Remote Sensing Monitoring Of River Ice Fraction In OB River Basin Based On MODIS

River ice change is one of the key climate variables of the Global Climate Observation System(GCOS)and is a component of the cryosphere.As an important freshwater ice on the land surface,river ice occupies a dominant position in the Arctic terrestrial environment.Its formation and rupture have greatly affected the ecosystem and socio-economic system of the entire Arctic region.In order to obtain the change pattern of river ice in the Arctic,this paper proposes the concept of river ice fraction to monitor the river ice.Based on a 30-m resolution land surface water dataset from 2010 together with global river width data and reference DEM elevation data,the river systems in each basin were identified.High-resolution image data were used for verification and the resulting accuracy was 93.96%,and identified river ice based on this data.This study takes the Ob River basin as an example,based on the vector data of Arctic rivers in Eurasia,combined with MODIS,using NDSI snow cover algorithm SNOWMAP to estimate the daily river ice under sunny conditions.Then,based on the spatial and temporal continuity of the freezing and breaking of river ice under cloudy conditions,a series of cloudless processing was performed to obtain daily river ice fraction data.Taking the 12.5/25/50 grid as the condition,extract the pixel values of the Ob River basin and calculate the proportion of river water,ice and cloud,and obtain the proportion of river ice coverage under each grid.Under grid conditions,the time series of river ice fraction has obvious periodic changes.The time series data can be used to estimate the river ice phenology parameters of Freeze Up End(FUE)and Break Up Start(BUS),and obtain the Ice cover duration.For river ice fraction data validation,the Landsat and MODIS images of the same day were selected,and the ratio of river water and ice in the same unit was calculated with 12.5km as the unit.The two were compared,and the overall accuracy of MODIS river ice fraction was calculated to be87%.The results show that the river ice fraction of the Ob River fluctuated from 2002 to 2018,and the annual river ice fraction averaged more than 45%.The river ice fraction increased from 2012 to 2015,but the overall trend was gradually decreasing.By calculating the river ice fraction in the sub-basins of the Ob River and combining the temperature changes,it can be seen that the six river basins,including the Lower OB,Middle OB,and OM river in the east direction,have shown an upward trend in temperature over the years and a gradual decrease in ice cover;in the four westward river basins,Iset,Tobol,and Ishym river,the temperature has been decreasing over the years,and the ice cover has been increasing.The river ice phenology of the Ob River varies with latitude,and the ice cover duration is gradually extending from south to north,but there are differences in sub-basins of Ob River basin.According to the change rate of the ice cover duration of each basin over the years,it can be concluded that the ice cover duration of the western rivers were shortening,and the ice cover duration of the eastern rivers were all extending.River ice fraction data provides fundamental data for polar climate,ecology,environment and water resources research.This method can be extended to river ice research in other basins,supporting polar research based on big earth data,and understanding the effects of river ice changes response to climate warming.