تعیین و بررسی سیرکهای یخچالی اشترانکوه با استفاده از شاخص سطح نرمال شدهی پوشش برف (NDSI)
هیدروژئومورفولوژی تابستان 1398 - شماره 19 علمی-پژوهشی (وزارت علوم)/ISC (18 صفحه - از 1 تا 18)
سیرک های یخچالی، حفره هایی با حجم متغیر و اغلب نیمه مدور هستند که با توجه به اندازه و ویژگی شکل ناهمواری ها، نمونه های مختلفی دارند. سیرک ها هم از نظر میراث طبیعی یخچال ها که در گذشته فعال بوده اند و هم از نظر فرآیندهای یخچالی که در زمان حال رخ میدهد، به عنوان شاخص ترین لندفرم حاصل از فرسایش یخچالی، حائز اهمیت میباشند. این لند فرم ها با توجه به مورفولوژی که دارند در مشاهدات عینی و همچنین در تصاویر ماهوارهای و نقشه های توپوگرافی شاخصه ی مناسبی برای تعیین محدوده ی فعالیت های یخچالی و برف مرزها و دنباله های زبانه ی یخچالی در یک منطقه می باشند، چرا که این اشکال با تأمین شرایط لازم، زمینه را برای ذخیره و شکل گیری برفچال ها و یخچال ها در خود فراهم میکنند. در این پژوهش، سیرک های یخچالی منطقه ی اشترانکوه با توجه به روند شمال غربی-جنوب شرقی آن به دو دسته ی سیرک های دامنه ی شمال شرقی و سیرک های دامنه ی جنوب-غربی تقسیم و توسط شاخص پوشش برف سطح نرمال شده (NDSI) و با استفاده از شکل خطوط منحنی میزان نقشه ی توپوگرافی منطقه تعیین شدند. پس از مشخص شدن سیرک های یخچالی منطقه، مورفومتری سیرک ها در هر دامنه تعیین و با یکدیگر مقایسه شد. در نتیجه ی این مقایسه مشخص شد که سیرک های دامنه ی شمال شرقی با توجه به تابش دریافتی کمتر و تأثیرپذیری کمتر از فرآیندهای مختلف شکل زا مانند هوازدگی و رواناب ناشی از ذوب، کمتر از دامنه ی مقابل دستخوش تغییر شدهاند و از سیرک های مشخص و تیپ یکتری با توجه به تعریف ایوانس برخوردار است.
<strong>Introduction </strong> <br />The history of reviewing Iran's glaciers may be less than 80 years old and there are not enough records and opinions before it. For example, Esther, a Finnish geologist, didn’t know about the Quaternary glacier in Iran (Peru, 1984). Cirques are depressions with variables and often semi-circular volumes which have several examples according to the size and shape of the rippling. The simplest form has a slender floor (Mahmoudi, 1988). Deepening of the cirques is the result of an exploration and gentle rubbing on the floor of the glacier. In fact, the formation of the glacier cirques is that at first the glaciers form in the slopes and grooves in the range. Then under the influence of the slope of the hillside, the glaciers move from their original location to the bottom of the grooves which are curved and their depth is increased. This phenomenon is due to the movement of snow ice in the lower part. As a result, the stones appear on the bottom of the cirques (Moghimi, 2008). In this study, the glacier cirques of Oshtorankuh area were divided into two sections of the northeastern and southwestern cirques with respect to the northwest trend of the eastern south. It is related to the normalized snow cover index (NDSI) using Landsat 8, scheduled for April 2016. <br /><strong> </strong> <br /><strong>Materials and methods</strong> <br />Normalized Difference Snow Index (NDSI) is a standard for determining snow levels and it is one of the remote sensing techniques for surveying the status of the mountainous and snowy areas. The basis of this index is the determination of the satellite images and the amount of light and heat reflected from a snow-covered surface. Considering the difficulties of accessibility to mountainous areas for the identification of snow-covered surfaces, using remote sensing will be very useful. NDSI is used as an automatic snow extractor algorithm with a set of thresholds and pixel-to-pixel values. This index is based on the fact that snow has a high reflectivity in the visible area and low reflectivity in the infrared range. It is used to detect snow from the cloud and areas without snow (Nolin et al., 2000). The NDSI is non-sensitive to exposure conditions and is adjustable to atmospheric effects. In other words, this index depends on both its reflection values in a band and the amount of digital reflection in pixels. <br /><strong>Discussion</strong> <br />In order to calculate the Normalized Snow Cover Index (NDSI), Landsat 8 was introduced in April of 2016, and Erdas Imagine combination of 6 bands was used. Then, using the formula (1), NDSI was calculated: <br /><strong>Equ (1) NDSI=</strong> <br />NDSI: Normalized-Difference Snow Index <br />OLI3: Band 3 Landsat 8 <br />OLI6: Band 6 Landsat 8 image <br />After calculating the NDSI and taking into account the altitudes above 3500 m with the help of the altitude digital model, the correction of the errors caused by the reflection of snow-like surfaces was carried out. Finally, the area covered with snow and ice of the Oshtorankuh area was determined. Given that the snow and ice in the cirques center in the <br /> <br /> <br /> <br /> warmer seasons melt away from the snow and ice of the inner walls of the cirques, the walls and boundaries of each cirques was determined by these snow and ice coverings. Therefore, normal snow cover levels (NDSI) can be used to identify and determine glacier cirques, but it is necessary to mention that the satellite images used in this method were related to the history of the boundaries of all cirques and the minimum amount of snow and ice which were detected by the NDSI. Finally, 33 glacier cirques were determined and positioned. Then, with respect to the Oshtorankuh along the northwest-southeastern direction, the cirques were divided into two strata of the northeastern slopes including 18 cirques and southwest slopes including 15 cirques. <br /><strong>Conclusion</strong> <br />Glacier cirques aresemi-circular cavities with variable volumes due to their size and shape that feature rugged and different samples. The cirques are also important for the glacier's natural heritage. They were active in the past and due to the glacial processes that occur in the present time they are the most important landforms of glacial erosion. In this study, the glacier cirques of Oshtorankuh area were divided into two sections of the northeastern and southwestern range cirques with respect to the northwest trend of the eastern south. It was related to the normalized snow cover index (NDSI) and Landsat 8 which was scheduled for April 2016 was used. The point which should be taken into account is that the determination of cirques by the Normalized Snow Covering Index (NDSI) of satellite imagery should certainly be related to the time period in which the limits of all cirques have a minimum snow level. It is self-identifying by the Normalized Snow Cover Indicator (NDSI). After determining the glacier cirques of the area, the characteristics of the cirques were determined in each domain and compared with each other. It was found that the cirques of the northeastern stretch have more developed and more typical cirques due to the less radiation received and the activation of glacial shaping processes in relation to water erosion along with glacial processes. <br /> <br /> <br />کلیدواژه ها:
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