پایش مکانی و زمانی سطح پوشش برف با تصاویر NOAA-AVHRR در بازه زمانی 93- 1385 (مطالعه موردی: حوضه آبخیز بالیقلوچای)

نویسندگان

چکیده

در کشور ایران و به خصوص در نواحی کوهستانی آن، بارش برف یکی از عوامل اصلی ذخیرة آب به شکل‌های مختلف برای مصرف در فصول گرم سال است. حوضة آبخیز بالیقلوچای، در استان اردبیل واقع شده که به‌دلیل کوهستانی بودن دارای زمستان‌های سرد و پربارش است. این حوضه، نقش مهمی در تأمین آب شرب و کشاورزی منطقه دارد. در پژوهش پیش‌رو، تغییرات سطح پوشش برف زمستانه در بازة زمانی 9 ساله (1393- 1385) بررسی شد. برای دستیابی به این هدف از تصاویر ماهوارة NOAA-AVHRR و الگوریتم NDSI استفاده شد. نتایج پژوهش نشان می‌دهد که شمال غرب و جنوب غرب منطقه به‌دلیل قرار گرفتن کوهستان سبلان و ارتفاعات بزقوش از وضعیت برف‌گیری مناسبی برخوردار هستند. همچنین نتایج نشان داد که حداکثر سطح پوشش برف حوضه، مربوط به دی‌ماه سال 1385 با مساحت 7/1182 کیلومترمربع (9/94 درصد) و حداقل مساحت برف نیز مربوط به دی‌ماه سال 1389 با مساحت 8/33 کیلومترمربع (7/2 درصد) است. این امر نشان‌ دهندة تغییرات بسیار شدید در میزان پوشش برف منطقة مورد مطالعه است و همچنین آسیب‌پذیری منابع آب وابسته به ذوب برف را نشان می‌دهد.

کلیدواژه‌ها


عنوان مقاله [English]

Spatial and Temporal Monitoring of the Snow Cover Area Using NOAA-AVHRR Images (Case Study: Balighlu Chay Watershed)

نویسندگان [English]

  • Bromand Salahi
  • Mahsa Nakhostinrouhi
چکیده [English]

Snow is one of the most important forms of precipitation in the mountainous regions. This parameter has an important role in supply of drinking and agriculture water resources, and energy production. Monitoring of the snow cover area creates the possibility of estimating the snowmelt runoff that is used in control and management of the watershed water. Required water of more than one sixth of the world's population is supplied through melting seasonal snow and glaciers. If the warming phenomenon happens, a huge number of populations will encounter the risk of the lack of water. So, monitoring the snow cover area is considered very important in order to understand regional hydrological cycles. The snowy areas of Iran are mainly located in mountainous heights where the monitoring and measurement stations for snow have not developed. Therefore, it is not possible to measure the accumulation and melting of the snow exactly. Obtaining enough information about snow reserves during a period can be useful for proper use of the water from snow melting, saving water and prevention of its waste, controlling the flood, and providing water for downstream lands. Snow survey stations and satellite images are used for estimating spatial and temporal distribution of the snow in watersheds. When the number of stations is enough and their data are complete, the statistics obtained from snow survey stations will be acceptable, while there is lack of such facilities in most mountainous regions of country. In this study, Normalized difference snow index (NDSI) was used for determining snow-covered areas. The index is applied as an automated algorithm to recognize the snow and calculated pixel by pixel. The final snow cover map is provided in the binary form. It means that study area is classified into two classes: snow and no snow. ArcGIS10.2.2 software was used for calculating snow cover area.
According to the produced maps, the northwest of the study area is always covered by snow in winter. Because this region includes the maximum elevation (more than 3000 meters) and Sabalan mountain is located there. So, in comparison with other regions, temperature is lower in this area which leads to snowfall and snow durability. Also, the southwest of the catchment is covered by snow in most of the maps, due to the existence of the Bozgush Mountain with approximate elevation of 2000 to 2700 meters. The minimum density of the snow was showed in the northeast of the catchment, due to being on the catchment outlet and having the lowest elevation. Also, the central parts of the catchment often include low snow cover. In this region, low elevation and high temperature cause low snowfall and high velocity of snowmelt. Existence of Ardabil city in the northeast of the catchment leads to high temperature in this zone in comparison with other zones. The percentage changes of the snow cover area per month were calculated during years 2011- 2014 in comparison with the same month in 2010 in order to survey the monthly changes of the snow cover area in winter in the study area. It was concluded that the snow cover area in the first month of the winter, during all 4 years, has been decreased in comparison with the same month in 2010 and maximum reduction was associated with the first month of the winter in 2011.
Provided maps and calculated snow cover areas showed that northwest and southwest of Balighlu Chay watershed is often covered with snow in winter, due to existence of Sabalan and Bozgush mountains in these regions. Low-elevation regions, including the end (northeast of the study area) and center of watershed had the lower snow cover. Maximum snow cover area was 1205.7KM2 (96.7%) in the period studied, and was related to 7/1/2010. However, during 5 years, maximum snow cover area has often occurred in February. Investigating snow cover area monthly in winter, during 5 years, showed that the amount of snow covers in the study area had intensive changes. The average winter snow cover area for each year indicated that this parameter has decreased and increased alternatively during 5 years. The maximum and minimum average winter snow cover area were related to 2012 (857.4KM2 ), and 2011 (414.6KM2). To sum up, intensive fluctuation in snow cover area reflects the vulnerability of the water resources in the study area, which are dependent on snowmelt.

کلیدواژه‌ها [English]

  • Balighlu Chay watershed
  • NDSI
  • NOAA-AVHRR
  • Remote sensing
  • Snow cover