عنوان مقاله [English]
Snow is a form of precipitation in hydrological cycle of mountain regions that plays an important role to supply drinking and agricultural water as the delay water resource during low-water seasons. Snow distribution is important to hydrological research of watersheds. The temporal and spatial distribution of snow cover has significant influence on snowmelt runoff. Knowing the extent of the snow is valuable information that provides insight as to the amount of water to be expected from snowmelt available for runoff and water supply. In this study there has been an attempt to investigate changes in snow cover of Koohrang watershed. However, the elevation effect on snow cover is also proposed. Koohrang’s catchment is one of sub-basins of northern Karoon with an area estimated over than 2700 km2 which is located in Chaharmahal-va- bakhtiyari Province of Iran. The permanent stream of the rivers; ultra-cold climate and high elevation are good and justifying evidences for solid precipitation in this area. Therefore, awareness of the amount of snow resources in this area is necessary in the storage of the equivalent amount of water, controlling seasonal floodwaters, anticipation of the river’s stream in supplying the required water of downstream land. This study aims to monitor the change of snow cover of the Koohrang watershed during 2010 to 2018 and to determine the association between snow cover and elevation zones.
Material and method
Two groups of data including remote sensing and Geographic Information System (GSI) have been used in this study, And with respect to the subject, different techniques have been utilized. Moderate Resolution Imaging Spectroradiometer (MODIS) Satellite imagery within the statistical period of 2010-2018 was obtained. Then by calling those images into ENVI software, snow covered areas were differentiated from other areas, using difference index and NDSI ratios band 4 and 6; and also their maps were provided. In order to examine the amount of snow border of basin, with the use of the principle of environmental degradation and regression relationship and linear functions and the map, the daily 0°C (snow border) isothermal lines were defined. Furthermore, we investigate trends and variability of snow cover changes in the Koohrang watershed at different temporal scales (monthly and annual) and at different elevation zones between 1656 to 4074 m.
The results show a decreasing trend of changes in snow cover in the region during the study period. The most decreasing trend among the months was related to February. October and March are on the second and third places. The average snow cover of the region during this period is 440 km2 (equal to 34%). 2010 and 2014 years showed the lowest and highest snow cover, respectively. The highest snow cover among the months of the year belongs to January with an average of 1132.3 km2 (equal to 88.6%). The results also indicate that there is a direct relationship between the elevation and the quantity of snow cover and its persistence. So the highest percentage of snow cover was related to higher elevation zones. The average percentage of snow cover in elevation zones of 1 to 6 were 10, 22, 32, 40, 48 and 58%, respectively.
The results of this study can be very useful and practical for planning water resources in the region, especially in the warm seasons. The trend analysis on the alternation of snow cover during the 9 years period from 2010 to 2018 has shown some important results. If the decreasing trend of the snow cover in February and March as observed in this study continue, this may result in significant changes in the river flows and water resources in the region, particularly in spring. This would have implications for aquatic ecosystems that depend on the seasonal melt water pulse, for irrigation dependent agriculture and, last but not least, for water resources in the densely populated downstream areas. Some of the trends observed in the snow cover changes can be explained by the high correlations observed between the snow cover and the observed temperature. It should be noted here that not only snow covered areas but also the snow depth and snow water equivalent will impact the snow melting, which subsequently influence the river flow regimes and water resources availability. Therefore, future studies should attempt to consider these factors for a more comprehensive assessment of the impact of Koohrang snow on the river flow regimes.