عنوان مقاله [English]
Nowadays, many factors such as destruction of the urban green areas and increasing the environmental pollutants in urban areas cause the solar radiation confinement and formation of urban heat island phenomenon. This has dangerous consequences, such as stagnation and lack of the air flow, transmission of air pollution and dispersed toxic particles in urban areas. The main purpose of this research is to analyze the vegetation status and land surface temperature in Urmia city, investigate the formation of urban heat islands and analyze their temporal and spatial variations. For achieving these purposes, Landsat 4 and 8 satellite images for 1989 and 2017 years were used. In this study, after applying the necessary corrections to the images and training them, land use change detection was accomplished. By using the red and the near infrared bands of the images, the vegetation index (NDVI) was calculated and then. Ground surface temperature (LST) was calculated by the image thermal bands. The results of this research showed that the temperature of Urmia city and its suburbs increased during the 28 years (1989-2017). It was also found that increasing temperatures within Urmia city was mainly due to the loss of the vegetation areas and to the conversion of the green area to urban lands by urbanization processes. The results showed that the vegetation index (NDVI) in Urmia city decreased as it reashed to-0.053 from 0.023 in the wasteland, which indicate the disappearance of thin green cover (about 9.24% of the vegetation cover of Urmia city disappeared during 1989–2017).
In the past decades, the rapid growth in the industrialization, the destruction of the urban green cover, and increasing environmental pollutants in the urban areas especially big cities, have confined the solar radiation in their atmospheres. This causes to trap the solar radiation throughout the day and reflect during the night (the heat islands). As a result, the natural process of cooling the earth's surface occurs less slowly during the night. In fact, the urban heat islands are discrete parts whose temperatures have risen throughout the urban surface and become more intense over the last century. The heat islands cause changing the pattern of winds in the urban areas and also suburbs, and occuring dangerous consequences such as stationary air or transmission of air pollution and dispersed toxic particles to different urban areas. These can ultimately disrupt the comfort of the urban dwellers. Recently, one of the major issues in the cities is how heat islands are formed and what their effects are on urban environment, as well as being aware of the vegetation status and urban surface temperature, which are two main goals of this research. The use of remote sensing imagery can provide powerful tools to evaluate and monitor environmental changes, including land use changes and vegetation covers, that allow the evaluation of the changes and calculation land surface temperature by image processing techniques and multi-temporal satellite image comparisons. In this study, Urmia city was selected to study and evaluate vegetation cover changes in relation to its heat islands.
To detect land use changes in Urmia city during the period 1989–2017, Landsat satellite images of the city were used and the coresponding land use maps were extracted. In the next step, land use changes were calculated and revealed in three classes of wasteland, vegetation and urban. It was found that most of the land use changes occurred in the urban areas. Vegetation and waste lands have been converted to urban lands about 1292 and 2721 ha, respectively. The research method consists of three general sections: The first part is to detect vegetation cover changes. To achieve this, ENVI5.1 software was used. In this software, after separating the study area, the radio metric and atmospheric corrections were performed. Then, the images were classified by maximum likelihood algorithm. To detect land use changes, ICW and TCW tools were used. In the second part, the vegetation index was measured using NDVI tool in Envi software. The vegetation index tool uses two bands, the red and near infrared bands, of Landsat satelite images. In the third part, ARCGIS software was used to obtain ground surface temperature (LST). To produce the surface temperature map of the study area, the termal bands of each period were used and converted into radians. Then the luminance temperatures recorded in the sensor were calculated by the corresponding equations. Finally, after determining the total temperature of the study area, an overlap was made between the temperature of each period and Urmia city map.
Results and discussion:
The temperature map obtained from GIS analyzes for year 1989 showed that the temperature range between 24–33 °C was extended to the subrbs of Urmia and the vegetation-covered areas. However, almost all parts of the city except the central part of it had average temperatures of 33-37 °C. The waste lands of Urmia were also within the temperature range of 37-42 °C. The temperature map of year 2017 showed that the cooler temperature range coverage was more scattered throughout the city as it covered many parts of the city, but the heat islands that were created in the city is important. The cool temperatures of 1989 have given way to warm temperatures in 2017. Some waste lands that were in year 1989 in range 37-42°C are now in the range 42-50°C. Inside the city there are areas with temperatures of 37-42°C and even 42-50°C. The vegetation index of the city has been steadily declining, reaching 0.2 from 0.4. Although some activities to improve and enrich the green space of Urmia city were carried out, but the vegetation area was reduced by 0.2 with Urmia urban growth and the city vegetation index decreased by 0.074 and reached to -0.051. The result of these changes was the loss of thin vegetation due to human activities. The calculation of the vegetation index of the study area showed that in the early stages of development of the city, many lands with green cover were destroyed and the corresponding cover was disappeared.
The results of the research showed that during the period (1986-1999) the temperature of Urmia city increased. It was found that the increase of temperatures within Urmia city is mainly due to the loss of the vegetation cover and the conversion of the green lands into urban areas. It was shown that the vegetation index in Urmia city decreased as it reached -0.051 from 0.023 in the waste lands that indicating the disappearance of thin green cover. In other words, in the areas that were previously covered by vegetation covers and had a cooling effect, by removing the cover and replacing the urban areas (9.24% of the vegetation cover of Urmia city disappeared during 1989–2017), the temperature has increased markedly and caused creating the heat islands. In order to reduce the environmental impacts and achieve sustainable development of Urmia city because of the heat islands, it was suggested that the expansion of green spaces in urban and industrial and forestry areas should be carried out for example in the urban areas of the east and south of Urmia. In order not to disturb the natural flow of the wind, it was suggested that Urmia urban growth should be prevented in the high quality lands of the north and east of the city and be reviewed the urban concession policy.