كليدواژه :
پوشش برف , زاگرس , MODIS , منابع آب
چكيده فارسي :
هدف از انجام اين پژوهش تجزيه و تحليل روند تغييرات پوشش برف در محدوده رشته‌كوه زاگرس جهت واكاوي نوسانات پوشش برف طي دوره زماني 2016-2001 است. در اين پژوهش از داده‌هاي روزانه سنجنده MODIS جهت استخراج و بررسي تغييرات سطح پوشش برف (SCA) و برفمرز (SL) براي ماه‌هاي داراي پوشش برف طي سال‌هاي 2016-2001 شده است. براي ارزيابي صحت تغييرات پوشش برف از آمار ميانگين بارش (بدون تفكيك برف و باران) ايستگاه‌هاي سينوپتيك قرارگرفته در ارتفاع بالاتر از 1500 متر استفاده شد و جهت ارزيابي دقت تصاوير MODIS در استخراج سطح پوشش برف از 12 فريم تصوير سنجنده ASTER به‌عنوان نقشه صحت زميني استفاده شد. تحليل روند تغييرات پوشش برف و برفمرز با استفاده از آزمون من-كندال نشان مي‌دهد كه پوشش برف و برفمرز به‌طور كلي چه در سطح معناداري 99 درصد و چه 95 درصد فاقد روند است. بررسي آمارهاي پوشش برف هرماه نشان مي‌دهد كه سال 2009 نقطه عطفي در پوشش برف زاگرس به‌شمار مي‌رود و از اين سال به بعد تا سال 2016 براي تمامي ماه‌ها به‌جز ماه نوامبر تغييرات پوشش برف كاهشي است. بررسي تغييرات ميانگين سطح پوشش برف طي سال‌هاي 2016-2009 نسبت به سال‌هاي 2008-2001 نشان مي‌دهد كه ماه ژانويه با بيشترين كسري پوشش برف به ميزان 11/55- درصدي در دامنه ارتفاعي 1162-4212 و كسري بارش 85/45- براي ايستگاه‌هاي سينوپتيك مواجه بوده است. ارزيابي دقت تصاوير پوشش برف MODIS نسبت به تصاوير ASTER نيز نشان مي‌دهد كه مقادير ضرايب R2، RMSE و MAPE بين سطح پوشش برآوردي توسط هر دو تصوير به ترتيب 99/0، 3/49 كيلومترمربع و 51/21 درصد بوده است.
چكيده لاتين :
Snow cover (SC) is the main part of global cryosphere that covers about 46 million square kilometer of earth surface. Seasonal SC and snow storage provides the required water of around 1/6 of world population. About 98 percent of global SC is in the northern hemisphere. Southern and southwestern mountain range of Iran, called Zagros is located in the mid-latitudes that is exposed to the northern hemisphere westerlies resulting in high amount of rain and snowfall. This region contains 40 percent of Iran's water storage. Monitoring and evaluation of seasonal SC is essential for managing winter tourism, transportation, environment, land cover, water and irrigation and also essential in runoff modeling. In our study area snow gauges are insufficient and sparse, therefore monitoring spatial changes of Snow Cover Area (SCA) is difficult. MODIS images lunched on TERRA and AQUA platforms are suitable data source for estimating SC, because they provide daily images with 500 meter spatial resolution. The aim of this study is detecte and analyze SC variations and trends in Zagros mountain range using daily images of MODIS in order to determine SC variation.
In this research, 348 daily products of MODIS for first days of snowy months (Months with some snow on the ground in Zagros; Dec, Jan, Feb, Mar, Apr, May, Jun) are obtained from National Snow & Ice Data Center (NSIDC) to calculate SCA. Digital Elevation Model (DEM) is used to determining snow line (SRTM 90m from Earthexplorer.usgs.gov) and total monthly precipitation of weather stations above 1500 meter elevation of the same period with SC data is used. Snow line is extracted by overlaying DEM on SCA. Then the snow line pattern is determined as the average of snow line for each day during 2001 to 2016. SCA pattern is also extracted as the average of each day during the same period. In order to calculate trends in SCA and Snow Line Man-Kendal test is used. Man-Kendal test is less sensitive too extreme values of time series.
The trends of SC are investigated using Man-Kendal test results show a decreasing trend in January, May and June in 2007-2016. The highest amount of SCA decrease and snow line retrogression for January, May and June are recorded in 1250-4500 m, 2300-4500 m and 3500-4500 m altitude belts, respectively. Based on SC departure from the average, two periods are recognizable: first, from 2001 to 2008, in this period SC is above the average and is more similar to the snow cover pattern of the whole period (2001-2016). Second period from 2009 to 2016, in this period SC is more below the average and is not similar to the snow cover pattern of the whole period (2001-2016). In the second period mean SC reduction compared to the first period in Dec, Jan, Feb, Mar, Apr, May and June are 4, 67.6, 24.5, 21, 10, 47.3 and 48.4 respectively. Mean monthly precipitation changes of weather stations above 1500 meter elevation is similar to the changes of SC changes.
