Introduction
Studying and measuring changes in snow levels is very important as one of the important sources of water supply. Due to the harsh physical conditions of mountainous environments, there is no possibility of permanent surface measurement to estimate snow sources and form a database. The use of satellite images and remote sensing due to their low cost, up-to-dateness, and wide coverage is a way forward in this field, and it can be a suitable method for identifying snow catchment areas and evaluating its changes to achieve this goal. The presence of snow in the basins affects the water resources that are stored in the form of frozen water on the surface Therefore, temporal and spatial monitoring of snow cover has been used for hydrological forecasts for years. Remote sensing images are a useful tool for estimating snow cover changes and analyzing the spatial pattern of this important environmental phenomenon, especially in high areas where there are few available ground stations or there are no stations

Methodology
In this research, data from V006-MOD10A1 of the Terra satellite and V006-MYD10A1 of the Aqua satellite were used for snow coverage from 2003 to 2020, which have a spatial resolution of 500 meters and a daily time resolution. The Modis sensor data in both Terra and Aqua satellites were converted from HDF format to TIF format with a threshold of 0.1-1 to binary and ASCII and with a geographic coordinate system. These data were processed using Python coding language. Cloud effect was reduced by using three algorithms: data combination, spatial, and temporal filtering. For the digital elevation model (DEM), the data of the Japan Space Exploration Agency (JAXA) called ALOS World 3D (AW3D) was used. After preparing the snow cover database, average snow-covered days (SCDs) were calculated on a seasonal scale and maps of the spatial distribution of snow cover in this period were produced. Then, the relationship between the two components of SCAs and SCDs and the relationship between SCDs and altitude in the north-western geographical area were investigated and analyzed.

Results and Discussion
According to the pattern of winter SCDs, the flat plains around Lake Urmia, the plains around Zanjan City, Moghan Plain, and the lands around Sanandaj City show the lowest rate of SCDs (less than 10 days of snow cover). The maximum of winter SCDs corresponds to Sablan Heights with more than 60 days of snow cover. According to the average spring SCDs in almost the entire northwest area, the drop of SCDs compared to the winter season is evident. This reduction is much more evident in low and flat areas and plains than in high and mountainous areas. Spring SCDs are still high in the main altitudes such as Sahand, Sablan, Qandil, Bozghoush, and Qara Dagh and reach about 54 days in the spring season in Sablan. The decline of SCD started from the south and covered more areas of the northwest region. Summer SCDs are facing a sharp decrease in the northwest region. Except for the heights of Sablan, Sahand, and the heights of the western border including the Qandil range, the rest of the areas have snow cover for less than 2 days in summer. It is noteworthy about the heights of Sablan, which still can maintain snow cover for up to 55 days in the summer season. The average autumn SCDs in the northwest region show an increase again in the autumn season. The heights of Sablan, Bozghoush, Sahand, Qara Dagh, and Qandil all show a significant increase in SCDs in the fall season, and in the high parts of these roughnesses such as Sablan, the fall SCDs increase to over 60 days. In other areas, mainly the main body of the roughness has SCDs above 20 days. Investigating the phenological changes of SCAs and SCDs in the seasonal period showed that the highest SC levels in the winter season have SCDs of about 15 days. This surface is about 18000 square kilometers. In general, the areas and regions that have SCDs between 5 and 25 days have the highest SC levels in this season. In the spring season in the northwest region, the SC levels, which have SCDs of less than about 8 days, reach a maximum of 160,000 square kilometers. In the summer season, lands with SCDs of less than 5 days occupy the highest levels, reaching a maximum of 160,000 square kilometers. In autumn, the regions with SCDs of about 7 days have the highest SC levels with an area of about 50,000 square kilometers.

Conclusion
The findings indicate a decreasing trend of SCDs from winter to summer. Meanwhile, the average autumn SCDs in the northwest area again take on an increasing tone in the autumn season. Analyzing the seasonal changes of SCDs in the studied area showed that this phenological component of snow is strongly influenced by the two factors of altitude and latitude. Sablan mountain range has special conditions to maintain snow cover in the study area even in the summer season. The analysis of seasonal changes of SCAs with SCDs shows that the maximum extents of SCs in winter, spring, summer, and autumn seasons have SCDs of approximately 15, 8, less than 5, and 7 days, respectively. In winter, areas with SCDs of more than 40 days include significant SC levels of about 2000 to 8000 km2. On the contrary, in the autumn season, the areas with SCDs higher than 40 days have almost negligible SC extents, whose values reach below several hundred square kilometers. Analyzing the changes of SCDs with the altitude measurement in the seasonal interval, showed the obvious relationship of this snow phenological component in association with the altitude; In such a way that the highest values of SCDs in winter belong to the altitude band of 2500-3500 meters, in spring to the altitude range of more than 2500 meters, in summer to altitudes above 4000 meters and in autumn to the altitude belt of 2000-3500 meters.