Monitoring the Spatial-Temporal Changes of Snow Surfaces in Zagross Mountains using NOAA-AVHRR Images

Spatial-Temporal changes of snowy areas extent is not only an important element of environmental and socio-economic, but is an important key in study of climate change and include an effective element in surface albedo and energy balance of Earth's surface and seasonal changes of it. Thus quantification of these changes has a special importance, that this is possible by using satellite images and spending energy and little cost.

In this research, Snow Area Changes in Zagros Mountains by using NOAA-AVHRR Images since 1996 from 2015 is studied, and appropriate models based on composition of reflection and thermal bands for extract of snow from NOAA-AVHRR imagery in Zagros Mountains is presented. These models use two different methods to detachment snow from non- snow according to the type of sensor. First, satellite images were geometric correction in ENVI software. Then, radiometric correction was performed. In the following calculated albedo of Reflective Bands (RB) and brightness temperature (BT) of thermal bands with the coefficients available in HDR files. For calculation land surface temperature (LST) we used formula Coll et al (1994). Then, calculate NDSI (normalize difference snow index) and NDVI (normalize difference vegetation index). Finally, for extraction snow pixel threshold was performed on RB, BT, LST, NDVI, NDSI and DEM models.

In this research, we concluded snow pixel that RB is more than 0.2, the difference BT (3.7µ) and BT (11µ) less than 11 and 265 > BT (11µ) <285. So, snow pixel are land surface temperature for April -4 < LST < 14, for May -2 < LST < 16, for June and July -2< LST < 18. For determination snow pixel, should be the amount of NDVI more than -0.1, NDSI more than 0.3 and DEM more than 2000 meter. The results of this research showed in April most snowy cover in 1997 was 9234.7 km2 and the lowest snowy cover in 2008 was 2509 km2. In May most snowy cover in 1997 was 27637.6 km2; the lowest snowy cover in 2008 was 231 km2 and the average snow cover is 1811 km2. In June most snowy cover in 1997 was 1471.4 km2; the lowest snowy cover in 2012 was 125.8 km2 and the average snow cover is 409 km2. During the studied period, the maximum amount of snow cover reduction occurred in May to June. In July most snowy cover in 1998 was 208 km2; the lowest snowy cover in 2001 was 3.6 km2 and the average snow cover is 55.6 km2.
The highest monthly variation in snowy cover in Zagros Mountains was -0.35 in July and the lowest monthly variation in snowy cover were was about of -0.17 in April. Overall, the reduce average of snowy cover in the study period was about of -0.22.
From the spatial point of view, the maximum of snowy covers has been observed in the highlands of Chahar Mahaal and Bakhtiari Province, east of Lorestan and border elevations of Isfahan and Kohgiluyeh and Boyer-Ahmad Provinces for most years. Of course, in April and especially in 1997, snow was observsd in most of the provinces within the study area (except Bushehr, Hormozgan and Yazd).

The results of Zagros snow cover survey during the studied period showed, the highest snowy cover was observed in April, May and Jun in 1997, and in July in 1998. From the spatial point of view, the maximum of snowy covers has been observed in the highlands of Chahar Mahaal and Bakhtiari Province, east of Lorestan and border elevations of Isfahan and Kohgiluyeh and Boyer-Ahmad Provinces for most years, but with increasing temperature, the area of the snowy cover is reduced. As in July, in most of the years, only Chaharmahal and Bakhtiari heights and mountainous areas of the border of Isfahan and Kohkiluyeh and Boyer-Ahmad provinces have been observed. As a whole, towards the warmer months of the year, changes in the snowy cover are increasing.