacts of revegetation solutions on water runoff levels (mm3/million m3)"). This reduction reaches 11%, 22% and 33% for revegetation rates of 25%, 50% and 75% of available ground surfaces respectively. The capacity of planted rooftops to manage rainwater varies according to season. In spring and in autumn, these features allow quantities of water runoff to be reduced by 18% and 10% respectively.

Finally, it is interesting to remark that beyond a certain global rate of city revegetation (11%), annual volumes of runoff water no longer offset the levels of water required for irrigation during warmer months.

Comparison of impacts of revegetation solutions on water runoff levels (mm3/million m3)

All open-ground revegetation solutions are effective in terms of water runoff (as less artificial and waterproof surface coverings mean more direct absorption of rain water into the ground). On the other hand, the capacity of TVE roof gardens to manage rainwater varies according to season. The presence of natural precipitation when added to systematic summer watering, as well as the fact that TVE designs tend to drain too much water, means that irrigated roof gardens produce a higher level of water runoff that non-irrigated roof gardens during summer and in early autumn.
Sources and credit: Cécile de Munck, CNRS

GLOSSARY
Artificialized ground surface: the construction of urban infrastructures or any other construction on surfaces that were previously natural, agricultural or covered by forest.
Carbon sink: areas or processes in which greenhouse gases are removed from the atmosphere, either to be destroyed by chemical processes or stored in another form. Example: Carbon dioxide is stored in ocean water, vegetation and underground spaces.
Thermal stress (heatstroke): a rise in body temperature that can lead to serious health risks.
Revegetation: in this article, the term refers to the voluntary process of replanting and reconstruction of ground surfaces in areas that have been disturbed by human activity, as well as the installation of planted roof gardens.

1. In the Île-de-France region, 96% of the population currently live in an urban district (source: Insee).
2. Muscade: Urban data modeling and adaption strategies for climate change, in order to anticipate energy demand and production. Project coordinated by the Agence nationale pour la recherche (National Research Agency, ANR) 2009-2013.
3. Épicéa Research Project: multidisciplinary study of the impacts of climate change on the Paris Urban Area. h t t p : / / w w w. c n r m . m e t e o. f r / s p i p.php?article271
4. www.utci.org