استفاده از داده هاي سنجش از دور در تعيين محدوده يخچال هاي زردكوه

Application of Remote Sensing Data in Measuring the Area of the Zardkuh Glaciers

يخچال هاي طبيعي متاثر از فاكتورهاي اقليمي بوده و لذا به عنوان يك شاخص موثر در بررسي تغييرات اقليمي، مطالعه مي شوند. اگرچه بررسي هاي مورفومتريك يخچالهاي طبيعي بر اساس تجزيه و تحليل داده هاي بازتابي سنجنده هاي ماهواره اي، امكان اندازه گيري رخنمون هاي يخ را فراهم مي سازد، اما شناسايي و تعيين محدوده يخچال هاي مدفون شده در زير واريزه ها و به تبع آن تعيين محدوده واقعي يخچال و ساير پارامتر هاي آن از جمله سطح و حجم يخچال بر اساس باند هاي اپتيكي تصاوير ماهواره اي امكان پذير نمي باشد. از طرفي وجود منبع سرمايشي يخ در زير واريزه ها كه منجر به كاهش قابل توجه دماي سطح واريزه نسبت به مناطق اطراف ميگردد، وجه تمايزي براي تفكيك يخچال هاي طبيعي پوشيده شده توسط واريزه ها محسوب ميگردد. در اين تحقيق گسترش يخچالهاي طبيعي ارتفاعات زردكوه با تحليل تلفيقي داده هاي حرارتي، اپتيكي (سنجنده هاي OLI و TIRS لندست 8) بر اساس روش تك پنجره و داده هاي رقومي ارتفاعي تعيين گرديده است. به اين منظور، پس از اعمال تصحيحات راديومتريك و هندسي، دماي درخشندگي محاسبه و سپس بر اساس شاخص پوشش گياهي تفاضلي نرمال شده و مقدار ضريب گسلندگي سطح زمين (LSE) ، دماي سطح زمين (LST) محاسبه گرديده است. بر اساس نتايج اين تحقيق سطح پهنه هاي يخچالي (سطح توام يخ و يخ مدفون زير واريزه ها) در مرداد ماه 1396 برابر با 201 هكتار بوده است كه در مقايسه با تحقيقات قبلي، كاهش چشمگير پهنه هاي يخي را در دهه گذشته نشان مي دهد.

Introduction: Glaciers influenced by climatic factors and therefore as an important indicator in the study of climate change are studied. Although morphometric analyzes of glaciers based on the analysis of optical satellite data can provide an opportunity to measure ice outcrops، but the identification and determination of the buried glaciers underneath the Supraglacial moraines (accumulated debris on top of the glacial ice) and، consequently، the determination of the actual glacial body and their parameters، including the level and volume of the glaciers، are not possible based on optical satellite imagery. On the other hand، the existence of an ice cooling source underneath the glacial debris، which leads to a significant reduction in the surface temperature of the glacial debris، is a distinguishing feature for the separation of covered glaciers. Materials and methods: The over 2000 km long NW-SE trending Zagros Mountain belt extends from eastern Turkey to the Makran Mountains، forming a morphological boundary between the Iranian plateau and the Mesopotamian and Persian Gulf basins. The Zardkuh Mountain lies along the central Zagros Mountain Range (32°14'-32°38' N; 49°50'-50°15' E) in Chahar-Mahal and Bakhtiari province، Iran. The highest summit of this mountain chain is 4220 m a.s.l altitude. Small glaciers (mostly cirque glaciers) were first reported on the northern slopes of Zardkuh during August 1933. Other research has pointed to some glacial cirques on Zardkuh which have altitude about 3000m and located in small valleys on the N faces of the Zardkuh. Some new researches on the active glaciers of Zardkuh show the greatest glacier concentration around 1) Joft-zarde and Shahe Shahidan (Zarkuh) summits 2) around Sirdan summit and 3) Haft-tanan (Iluk) region. Based on the latest study، the current size of Zardkuh’s glaciers have estimated about 7.1 km2. In this research، the expansion of natural glaciers of Zardkuh Mountain has been determined by the combined analysis of thermal (Thermal Infrared Sensor (TIRS) of Landsat 8)، optical data (Operational Land Imager (OLI) of Landsat 8 and Quickbird image) and digital elevation data، based on Split Window Algorithm. For this purpose، the following steps are carried out: 1) Preparation of Landsat 8 TIRS and OLI images، Quickbird image and Digital Elevation Model with 10 m resolution 2) Radiometric correction and rectification of images to WGS-1984-UTM-Zone_39N 3) Detection of exposed ice of active glaciers using by short wavelength infrared and green spectral bands of Landsat and optical images 4) Creation of 3D model of active glacier by combination of DEM and Quickbird images / Landsat images 5) Estimation of Land Surface Temperature (LST) using by Normalized Difference Vegetation Index (NDVI) and Land Surface Emissivity (LSE) based on the Single Window Algorithm. The NDVI is calculated as a ratio between measured reflectivity in the red and near infrared portions of the electromagnetic spectrum. The land surface emissivity (LSE (𝜀)) is calculated based on the Proportion of vegetation (Pv) and NDVI. 6) Supervised classification of LST layer based on the sampling of ice and supraglacial deposits temperatures for each glacier (the presence of ice under sediments has already been confirmed in field visits) 7) Area calculation of ​​the glaciers based on the classified LST layer. Results and discussion: In this study، Land Surface Temperature (LST) layer were calculated for Zardkuh Mountain based on the single method algorithm. Furthermore، Land Surface Emissivity (LSE)، Proportion of vegetation (Pv) and normalised difference vegetation index (NDVI) layers were calculated for the study area. The surface area of exposed ice of Zardkuh glaciers is detected by using the short wavelength infrared and green spectral bands by making RGB753 color composite which its spatial resolution has improved with Panchromatic band to 15 m and 3D model of glaciers. Based on the results of this study، the area of the exposed ice of glaciers were 123 hectares. The surface area of exposed ice and buried glaciers underneath the Supraglacial moraines of Zardkuh glaciers based on the described methodology، were measured about 201 hectares in August 2017. The area of ​​the Haftanan، Joftzarde، Kuhrang3 and Khersan glaciers has decreased and reached to 32، 31، 30 and 29 hectares، respectively. Conclusion: In this paper، potential of remote sensing to study the actual extent of Zardkuh glaciers in Zagrous Mountains in Iran by estimating LST distribution with the help of Landsat 8 OLI and TIRS sensor bands provided. Remote Sensing technology data such as Landsat 8 TIRS proved as an efficient one to estimate LST. Single window algorithm methods are applied to calculate the LST from TIRS data. Based on the results of this study، the area of the glaciers (surface and buried ice) in comparison with previous studies، shows a significant decrease in the past decade.