Study of Urban Heat Island Phenomena for Baghdad City using Landsat- 7 ETM+ Data
Alaa M. Al-Lami
Department of Atmospheric Science/ College of Science/ AL-
Mustansiryah University/ Baghdad/Iraq
Diyala Journal For Pure Science - Diyala University - April 2015 - Volume: 11 - Issue: 2 - Pages: 82-98:
Abstract
Urban areas have well documented effects on climate, such as the urban heat island effect, reduction of wind speeds, enhanced turbulence and boundary layer heights, and changes in cloud cover and precipitation. The objective of the present work is to study the Urban Heat Island in Baghdad city, by finding out the spatial variation of four indicators, Normalized Difference Vegetation Index (NDVI), soil adjusted vegetation index (SAVI), Leaf Area Index (LAI) and Land Surface Temperature (LST) using Landsat-7 ETM+ imagery. Land use/ Land cover map for the study area were retrieved using supervised classification by ERDAS(9.1) software. This paper classifies Baghdad city area as five type of land built up area with trees, built up area without trees, water area, low vegetation and high vegetation. The results shows that the maximum difference of land surface temperature between the built-up and the surrounding area reach to 11.97 oC. The maximum value of the land surface temperature appeared in the built-up area without trees in the downtown of the city (29.96 oC ),while the water and high vegetation area show 17.93 oC of minimum land surface temperature values. The result of the spatial analysis of the NDVI, LAI and SAVI indicated that there is a negative correlation with the built-up area and positive correlation with vegetation area.
Key words: Urban heat island, land surface temperature, normalized difference vegetation index, Landsat-7, Land use land cover, Baghdad.
دراسة ظاهرة الجزيرة الحرارية الحضرية في مدينة بغداد باستخدام بيانات القمر الصناعي لاندسات-7 (+ETM)
علاء مطر اللامي
قسم علوم الجو - كلیة العلوم - الجامعة المستنصریة - بغداد -العراق
مجلة ديالى للعلوم الصرفة - جامعة ديالى -المجلد 11 - العدد 2 -أبريل 2015 - ص ص 82 - 98 :
الملخص :
أن للمناطق الحضرية تأثيرات موثقة على المناخ مثل ظاهرة الجزيرة الحرارية الحضرية وتقليل سرعة الرياح والزيادة في الاضطرابات الجوية وارتفاعات الطبقة المحاددة والتغيرات في الغطاء الغيمي والتساقط. أن الهدف من هذا البحث هو دراسة ظاهرة الجزيرة الحرارية في مدينة بغداد من خلال حساب التغيرات المكانية في أربع مؤشرات وهي مؤشر الفرق الخضري (NDVI)، مؤشر مراعاة التربة للنبات (SAVI)، مؤشر الكثافة الخضرية (LAI) ودرجة حرارة سطح الأرض (LST) باستخدام بيانات القمر الصناعي لاندسات-7 (ETM+).في هذه الدراسة تم وضع خارطة للاستخدامات الأرضية في مدينة بغداد باستخدام تقنية التصنيف المراقب في برنامج الايرداس 9.1، حيث تم تصنيف منطقة الدراسة إلى خمسة أصناف وهي مناطق حضرية بدون أشجار، مناطق حضرية مع الأشجار، مساحات مائية، غطاء خضري قليل وغطاء خضري كثيف. أظهرت النتائج أن أقصى فرق لدرجة حرارة سطح الأرض بين المناطق الحضرية والمناطق المجاورة لها وصل إلى 11.97 درجة مئوية. أقصى قيمة لدرجة حرارة سطح الأرض ظهرت في المناطق الحضرية بدون أشجار وكانت 29.96 درجة مئوية، بينما سجلت المساحات المائية والمناطق الخضراء أقل قيمة وكانت 17.93 درجة مئوية. نتائج التغير المكاني للمؤشرات (NDVI,SAVI and LAI) أشارت إلى علاقة عكسية مع المناطق الحضرية وعلاقة إيجابية مع المناطق الخضرية.
الكلمات المفتاحیة: الجزیرة الحراریة الحضریة، درجة حرارة سطح الأرض، مؤشر الفرق الخضري, لاندسات- 7، الاستخدامات الأرضیة، بغداد.
Introduction
Urban areas are one of the most obvious examples of human modification of the Earth’s surface. Despite covering only 1.2% of the Earth’s surface [1, 2], it is estimated that in 2003 about 48% of the World’s population resided in urban settlements [3]. By 2030 it is expected that 61% of the World’s population will be living in urban areas. Urbanization is an extreme example of human land use modification, since it radically alters the physical properties of the Earth’s surface and may also affect the thermal, radiative and aerodynamic character of the surface [4]. Urban areas have well documented effects on the environment, such as changes to the local winds and turbulence [5], changes in cloud cover and precipitation [6] and the urban heat island phenomenon.
The urban heat island (UHI) is a particularly important example of how the urban area can influence climate and the most obvious climate manifestation of urbanization [4, 7 and 8]. It is caused by a variety of factors which contribute to higher temperatures in the urban centre, either of the surface or the atmosphere, compared to the surrounding rural areas. The UHI can be particularly significant in exacerbating the effects of summer heat-waves, with consequent problems such as increased mortality [9] and marked air pollution events [10]. A UHI can also affect the regional scale flow by means of a thermodynamically driven circulation pattern [11, 12 and 13] caused by the UHI modifying the local pressure field and the stability. Under best conditions the Urban Heat Island may be up to 10-15 oC [14].
Urban heat island mainly appeared in the spatial distribution of land surface temperature (LST), which is governed by surface heat fluxes and obviously affected by urbanization [15,16]. Consequently, acquiring LST is the primary and key step to the urban heat island analysis. The LST difference is usually larger at night than during the day. Seasons influence the LST difference, too. Heat island cities located in the mid latitude usually are strongest in the summer seasons. In tropical climates, the day season may affect the large island magnitudes. Thus, there are differences in day, night and seasonal measurements of LST.
The concept of urban heat island was described by Luke Howard [4], and since then this research topic has received ever more attention. Recently, with the development of society and acceleration of the process of urbanization, the urban heat island has become more and more significant and has had severe impact on urban development and human living environments.
Urban Heat Island can be marked by in situ measurement of the air temperature in the city or by using remote sensing techniques, which needed information about the overlying atmosphere and the surface heat radiation flux properties. Several relevant researches i.e. [17- 19], estimated Urban Heat Island using remote sensing techniques.
The aim of the present study is to assessment the specific characteristics of Urban Heat Island in Baghdad city, by analyzing the spatial variation of Normalized Difference Vegetation Index (NDVI), Leaf Area Index (LAI), Soil adjusted vegetation index (SAVI) and Land Surface temperature. (LST), using Landsat-7 ETM+ imagery and digital image processing techniques.
Area of study
Baghdad is the largest and most heavily populated city in Iraq with an area of about 900 Km2 , whereas the total area of Baghdad Governorate reaches 5159 Km2 [20]. The estimated population is in the order of 6 million [21]. Baghdad lies in the middle of Iraq within the Mesopotamian Plain. The Tigris River passes through the city dividing it into two parts; Karkh and Rasafa. The area is bounded from the east by Dyala River which joins the Tigris River southeast of Baghdad. The Army Canal, 24 km long, recharges from the Tigris River in the northern part of the city and terminates in the southern part of Dyala River [22]. The study area is restricted to latitudes (33º 15′ -33º 28′N) and longitudes (44º 15′- 44º31′E ) with an area of about 540 km2 approximately( Figure 1). Several land uses can be noticed from Baghdad maps, specifically those prepared by Baghdad Environment Directorate [23], According to these statistics the percentages of the urbanized, agricultural, and industrial areas from the total area are 72.69%, 25%, and 2.31% respectively.
Material and Methods
For the present study the following methodology is adopted (Figure 2) ,which involves satellite data collection, classification of the imagery, development of land use/cover maps, preparation of (NDVI) maps, (LAI )map, (SAVI) map and retrieval of (LST) maps. These are briefly outlined here.
Data preparation
Landsat satellite data of 2001 for the study area has been downloaded from United States Geological Survey (USGS) Earth Explorer website. All the remote sensed data were preprocessed and geometrically corrected in the datum WGS84 and projection UTM zone N38. The details of the satellite data collected are shown in the Table.1. The multispectral image data consists of seven spectral bands and has a spatial resolution of 30 meters for the reflective bands and 60 meters for the thermal bands. The overpass time is about 10 o'clock A.m., and the sun elevation angle is 48.035°.
The data pre-processing is performed using ERDAS Imagine 9.1 software. Each ETM+ file is composed of the independent single-band images. Thus, it is firstly used to combine the single-band images to a multi-bands image of ETM+ using a layer stacking tool. Secondly, the geometric correction was made for the ETM+ images, by which each point on the image would have only latitude or longitude geographical coordinates. This is the most important step in pre-processing. Thirdly, after geometric correction, the image subset tool was used to clip our study area. After the pre-processing, the study area images of Baghdad were obtained for the data processing and analysis of urban heat island.
Development of Land use/Land cover map
Using bands 1-6 and 7 of the pre-processed images the land use / cover pattern was mapped by supervised classification with the maximum likelihood classification algorithm of ERDAS software. The Five classes considered for the study area are built up area with trees, built up area without trees, water area, low vegetation and high vegetation. The supervised classification involves pixel categorization by Training, Classification and Output.
Conclusions
In this work Landsat ETM+ images of Baghdad city were collected from USGS earth explorer web site. The land use land cover maps of the study area are developed by supervised classification of the images. Land use classes have been identified as Built-up area with trees, built-up area without trees, Water area, low Vegetation and high vegetation land. LST, NDVI, SAVI and LAI, Four indicators are using to evaluate the urban heat island effect for Baghdad city. From the LST images it is clearly understood that surface temperature is more in urban area compared to rural areas, the difference reach to 11.97 oC, the results also
shows that the LST is strongly and negatively correlated with NDVI, SAVI and LAI. The physical properties of urbanization structures play a main factor for transferring the land surface thermal properties to the air through the vertical exchange of surface radiation heat flux, when compared to the sub- urban surrounding areas for the difference in temperature, where the vegetation cover and water bodies play a major role in reducing surface temperature.
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