Session Overview
CCMA7: UHI mitigation strategies III : watering processes studies
Thursday, 23/Jul/2015:
11:00am - 12:30pm

Session Chair: Cécile, Sylvie DE MUNCK, CNRS-Météo France
Location: Cassiopée Room


The implementation of biofiltration systems, rainwater tanks and urban irrigation in a single-layer urban canopy mode

Matthias Demuzere1,3, Andrew M. Coutts2,3, Maren Gohler4, Ashley M. Broadbent2,3, Hendrik Wouters1,5, Nicole P.M. van Lipzig1, Luke Gerbet2,3

1KU Leuven, Department of Earth and Environmental Sciences, Celestijnenlaan 200E, 3001 Leuven, Belgium; 2School of Earth, Atmosphere and Environment, Monash University, Melbourne, Victoria, Australia; 3CRC for Water Sensitive Cities, Australia; 4Department of Computational Hydrosystems, UFZ Helmholtz Centre for Environmental Research, Leipzig, Germany; 5Department of Environmental and Atmospheric Modelling, Flemish Institute for Technological Research (VITO), Mol, Belgium

Urban vegetation is generally considered as a key tool to modify the urban energy balance through enhanced evapotranspiration (ET). Given that vegetation is most effective when it is healthy, stormwater harvesting and retention strategies (such as water sensitive urban design) could be used to support vegetation and promote ET. This study presents the implementation of a vegetated lined bio-filtration system (BFS) combined with a rainwater tank (RWT) and urban irrigation system in the single-layer urban canopy model Community Land Model-Urban. Runoff from roof and impervious road surface fractions is harvested and used to support an adequate soil moisture level for vegetation in the BFS. In a first stage, modelled soil moisture dynamics are evaluated and found reliable compared to observed soil moisture levels from biofiltration pits in Smith Street, Melbourne (Australia). Secondly, the impact of BFS, RWT and urban irrigation on ET is illustrated for a two-month period in 2012 using varying characteristics for all components. Results indicate that (i) a large amount of stormwater is potentially available for indoor and outdoor water demands, including irrigation of urban vegetation, (ii) ET from the BFS is an order of magnitude larger compared to the contributions from the impervious surfaces, even though the former only covers 10% of the surface fraction and (iii) attention should be paid to the cover fraction and soil texture of the BFS, size of the RWT and the surface fractions contributing to the collection of water in the RWT. Overall, this study reveals that this model development can effectuate future research with state-of-the-art urban climate models to further explore the benefits of vegetated biofiltration systems as a water sensitive urban design tool optimised with an urban irrigation system to maintain healthy vegetation.

Impacts of urban heat island mitigation strategies on surface temperatures in downtown Tokyo

Nobumitsu Tsunematsu1, Hitoshi Yokoyama1, Tsuyoshi Honjo2, Arata Ichihashi1, Haruo Ando1, Futoshi Matsumoto1, Yoshihito Seto3, Nobuhiko Shigyo2

1Tokyo Metropolitan Research Institute for Environmental Protection, Japan; 2Chiba University, Japan; 3Tokyo Metropolitan Research Institute for Environmental Protection/Tokyo Metropolitan University, Japan

Annual mean air temperatures in downtown Tokyo have increased about 3 degrees Celsius in the past 100 years due to global warming and urban heat island (UHI). Also, the frequency of heat stroke outbreaks in Tokyo tends to increase in recent years. We have therefore investigated the impacts of UHI mitigation and adaptation strategies such as making highly reflective pavements, creating green and water spaces, etc.

As part of the investigations, we performed thermal infrared (TIR) remote sensing in downtown Tokyo on three different extremely hot days: Aug. 7, 2007, Aug. 19, 2013, and Aug. 19, 2014. The TIR measurements were carried out in the daytime (12-13 local time: LT) and the nighttime (around 21 LT) under similar weather conditions, using a long-wave infrared (8-14 um wavelength) camera (NEC Avio; TS7302) installed on a helicopter. The helicopter was flying at Flight Level 20 (2,000 ft: 610 m). This allows horizontal spatial resolution of data from the thermal imaging camera to be significantly high (approximately 2 m) in spite of airborne TIR measurements. Daily maximum air temperatures on those days reached 32-34 degrees Celsius. Sea breezes prevailed over downtown Tokyo.

An area of the measurements in 2013 includes the area of 2007. To evaluate impacts of recent UHI mitigation strategies in downtown Tokyo, we analyzed changes in the image-derived surface temperatures between 2007 and 2013. The results show that daytime surface temperatures in 2013 are relatively high in the greater part of the area, compared with the ones in 2007, nevertheless, lower surface temperatures can be observed in some redevelopment areas where new buildings were constructed between 2007 and 2013. This appears to be due to green and water spaces created in the redevelopment areas through the UHI mitigation strategies, indicating some measures for UHI mitigation are effective.

In addition, we pick out hot spots where strategies for lowering temperatures should be required. The TIR images projected on Google Earth show higher surface temperatures on intersections and the northern parts of streets running from east to west. To clarify the causes of those hot spots, relationships among surface temperatures, sky view factors, etc. are investigated. Also, we analyze thermal environment around venues of the 2020 Summer Olympic and Paralympic Games.


Watering practices and urban thermal comfort improvement under heat wave conditions

Maxime Daniel1, Aude Lemonsu1, Vincent Viguié2

1CNRM-GAME / Météo-France, France; 2CIRED, France

Increasing heat-wave risk due to regional climate evolutions, exacerbated with urban heat island effect, is a major threat for the inhabitants of many cities. However, relevant urban planning choices and adaptation policies can help limit population vulnerability.

Adding more vegetation to urban environment is often proposed as a potential tool, as it enables to regulate the microclimate by evapotranspiration. But the efficiency of such strategies tightly depends on water availability for green areas, so that this raises the fundamental issue of water supply for irrigation.

Using Paris urban area as a case study, we build several contrasted scenarios of possible city evolutions until 2100. Urban climate is simulated for each of them, which enables to evaluate both urban heat island (UHI) and heat stress under heat-wave conditions in 2100. To understand the characteristics and efficiency of different irrigation practices, three vegetation watering alternatives, as well as a scenario of pavement watering, are studied and compared. Using different indicators of UHI and heat stress, an optimized water-use scenario is finally proposed.

The effect of irrigation on air temperature during heatwave conditions

Ashley Mark Broadbent1, Nigel Tapper1, Andrew Coutts1, Jason Beringer2, Matthias Demuzere3

1Monash University; 2University of Western Australia; 3KU Leuven

In conventional urban areas, the majority of rainfall is exported out of the environment through the stormwater drainage system. This loss of stormwater is balanced by importing potable water, which is used for irrigation and gardening watering. Stormwater runoff can reduce urban moisture availability, leading to reduced evapotranspiration and increased sensible heat flux, which may cause higher urban air temperatures. This is particularly relevant in Australia, which has experienced extended dry periods and heat waves over the last two decades, especially in the major southern cities: Perth, Adelaide, and Melbourne. The ongoing drought has placed pressure on potable water resources and led to water restrictions and irrigation bans. These compounding consequences of drought: water restrictions, xeric gardening practices, and reduced health of urban vegetation, further exacerbate urban warming and energy demands. Reintegrating stormwater into the urban environment, may help to modulate the effects of urban warming, while also improving stream ecology and conserving valuable potable water resources. However, the climatological implications of water scarcity, stormwater reintegration, and changing irrigation practices, for urban climate are often ignored in urban scenario and mitigation modelling research.

The objective of this research is to understand the cooling potential of irrigation on local-scale air temperature in a mixed-residential environment. The analysis is conducted using the Town Energy Balance (TEB) model. We used TEB’s built in irrigation scheme to test the cooling potential of a series of irrigation regimes during a heatwave case study. Hypothetical scenarios (where large volumes of water were used) suggest the average maximum cooling associated with irrigation during the 2009 heatwave was up to 3.7 °C at 3pm, and 1.1 °C at 5 am. It was also found that night-time irrigation (11pm – 5am) was more effective than daytime irrigation (11am – 5pm) at reducing exposure to adverse heat health conditions (daily average air temperature > 34 °C). Overall, irrigation and stormwater reintegration (processes which also have ecological benefits) show potential to cool daytime local-scale climate in suburban areas during heatwave conditions.

Rain Water Catchment Design Applied to Educational Centers in Mexico

Jesús Gómez Velázquez, Abel Antonio Morales Santiago, Rosa Maria Velázquez Sanchez, Armando Pérez Delgadillo

Universidad Autónoma Benito Juárez de Oaxaca, Mexico

Rain water is a constant resource that is not well approached. We know that most of the major cities have periods of rain which have become random because of the climate change. In Mexico, this causes a problem of imbalance in the relationship between the rain water falls and the adaptation from the cities to manage this misunderstood resource. This problem raises when the drainage facilities got covered by garbage in the streets, which is another problem in Mexico. This causes flooding in cities and an unhealthy environment. So, Why not apply a design to take advantage of this situation instead of continue with this matter?

Water Catchment has been an ancient practice of our Mayan forefathers. They used to live in a peninsula with calcified water, so they need to create a system to take advantage of the rain water, the same that actually is used in some houses in this peninsula and have become a very useful practice along Mexico.

By the other side, most of the schools and public government institutions along Mexico have a lack in water for human use. The water use in this institutions is simple and defined by use in restrooms, green areas irrigation and cleaning.

There are many filter water systems, some of them may be used to rain water applied to this use in institutions. Then this study expect to look for the appropriated system for rain water catchment in public institutions which will be economically feasible, environmentally sustainable and socially applicable in Mexico.

Keywords: Rain Water, Catchment, Educational Centers


Passive irrigation of street trees to improve tree health and support urban cooling

Andrew Coutts1,2, Stephen Livesley3, Christopher Szota3, Jasmine Thom1,2

1Monash University, Australia; 2CRC for Water Sensitive Cities; 3University of Melbourne, Australia

Urban vegetation is commonly identified as a key measure for mitigating urban heat and improving human thermal comfort. Trees are particularly effective as they act as a conduit for water loss from the soil to the atmosphere via transpiration and they provide shade. However, to be most effective, trees need a healthy canopy and be actively transpiring. At times, high rainfall runoff and limited infiltration from extensive imperviousness may reduce water availability in urban areas, and when combined with drought conditions and water restrictions, leads to a water deficit. This compromises the ability of trees to transpire at maximum rates and provide cooling. Further, high heat loads and greater vapour pressure deficits (VPD) in urban areas places greater evaporative demand on trees. Limited water availability can compromise tree canopy health, leading to leaf loss and reduced shading potential.

In partnership with the City of Monash local government authority, we explored the role of passive irrigation on soil water availability and water use of street trees in Melbourne, Australia. Two different passive irrigation kerbside treatments were retrofitted adjacent to established trees (Lophostemon Confertus) to divert stormwater runoff from the road directly to the root zone of the trees in a bid to increase soil water availability. The study aims to explore the effectiveness of the passive irrigation design, changes in soil moisture, and subsequent variations in tree water use. Over the 2013-14 Austral Summer (prior to treatment installation) and over the 2014-15 Summer, we established an extensive field campaign to observe the effects of the passive irrigation treatments. Soil moisture was monitored at each tree where the passive irrigation kerbside treatments were installed, along with several control trees. Tree water use was measured using sap flow sensors. Meteorological variables were also observed at the site including air temperature, VPD, solar radiation, mean radiant temperature, wind speed, soil temperature and CO2 concentrations, in order to consider their influence on tree water use.

Results will be presented for the 2013-14 and 2014-15 Austral Summers, with a particular focus on extreme heat conditions when air temperature can exceed 40 °C in Melbourne and VPD is high. Results will demonstrate the effectiveness of the passive irrigation treatment designs on soil water availability and tree water use. The results from this study will help inform the roll out of passive irrigation treatments to support tree health throughout Melbourne. The important dynamics of tree water use under changing soil water and meteorological conditions can help inform urban land surface schemes in which modelling the latent heat flux proves particularly challenging.