نوع مقاله : مقاله پژوهشی

نویسندگان

• فاطمه قربانی ¹
• حسن سجادزاده ²

¹ دانش‌آموخته کارشناسی ارشد، گروه شهرسازی، دانشکده هنر و معماری، دانشگاه بوعلی سینا، همدان، ایران.

² استاد گروه شهرسازی، دانشکده هنر و معماری، دانشگاه بوعلی سینا، همدان، ایران.

چکیده

با توجه به تغییرات اقلیمی و همچنین توسعه های شهری بدون توجه به کیفیت زیست محیطی، شکل­گیری جزیره گرمای شهری به یکی از چالش­های جدی عصر حاضر تبدیل شده است. هدف عمده این پژوهش، تحلیل وضعیت پوشش گیاهی و دمای سطح زمین برای بررسی شکل گیری جزایر حرارتی شهری در شهر کرج است. به این منظور و در راستای آشکارسازی تأثیر فضاهای سبز در محیط­های حرارتی شهر کرج، از داده­های سنجش از دور برای استخراج فضاهای سبز، دمای سطح زمین و پوشش زمین استفاده شده است. دمای سطح زمین با استفاده از تصاویر مودیس محاسبه شد. تغییرات پوشش گیاهی با استفاده از اطلاعات برداشت شده توسط سنجنده­های مربوطه ماهواره لندست 7 و 8  مربوط به سال های 2001 و 2021 استخراج شده و بعد از اعمال تصحیحات لازم و بررسی تصاویر، اقدام به شناسایی تغییرات کاربری اراضی و نوع تغییرات آن گردید. نتایج بررسی­ها نشان داد که اراضی بایر پیرامون شهر به صورت پیوسته دارای بالاترین دما و لکه های گرم هستند و محدوده­های دمایی بسیار گرم را تشکیل می­دهند. در محدوده­ مسکونی شهر نیز جزایر حرارتی بر بافت فرسوده و محدوده های متراکم منطبق است. همچنین تحلیل و بررسی نقشه­های دمای سطح زمین نشان­دهنده­ افزایش دمای سطح در شهر کرج است. درعین حال، واکاوی نقشه­ها حکایت از رابطه­ قوی بین پوشش گیاهی و دمای سطح زمین در کلانشهر کرج دارد. نقشه­های کاربری اراضی نیز نشان از کاهش مساحت پوشش­گیاهی و افزایش کاربری انسان ساخت دارد. این موضوع به طور واضح نشان­دهنده­ این است که مهم ترین عامل در گسترش جزایر گرمایی و افزایش دما در بازه­ زمانی مورد بررسی، کمبود پوشش­گیاهی بوده است. در نتیجه فضاهای سبز و پوشش گیاهی در شهرها به طور چشمگیری سبب تعدیل جزایر حرارتی شهری هستند. به این صورت که نواحی همچون اراضی بایر و نواحی دارای پوشش گیاهی کمتر، دارای دمای بیشتری نیز هستند. بنابراین گسترش بام سبز و استفاده از پوشش گیاهی سازگار با اقلیم هر منطقه به عنوان راه حلی برای کاهش جزیره گرمایی پیشنهاد می­شود.

کلیدواژه‌ها

• جزایر حرارتی شهری
• پوشش گیاهی
• تصاویر ماهواره ای
• کرج

موضوعات

• محیط زیست شهری

عنوان مقاله [English]

The impact of vegetation on urban heat island reduction in the city of Karaj

نویسندگان [English]

• fatemeh ghorbanileylestani, ¹
• Hassan sajadzadeh ²

¹ Department of Urbanism, Faculty of Art & Architecture, Bu-Ali Sina University, Hamedan, Iran.

² Department of Urbanism, Faculty of Art & Architecture, Bu-Ali Sina University, Hamedan, Iran.

چکیده [English]

Highlights

Comprehensive Analysis: This study investigates the Land Surface Temperature (LST) and Normalized Difference Vegetation Index (NDVI) over two decades (2000-2021) in Karaj, highlighting key trends.
Correlation Between Vegetation and Temperature: NDVI shows a strong negative correlation with LST, indicating vegetation’s cooling impact.
Built-Up Areas vs. Green Spaces: High-density urban areas exhibit elevated temperatures, while regions with dense vegetation are noticeably cooler.
Key Findings: The primary drivers behind Karaj’s Urban Heat Island (UHI) effect are the reduction of green spaces and the rise in urban land use.

 
Introduction
With rapid population growth, urbanization is transforming landscapes, often at the expense of natural green areas. Cities are expanding at unprecedented rates, bringing changes in land use, infrastructure, and building density. Natural permeable surfaces, such as vegetation and open spaces, are replaced by impermeable concrete structures that absorb and retain heat, leading to urban microclimate shifts.
This transformation has profound environmental impacts. One of the most concerning consequences is the Urban Heat Island (UHI) effect, where urban areas exhibit higher temperatures compared to their rural counterparts. The UHI effect intensifies as cities grow, impacting energy consumption, air quality, and public health. In this context, reducing UHI has become a priority for sustainable urban planning. Cooling strategies—especially the integration of vegetation—are essential to enhance urban resilience, adapt to climate change, and improve quality of life for city dwellers.
Materials and Methods
Remote sensing technology was used in this study as a powerful tool for analyzing urban temperature dynamics and land-use changes. Using Landsat satellite images from 2001 to 2021, we examined LST and NDVI for Karaj. Landsat imagery, with a resolution of 30 meters, was sourced from the USGS database. To ensure consistency, cloud-free images were selected from warm-season months for each year.
Data processing involved the extraction of LST and NDVI values for Karaj’s administrative boundaries. The satellite images were preprocessed, and land-use classification was carried out using the maximum likelihood approach, categorizing land into three classes: built-up areas, vegetation, and barren land. This classification provided a foundation for assessing the relationship between LST and vegetation cover.
Through correlation and regression tree models, we analyzed the interplay between LST and NDVI. By examining changes in both indices, we aimed to understand vegetation’s role in moderating urban heat in Karaj. Our two main objectives were to (1) assess LST and NDVI variations over time, and (2) explore their interdependencies to determine vegetation's influence on UHI.
Discussion of Results
The analysis reveals a clear trend of rising temperatures across Karaj over the study period. This temperature increase is strongly associated with urban expansion and the decline of natural vegetation. Our findings highlight that land-use type significantly influences LST, with barren and built-up areas having markedly higher temperatures.
Central Karaj, a dense urban area with heavy infrastructure and traffic, recorded the highest surface temperatures. These “hot spots” are concentrated around industrial areas, transportation hubs (airports, metro stations, highways), and zones with minimal vegetation. In contrast, areas with vegetation, such as parks and green belts, exhibited substantially cooler surface temperatures. This difference underscores vegetation’s role in absorbing less heat and promoting natural cooling.
The spatial distribution of LST shows the hottest zones in arid lands surrounding the city and densely built-up urban areas. The NDVI data further supports this observation; as NDVI values increase, LST values decline, illustrating a negative correlation between vegetation density and surface temperature. The correlation analysis reveals that regions with higher NDVI values, particularly in the eastern and northeastern parts of Karaj, experienced significantly lower temperatures.
The land-use maps demonstrate a significant reduction in barren and vegetated areas, accompanied by an increase in urbanized land. These patterns point to a direct relationship between declining vegetation and rising temperatures, reinforcing the critical need for green spaces in Karaj to mitigate UHI effects. Vegetation, as indicated by NDVI, plays a significant role in temperature regulation, with green areas acting as cooling zones in an increasingly built-up landscape.
Conclusions
This study underscores the pivotal role of vegetation in controlling urban temperatures in Karaj. Through detailed LST and NDVI analyses, the results confirm that vegetation coverage is inversely related to LST, with green spaces helping to mitigate the UHI effect. In contrast, barren lands and dense built-up areas contribute significantly to higher temperatures, highlighting the thermal impact of urban development without adequate vegetation.
To address the UHI issue, urban planners and policymakers should prioritize sustainable solutions such as increasing green spaces, incorporating green roofs, and developing urban vegetation initiatives tailored to Karaj’s climate. These approaches not only lower urban temperatures but also enhance the city’s environmental resilience, support biodiversity, and improve the overall quality of urban life.
Based on the findings, Karaj’s urban planning efforts should focus on preserving existing vegetation and expanding green infrastructure. Effective land-use policies that integrate vegetation can help counteract the adverse effects of rising temperatures, contributing to a more sustainable and livable urban environment.

کلیدواژه‌ها [English]

• Urban Heat Island
• vegetation
• satellite imagery
• Karaj
• land surface temperature