بررسی الگوی ناهمگنی محیط حرارتی شهر و ارزیابی اکولوژیک آن در منطقۀ شهری کرج

در اکوسیستم‏های شهری، بررسی الگوی ناهمگنی محیط حرارتی و عوامل موثر بر آن در ارزیابی اکولوژیک و آسایش حرارتی محیط شهری ضروری است. از این‏ رو هدف پژوهش حاضر بررسی ناهمگنی مکانی جزایر حرارتی و برودتی منطقۀ شهری با تاکید بر نقش پوشش اراضی در ایجاد آن و همچنین ارزیابی اکولوژیک محیط حرارتی است. از آنجا که اثرات جزایر حرارتی در فصل گرم سال اهمیت بیشتری دارد، این مطالعه در تیرماه سال 1399 با استفاده از تصاویر ماهواره لندست‌8 در منطقۀ شهر کرج انجام شد. در این پژوهش، نقشۀ دمای سطح زمین با استفاده از الگوریتم تک‌پنجره و نقشۀ پوشش اراضی با روش حداکثر شباهت استخراج شد. سپس برای تعیین نقش پوشش اراضی در ایجاد جزایر حرارتی و برودتی، شاخص سهم برای سه طبقۀ عمدۀ پوشش اراضی محاسبه گردید. در ادامه، از نمایۀ واریانس زمینۀ حرارتی شهر (utfvi) به‏منظور ارزیابی اولیۀ اکولوژیکی استفاده شد. نتایج محاسبۀ شاخص سهم نشان داد اراضی بایر و رهاشده بیشترین نقش (1.53) را در تشکیل جزیرۀ حرارتی داشت، درحالی‏ که کمترین سهم به‏ترتیب مربوط به اراضی انسان‏ساخت (0.2) و طبقۀ باغات، فضای سبز و کشاورزی (0.76) بود. همچنین یافته‏ها نشان داد نقاط داغ حرارتی در شهرک‏های صنعتی حاشیه‏ای و اراضی بایر (حوالی فرودگاه پیام) در جنوب‏ منطقۀ مورد مطالعه واقع شده‏ است. نتایج نمایۀ واریانس زمینۀ حرارتی شهر نشان داد مناطق داخل شهر (به‏ویژه بافت قدیمی و متراکم) از لحاظ اکولوژیکی و آسایش حرارتی، وضعیت بهتری از مناطق تازه‏توسعه‏یافتۀ حواشی شهر دارند. به‏طور کلی از نتایج این پژوهش می‏توان در فرایند برنامه‏ریزی شهری، به‏منظور بهبود وضعیت اکولوژیک شهر از دیدگاه حرارتی، استفاده کرد.

Heterogeneity of urban thermal environment and its ecological evaluation in urban region of Karaj

Temperature is one of the critical environmental parameters which is spatially heterogeneous, affecting biological, physical, and social interactions within an urban ecosystem. Therefore, the main purpose of this article is to investigate the contribution of various land covers in forming urban heat islands (UHIs) and urban cool islands (UCIs) and determining the pattern of UHIUCI in an arid and semiarid urbanization region of Karaj, Iran in July 2020. To achieve the goal, initially, the land surface temperature (LST) was retrieved using monowindow algorithm; maximum likelihood method was applied to generate the land cover/land use (LULC) map using Landsat 8 OLI TIRS data. Then, the contribution index (CI) of each LULC in creating UHI and UCI was calculated. The thermal environment of the city was evaluated using Urban Thermal Field Variance Index (UTFVI). The results showed that builtup surface (0.2) and green space (0.76) contributed the most in creating UCI, while the barren cover played a major role (1.53) in creating UHI. In addition, the urban hot spots were seen in the industrial zone and bare land (adjacent to the Payam airport) in the southwest of the region. The UTFVI analysis showed that the central areas of the city (old and dense residential areas) were ecologically better as compared to the urban periphery. Therefore, it is necessary to implement mitigation strategies in the marginal areas of the study area. In general, the results of this research can be helpful in urban planning to moderate urban temperature in ecologically stressed zones.Extended Abstract1IntroductionTemperature is one of the critical environmental parameters affecting biologicalphysicalsocial interactions of the urban ecosystem. In urban areas, the temperature is spatially heterogeneous due to extreme variation of land cover. One of the most wellknown temperaturerelated phenomena in cities is the formation of urban heat islands (UHI), which endangers thermal comfort and ecological status. Moreover, determining hotspots with the aim of reducing the temperature is one of the important measures to maintain the temperature balance of the city. Current knowledge of the urban thermal environment is principally based on land surface temperature (LST) maps retrieved from satellite thermal sensors. Therefore, the main purpose of this paper included; 1) extraction of land surface temperature (LST) using monowindow (MW) algorithm from Landsat 8 satellite image; 2) determination of urban heat island and urban cool island (UHI and UCI) and thermal hotspots based on LST; 3) investigation of the relationship between land cover/ land use (LU/LC) with UHI and UCI by calculating contribution index, and 4) ecological assessment of the thermal environment using Urban Thermal Field Variance Index (UTFVI). 2Materials and MethodsUrban region of Karaj includes a group city, a climatically arid and semiarid region, which was taken as the study area in current research. Since the effects of the thermal environment are more important in the hot season, this study was conducted in July 2020. The land cover/ land use map was generated using the maximum likelihood method in five classes of builtup, agriculture, orchards and green space, rangeland, bare land, and forest. The LST was retrieved using a monowindow algorithm to identify UHIUCI. In addition to the identification of UHI and UCI, thermal hot spots were also determined across the city. Then, the contribution index was calculated in order to quantify the role of the three major land cover classes of builtup, vegetation, and bare land in forming UHIUCI. After all, we used the Urban Thermal Field Variance Index to evaluate the ecological circumstance of the environment. Based on the thermal threshold of the UTFVI in terms of ecology and thermal comfort, the area was divided into three classes called good, normal, and poor. 3 Results and DiscussionThe overall accuracy and kappa coefficient of the land cover map were 87% and 83%, respectively. The accuracy of the LST map was evaluated using the data of two meteorological stations, indicating MAE and RMSE of 0.4 and 0.44, respectively. The results of this research showed that builtup cover (0.2) and agriculture and green space (0.76) contributed the most to creating UCI. However, the barren cover and abandoned farmlands played a major role (1.53) in creating UHIs due to the lack of shading effects of the building and evapotranspiration compared to builtup area and vegetation cover. Surprisingly, UCIs were formed in the densely builtup area and green spaces. The Urban Thermal Field Variance Index analysis showed that the inner and central areas of the city (old and dense part) were better in terms of ecological circumstances and thermal comfort than the newlydeveloped areas on the outskirt of the city. In addition, hot spots were seen in the industrial zone and bare land (adjacent to the Payam Airport) in the southwest of the region. In other words, the pattern of LST in the study region, which has an arid and semiarid climate, is different from cities located in humid climate with abundant vegetation cover such as European and tropical cities. 4 ConclusionIn general, it can be concluded that the monowindow algorithm is an appropriate method to retrieve LST in urban areas. In the study area, which is climatically located in an arid and semiarid zone, dense manmade areas were cooler than marginal and lowdensity manmade cover during the day in the warm season. The initial ecological assessment was performed using Urban Thermal Field Variance Index. In order to improve the ecological situation in poor areas, temperature mitigation measures such as developing green space can be used according to cost analysis and expert opinions. As a recommendation, establishing complementary research focusing on the arrangement and configuration of the different landscape elements can lead to providing a more accurate analysis of the thermal environment pattern.