Spatio-Temporal Analysis of UHI using Geo-Spatial Techniques: A case study of Ahmedabad
Keywords: Urban Heat Island (UHI) , Mono window algorithm, Land Surface Temperature (LST), NDVI, Urban Thermal Field Variance Index (UTFVI)
Abstract. As per the current estimates, nearly half of the world’s population lives in the cities, by 2030 it is calculated to increase to 70%. This calls for a need of more sustainable structure in the urban areas as to support increase in the urban population. Urban Heat Island is one such conspicuous phenomenon which has its significance at local regional and also at the global levels. It is a microscale temperature variation between urban and rural areas, in which urban area are warmer compare to surrounding rural area. The temperature difference between the urban and the rural areas are usually modest, averaging less than 1°C, but occasionally rising to several degrees when urban, topographical and meteorological conditions are favorable for the UHI to develop. It is defined as the phenomena where in the occurrence of surface and atmospheric modifications due to the urbanization causes modification in the thermal climatic conditions which results into warmer areas as compared to the surrounding non urbanized areas, particularly in night. In that case urban built forms such as buildings, roofs, pavements etc. absorb more solar heat/radiation and remain warmer throughout the day time and slowly release energy during night time. The two major causes are rapid urbanization and anthropogenic heat generated due to transport and industrial activities. Urban Heat Island is a crucial subject for global environment. Urbanization has significant effects on local weather and climate. Among these effects one of the most popular is the urban heat island, for which the temperatures of the central urban locations are several degrees higher than those of nearby rural areas of similar elevation. Satellite data provides important inputs for estimating regional surface albedo and evapo-transpiration required in the studies related to surface energy balance.
The phenomenon of UHI affects environment and population in so many ways it can also be considered as an active element that cause vulnerabilities to human health, the marginal population affected largely as the natural environment is their only home or their main shelter. Furthermore elderly people also affected in greater amount as their weakening immunes system. Major effects of UHI on environment include: a) Air Quality, b) Energy consumption and c) Human health.
To study the causes and effect of UHI of any urban area, the first step is to demarcate the spatial distribution of UHI and its intensity over different time period of the day as well as difference in the temperature of urban area with the surrounding rural areas. Secondly, study of land use land cover change in the area also helps in identifying causes of heat accumulation for particular region. After marking up of intensity, analysis of different zones for understanding the relationship between UHI and urban morphological features can be done which further became suggestive towards planning of urban center that mitigates the effect of UHI. Mainly two approaches are there to demarcate UHI study as:
– Field data collection and observations
– Remote sensing data analysis
For a long period of time observations from interior of the city and outwards of it can analyze by a climatic methods, by observing many days as well as many times of a day continuously to analyze the daily variation law of the heat island effects. As the city is for its developmental approaches may cover an area of hundreds of square kilometers, the ground observation data is not able to provide enough detail about the urban heat island distribution characteristics. The most precise method is the Satellite Remote Sensing method. The UHI phenomenon can be analyzed by using the thermal infrared data obtained meteorological satellite sensing. The atmospheric attenuation can be corrected for the remote sensing data by use of meteorological soundings and ground observation data. Ideally the heat island effect over a city is not same for any other city.
Satellite images from AVHRR Advanced Very High Resolution Radiometer) or ENVISAT AATSR provides thermal infrared data and comparatively easy to acquire, process and analyze. In the case of Ahmedabad city, land cover changes over the time is to be studied by classifying the image and then temperature can be derived by using a quadratic regression model from Malaret at al. (1985). Band 6 produces the images that show the relative difference emitted thermal energy that correlate in part with the effects of solar heating on surface of varying composition and orientation. The surface temperatures are suitable to detect UHI at Urban canopy level. Nichol (1996) found that surface temperatures extracted are moreover similar to the actual ambient air temperatures recorded.
The paper has narrated analylitical framework on which the research has been carried out. The result derived on Land Surface Temperature variation causing Urban Heat Island, its relationship with the land use land cover. A time series data has been used. Authors are thankful to Ms. Darshana Rawal, Ms. Pallavi Knahdewal and Mr. Hardik Panchal.