برآورد آب معادل برف با استفاده از داده‌هاي سنجنده AMSR-E و مدل GLDAS (مطالعه موردي: حوضه‌هاي شمال‌غرب ايران)

Snow Water Equivalent Estimation Using AMSR-E and GLDAS Model (Case study: Basins of Northwestern Iran)

آب معادل برف اطلاعات مهمی برای مدیریت منابع آب ارائه می‌دهد و اخیرا مورد توجه بسیاری از محققین سنجش از راه دور قرار گرفته است. اگرچه سعی شده است كه با استفاده از مدل‌های بزرگ مقیاس جهانی، آب معادل برف تخمین زده شود، ولی اثرات منطقه‌ای مانند چگالی برف، توپوگرافی و شرایط هواشناختی محلی ممكن است به عدم قطعیت منجر شوند. در این مطالعه از داده‌های روزانه سنجنده AMSR-E ماهواره آكووا و مدل جهانی سطح زمین (GLDAS) برای تخمین آب معادل برف روزانه در ایستگاه‌های برف‌سنجی حوضه‌های شمال‌غرب ایران (حوضه دریاچه ارومیه و غرب حوضه آبریز مازندران) در طول سال‌های آبی 86-85 الی 90-89 در تاریخ‌هایی كه اندازه‌گیری برف صورت گرفته بود، استفاده گردید. دلیل انتخاب این محدوده، كوهستانی بودن، بارش زیاد برف و تراكم ایستگاه‌های برف‌سنجی نسبت به سایر مناطق كشور بود. با توجه به نتایج به دست‌ آمده، داده‌های محاسباتی آب معادل برف همبستگی معنی‌داری در سطح 1 درصد با داده‌های مشاهداتی داشتند. استفاده از چگالی برف اندازه گیری شده در داده‌های AMSR-E، باعث افزایش ضریب همبستگی از میزان 27/0 به 55/0 گردید. نتایج نشان داد كه بهترین تخمین آب معادل برف در ایستگاه‌هایی بوده كه در سطوح ارتفاعی 1350 الی 1600 متری قرار داشته‌اند و با افزایش ارتفاع، دقت تخمین به‌طور قابل توجهی كاهش یافته ‌است. با استفاده از نقشه‌های آب معادل برف ماهانه مدل GLDAS، آب معادل برف ماهانه برای دوره 2000 الی 2015 برای منطقه مورد مطالعه محاسبه گردید. در اكثر سال‌ها مورد بررسی، بیشترین میزان آب معادل برف برای ماه‌های ژانویه و فوریه به دست‌آمد و در محدوده زمانی ژوئن تا سپتامبر، منطقه مورد مطالعه فاقد ذخیره برفی بود. با توجه به میانگین آب معادل برف سالانه و نمودارهای میانگین‌های متحرك سه، پنج و هفت ساله، میزان آب معادل برف حوضه‌های شمال‌غرب ایران در دوره آماری 2015-2001، روند كاهشی داشته است.

Introduction: Snow water equivalent (SWE) provides important information for water resources management and recently has attracted the attention of many researchers using remote sensing. Remote sensing presents a possibility for observation of snow characteristics, like water equivalent, over larger areas. Validation of remote sensing data of snow water equivalent (SWE) has always been an important issue for the researchers. Previous studies have assessed the global SWE data. Although it has been tried by using large-scale models of the world to estimate SWE, but regional effects such as snow density, topography and local meteorological conditions may lead to uncertainty. Materials and Methods: The Northwestern Iran was selected as the study area in this research. Reasons for choosing this area are being mountainous with much snowfall. Also this region compared to the other parts of Iran, has more dense snow survey stations. In this study the AMSR-E sensor data and Global Land Data Assimilation System (GLDAS) was used to estimate SWE in the basins of the northwestern Iran. After processing AMSR-E sensor data and GLDAS model with related software, SWE was estimated in the snow survey stations and evaluated with observed data. To specify the snow density effect on SWE data in AMSR-E sensor from the snow density data, the stations were used. To determine the accuracy of estimation of SWE at different heights, snow survey stations is arranged by considering height and were divided into four height classes that contain enough observational data to evaluate computational data in each height class. To verify SWE obtained estimations in the stations, Root Mean Square Error (RMSE) and Pearson correlation coefficient (r) assessment criteria were used. After evaluating, the SWE data of AMSR-E sensor and GLDAS model for the GLDAS model monthly data to estimate SWE was used for the period 2000 to 2015. With calculating average annual SWE from monthly data, SWE trend changes in mentioned period, the moving averages graphs 3, 5 and 7-year-old was drawn. Results and Discussion: According to the obtained results, SWE computational data with observational data had significant correlation at the 1% level. Using in situ snow densities, the correlation coefficient between AMSR-E and situ SWE increased from 0.27 to 0.55. The results showed that the best estimation of SWE is in the stations, which have the height of 1,350 to 1600 meters. Also with increasing altitude, the estimation accuracy is significantly reduced. In most years maximum of the SWE was obtained in January and February and in the period of June to September, the area was out of snow storage. According to the average annual SWE and moving averages graphs 3, 5 and 7-years old, the SWE of Northwestern Iran basins in period 2015-2001 has a reducing trend. Conclusions: In the regions like the Northwestern Iran mountainous where snowfall constitutes a significant fraction of total precipitation, the snowpack delays the resulting runoff into the time of year where water demand is greater. So measurement of snow on the ground has been an important component of hydrologic forecasting for a century. Various remotely sensed snow data have been widely utilized for cold regions to explore the relationships between snow distribution, river discharge, and climate change. The accuracy of remotely sensed snow products should be well understood and incorporated in any investigations using such data. The main objective of the present study was to quantitatively compare the AMSR-E and GLDAS model for an understudied region of the earth. AMSR-E global SWE data and GLDAS data were compared by situ SWE measurements performed in the snow courses. The results showed that the snow density is an effective factor in derived algorithm for the SWE AMSR-E data. Also with increasing height, precision of the estimation significantly decreased. The determination of SWE from satellite imagery in progress updated with new learning. The obtained results from passive microwave in smooth terrain are promising, but involvement of different mechanisms become more complicated as the terrain gets more complex. Nevertheless, it is believed that if the above discussions are taken into account, AMSR-E would provide valuable SWE information even for a mountainous region like Northwestern Iran. It is also hoped that this study would be a starting point in the water scarce, developing Iran to plan and use the limited supply in a suitable manner.