Effect of Vegetation on Microclimate of Residential Blocks

Abstract

In the fast urbanizing globe, where concrete built-up is making a way through the green cover, it is imperative to study the impacts of such urbanization and design the suitable counter measures vital for a sustainable development. Assessing the effect of vegetation on microclimate of residential blocks would help in gauging the ecological benefits of vegetation, relevance of properly planned urban green spaces, suitable measures to optimize the urban vegetation structure, and further strengthen ecological construction. A growing number of cities around the world are developing strategic and integrated green infrastructure strategies to cool urban climates.
Urban Heat Islands, due to the changing built environment and lifestyle, and the effect of reduced vegetation on built environment are one of the major issues that need to be thoroughly researched and analyzed. Healthy lifestyle, comfortable living conditions, energy-efficient performance of the residential blocks might greatly be affected by amelioration in microclimate as a result of vegetation cover and practices prevalent in an area. Identifying the role of vegetation on ambient conditions has been undertaken in this study to evaluate the benefits of vegetation towards cooling. Proactive measures need to be undertaken in the process of development of the countries, curtailing adverse impacts at later stages. This study aims to pave a path for sustainable development for Indian context where the research area is at very nascent stages and for German context where the research is in process of implementation.

In Germany, several cities including Stuttgart, Bremen, Dresden, Nurnberg and Berlin are preparing, or have developed, strategies for climate change adaptation and mitigation (J. Baumueller, N. Baumueller, 2011). One of the key objectives of Stuttgart’s strategy is to accelerate the movement of cool air at night time through the urban areas by expanding open spaces and increasing vegetation cover. No massive construction is permitted in the areas critical for maintaining air-flow, the clear-cutting of large trees is prohibited, and restrictions have been placed on laying of impervious surfaces. As a result, 39% of the city’s surface area is protected by nature conservation orders (A. Kazmierczak, 2010). The cold cities like Paris witnessed a death of 15,000 people due to 2003 European heat wave, and in Western Europe the total toll went up to 70,000. This is because of the zero capacity to withstand the rising heat outside as buildings in colder countries are not equipped with cooling measures. Countries like India experience extremely sweltering summers, sometimes temperature rising even up to 48 deg C. The key highlight of the study is identifying the role of vegetation in a hot and cold country. After uncontrolled urbanization in the city of Roorkee, India especially in the residential properties, evident climate change has become an issue of utmost emergency. But it is alarming to see how cold places like Munich, Germany also experienced one of the hottest summers in the year 2014. India, which is principally a hot country and Germany, which is predominantly a cold country have been taken as the study areas.

Based on climatic variances, difference in built typology and landscape structure of residential blocks, sites were selected in Roorkee, India and in Munich, Germany. The study focuses at analyzing the role of vegetation in curbing the climate change issues for both tropical and temperate climates. An effort has been made towards identifying, structuring and analyzing different vegetated scenarios on three residential sites selected in Roorkee, India and a site in a busy city center of Munich, Germany. Vegetation can be beneficial for lowering temperatures in hotter climate which will eventually reduce the cooling loads and be energy efficient. Whereas, it lowers cooling loads for hot areas, it might as well cause increase in heating load for colder areas. This study although confines to determining the effect of vegetation in summer season in both temperate type and tropical type countries. Therefore, “Effective vegetation is a much needed solution in hotter climates but it is also a very vital requirement for the colder regions, as there has been a considerable increase in the hot spots in summer seasons”.

Extensive literature review and relevant case studies have been foundational in conducting the study. A total of four cases have been designed each for the three sites in Roorkee. While total seven cases have been designed for the site in Munich. A thorough site survey to gather physical data in terms of built, paved and vegetated (BPV) surfaces has been conducted and the base maps for input area files have been generated. An extensive work has been carried out to measure the physical information of trees on site (LAI, Height, canopy size) and field survey has also been conducted to assimilate micro climate data from the site. The automatic weather station data has been collected as macro climatic data for the simulation processes. The Indian trees have been modelled by calculating leaf area density through integral equations. The cases have been designed using the software ENVI-Met (a three-dimensional microclimate model). The design decision for all the cases are based on the base case (existing scenario) simulations run on ENVI-Met which helped in identifying the nature of microclimatic conditions on site and the existing hot spots. One of the cases has been designed by integrating decisions based on vegetation indices like biotope area factor and green plot ratio. This gives a scope of different types of landscaped scenarios to be evaluated for their thermal implications.

Various configurations on site (roads, built and open canyons, specific points) have been selected and studied in detail to elaborate the potential of cooling by increased and engineered vegetation scenarios.
The results have been analyzed using the visualization software LEONARDO. Based on the results it is seen that the vegetation has both positive and negative effect on the microclimatic parameters. The results have been analyzed in detail to establish the cooling potential of vegetated areas in terms of Air temperature (Ta) lowering. Effect on Relative humidity (Rh) and Mean Radiant Temperature (Tmrt) has also been analyzed to support the study, the two being very crucial parameters in determining the outdoor thermal comfort. The analysis has been carried out for 15:00 hour in detail but 3:00, 9:00 and 19:00 hour have also been analyzed for air temperature modification.
Maximum cooling of air temperature by 2.25 deg C is observed for Type 1 in Roorkee. It exhibits more air temperature reduction than Type 2 and Type 3 (least being for Type 3). This is attributed to compactness of built layout and least exposure to harsh solar radiations due to the orientation of the site combined with vegetation present on the site. It is observed that temperate area like Munich displays lesser cooling potential (maximum 1.70 deg C despite increased wetness of soil) than tropical area like Roorkee. Tmrt shows significant reduction in all the cases (as high as 35 deg C at peak hour i.e. 15:00 hour for some of the cases) indicating effective shade effect of trees. Relative humidity is seen to rise with increase in vegetation although it is considerably offset by the Ta and Tmrt reduction. A night time UHI effect is also seen in few cases due to increased vegetation on site but it is not substantially high as compared to cooling at daytime.

Based on the design decisions and results, various measures have been interpreted for effective vegetation on a site. It is noticeable after the detailed analysis that impact of vegetation is largely location specific and is strongly driven by the factors like wind speed, wind direction, wetness of the soil, time of the day and correct siting of it. The validation of simulation results is also done and a highly strong correlation coefficient (R) of more than 0.95 is found for all the cases except for Type 3 which has a correlation coefficient (R) between 0.83- 0.89 (also accepted as a strong correlation). Thereby, a study on the effect of “vegetation will not only help us gauge the cooling potential in hot countries, it will also be a very crucial tool towards building resilience in the summer seasons to mitigate the rise in UHIs in the colder regions and in reinforcing more urban cool islands in the cities. ”