عنوان مقاله [English]
The glaciers are among the phenomena on the Earth's surface that with the advance and retreat of the planet show an appropriate response to climate change so that in recent decades due to increased emissions of greenhouse gases in the atmosphere and consequently increasing global temperatures, melting glaciers increased the volume greatly reduced. Check the status of glaciers due to their placement in the shadow of high mountains with hardships and difficulties faced, as a result the advantages of using remote sensing techniques for these areas is obvious. NDSI or Normalized Difference Snow Index is the criterion to determine levels snow cover and the basis for determining the index above the snow reflectance in the visible region and low reflectance in the middle infrared range and used to identify clear snow and snow-free areas that considering the problems of access and impassable mountainous regions to identify areas covered with snow and snow levels are used to determine the exact area. In this study was to determine the changes snowline in Oshtorankuh region, the cirques glacier were designated area and positioning. After regional status the cirques by the digital elevation model and using the cirques floor height Porter and Wright, the permanent snowline at 2505 and 2549 meters, respectively Late Quaternary period was determined. Then with the help of satellite images Landsat 8, the Normalized Difference Snow Index(NDSI) calculation and determined Snow levels for both the southwest and northeast and range for four consecutive years(2013 to 2016). Eventually the height of 3346 meters was already wrapping Snow levels. Accordingly, taking into account the height of the Porter method snow levels are already rising to 841 meters of Late Quaternary and considering the Wright method shows rising 797 meters to the height of the snowline in the Late Quaternary.
The glaciers are large masses of ice and snow which are formed in regions with cold climates with frost. In these areas snow is more than melting and evaporation, and the glacier feed is positive. Generally, the formation of natural glaciers are such that snow falls in the form of crystals of various shapes on the surface of the ground, accumulated in areas that are prone to the formation of a glacier due to the environmental and atmospheric conditions and the three factors The ambient temperature, sublimation, and the effect of increasing pressure are subject to complex transformation and during this process, ice frosts are formed(Vaziri, 2000). Snow reserves in the upstream mountains affect the downstream seasonal run off pattern, especially in areas with a dry summer season, where snow melt and glacier run off is a major source of water supply(Yang et al, 2005).
To determine the extent of snow line changes in the Oshtorankuh region, we first determine the height of snow line of the region in the later quaternary. For this purpose, the glacier cirques of the region should be determined as the most prominent glacial landforms. After identification of the cirques of the Oshtorankuh region, by using Porter method and Wright method Snow lines of the Quaternary period were determined. Then, by using the Landsat images related to July from 2013 to 2016, the NDSI determined for each year, using the of these, the approximate snow line height of the region is now determined.
Results and discussion:
First, the glacier cirques of Oshtorankuh region were investigated and using the topographic map of 1/2500 and the digital model elevation of 10 meter, 33 the glacier cirques was determined and positioned. Then, with respect to the Oshtorankuh stretching northwest-southeastern, the cirques are divided into two hillside of northeastern cirques, which include 18 cirques and southwest hillside, consisting of 15 circuses. The difference between these two hillsides is the intensity of the radiation and the duration of the radiation received. The southwestern range of cirques are superior in terms of duration and intensity to the northeast due to solar radiation, which increases glacial melting in warm seasons, leaving no effect on snow line and glaciers in the late summer. Gets After determining the position of the cirques in the region, using Porter method and Wright method, the limit of permanent snow line in the quaternary period 2505 and 2549 meters was determined. Then, by calculating the NDSI value in late July 2013 through 2016, in the Erdas Emagine 2014 software and by using digital model elevation of 10 meter, the snow cover surface and snow line height region in now equal to 3346 meter was determined.
In this research, after determining the position of the cirques, by digital elevation model, using the Porter and Wright method, the limit of snow line in the quaternary period was 2505 and 2549 meters respectively. Then, using the NDSI, the altitude of snow line levels for the two southwest and northeastern regions was calculated for 2013-2016, and finally the altitude of the snowy surface is now equal to 3346 m. As a result, the height of snow levels is now higher than the quaternary elevation of 841 meters, taking into account the Porter's method and 797 meters, considering the Wright method relative to the snow line height in the later Quaternary, which expresses the increase in mean the temperature in this area is relative to the quaternary period. As a result of this rise in temperature, the occurrence of hydro geomorphological changes in the region is as a result of the change in the forces forming the surface of the glacier to run off at the height of the snow line, which is also evident in the region.