Identification of snow reservoirs in Iran

At the high elevations of river basins, precipitations are mainly in the form of snow and its accumulation provides water of the rivers in warm seasons. Having accurate and on time information is of great importance for flood controlling, estimation of snow water equivalent. The extent of snow cover and its variations are important parameters in hydrologic and climatic systems. Suitable and accurate evaluation of snow cover on both small and large scales is very crucial. Lack of information on the high elevations is an issue which cause increasing concerns due to climate change as a great number of large rivers originate from these highlands. In the last few decades many researches have been carried out on the study of snow cover using remote sensing data. For instance Maskey et al (2011) used MODIS Terra data to examine seasonal snow cover in Nepal for the period from 2000 to 2008. His findings revealed that snow cover is more in the elevation zone of 3000 to 4000 meters compared to the elevation zone of 4000 to 5000 and 5000 to 6000. Khadka et al (2014) evaluated snow cover in different seasons in Tamakoshi in the highlands of Himalaya using MODIS data for the time coverage from 2000 to 2009. The results indicated that snow cover below the elevation of 4500 meters above sea level is not much significant. In winter and spring at the elevation above 4500 meters the snow cover areas are very noticeable. However in summer the elevation zones above 5500 meters have significant extent of snow cover.
In the present paper MODIS Terra and MODIS Aqua data were used to detect snow reservoirs of the country. The selected study period covers the years from 1382 to 1393. As MODIS Aqua data are missing before the year 1382, we had to limit the study period only to the aforementioned years. Before the analysis of the data, we applied two different algorithms to minimize cloud contamination that is a big obstacle against snow cover monitoring. One of the applied algorithms is based on three days filtering and the second is made on the combination of the two products. By merging the two products we managed to develop a regional snow cover data set over Iran. We also used a Digital Elevation Model that was exactly like the snow cover data both on the special resolution and projection system.
The findings of the present study revealed that there are three principle snow reservoirs that are very suitable for the accumulation of snow cover. The snow reservoir is an area which is snow-covered in long period of time in a year. The three main snow reservoirs of the country are Alborz, North-west and Zagrous and the most number of snow covered days on the heart of these snow reservoir is 153, 132 and 127 days respectively. The analysis revealed that the heart of Alborz snow reservoir is a point in Alamkooh which has a north facing slopes that is suitable for snow cover accumulation. The findings also revealed that in the snow reservoir of Zagrous the relation between snow covered days and elevation is not very matched in west to east direction. And this is due to decrease of precipitation from west to east in this area.
In this study the daily time series of MODIS Terra and MODIS Aqua data were applied to detect snow reservoirs of the country. Before using the daily data, some cloud removal technics were applied on the raw daily data to minimize cloud cover effects. The findings revealed that in Iran there are three main snow reservoirs which are Alborz, North-west and Zagrous. The most number of snow covered days was detected to be on Alborz snow reservoirs. It has been detected in this study that in eastern Zagrous the changes of snow cover with elevation is not a positive direct relation and it tends to be reduced as elevation increase. We also concluded that the most number of snow covered days are not necessarily seen on the highest mountains in Iran but in lower elevations. It was discovered that role of topographic conditions is of great importance for the accumulation of the snow cover. The eastern and northern aspects are suitable for the persistence of snow cover days and the highest number of snow covered days was detected in these aspects in the country.