Investigating spatiotemporal snow cover variability via cloud-free MODIS snow cover product in Central Alborz Region

Snow is a major source of streamflow in Central Alborz Mountains in northern Iran. Hence, understanding the spatial distribution and temporal variation of snow is crucial for proper management of limited water resources in this region. Since the in-situ measurements in the area are not sufficient, we used the Moderate Resolution Imaging Spectroradiometer (MODIS) snow cover products, for which their capabilities in providing a spatially distributed and accurate mapping of snow have been well demonstrated. In this analysis, we considered all the available MODIS acquisitions from both Aqua and Terra satellites for a total of 4873 daily tiles of MOD10A1 and MYD10A1 with 500-meters spatial resolution (version 5), collected during the winter seasons between years 2002 and 2015. The problem of cloud coverage, with serious constraints to the snow mapping in the area, is addressed by developing a sequential cloud removal algorithm, which applies different methods subsequently considering their performance in each month. The results indicate that the proposed algorithm is able to reduce 94% of cloud coverage and preserve the accuracy of cloud-removed area by 93%. In addition, a clear decreasing trend in snow cover is observed in the region. For example, trend analysis (at 10% significance level) shows that 61% of areas between 2000 and 2300 m elevation levels experienced a reduction in duration of annual snow cover. Moreover, a general reduction in the duration of snow season is also evident over the entire study area. In spite of short duration (13 years) used in this research, outcomes indicate a severe snow cover reduction during recent years.