| The Yarlung Zangbo River(YZR)is one of the highest rivers in the world and an important international river.However,due to its location in remote mountainous areas,harsh terrain and environmental conditions,and sparse distribution of survey stations,people have limited understanding of the dynamic changes of the YZR hydrological characteristics.By providing multi-temporal observation data,remote sensing satellites make it possible to monitor and analyze the dynamic changes of river hydrological characteristics in long time series.Based on the Global Land Analysis and Discovery(GLAD),this study filled the missing part of water body in the YZR by using the threshold segmentation method of river boundary flooding frequency,and reconstructed the monthly complete water area of the YZR from 2000 to 2020.This is the basis for rapid and continuous monitoring of the spatial pattern and spatio-temporal variation characteristics of its hydrological elements.On this basis,the area-water level fitting relationship was constructed by combining the short-term sequential water level results from Hydroweb virtual stations located on the YZR,and the long-term temporal variation data of water level of the virtual stations were reconstructed.Then,the reservoir capacity curve is constructed by combining the reconstructed water level and river water area to determine the variation of river water storage.Then the temporal and spatial variation of river width since 2000 is estimated based on the river center line.Finally,the application of ICESat-2 data in the YZR water slope estimation is discussed in combination with the acquisition of more water level information of different locations of the YZR by laser altimeter satellite ICESat-2.The main research contents and conclusions are as follows:(1)The GLAD and GSW datasets were used to reconstruct the complete water area of the YZR from 2000 to 2020 in combination with the threshold segmentation method of river boundary flooding frequency.The PA value and UA value of reconstructed water body classification were over 82%and the Kappa coefficient was over 84%in combination with Sentinel-2 verification.The results show that the water area of different reaches of the YZR has obvious differences in the period from 2000 to 2020,showing obvious seasonal changes.The annual average area of the upper,middle and lower reaches is 170.89 km~2,436.07 km~2and 73.68 km~2respectively,and generally shows a trend of decreasing first,increasing,and finally decreasing.The results of the YZR water area reconstruction can be used to further analyze the temporal and spatial dynamic characteristics of hydrological characteristics.(2)Based on the complete water range of the YZR since 2000 and the complete monthly river range of the YZR from 2018 to 2020,the fitting relationship between water range and water level of river segments near each virtual station can be established,and the water level of the YZR from 2000 to 2020 can be reconstructed.The river section is simplified into inverted trapezoid,and the pyramid estimation method is adopted to further estimate the variation of water storage in each reach.The time series of river width is estimated based on river center line.This paper makes up for the lack of hydrological characteristic data of the YZR and the lack of understanding of its temporal and spatial dynamic changes.The results show that the water level reconstructed from the YZR virtual station is in good agreement with Hydroweb,with an average R~2of 0.79.The variation of the YZR water storage is about one tenth of the total water storage in the basin.Limited by the minimum spatial resolution of Landsat satellite images,this study found that the river width at the narrowest part of the mainstream of the YZR was only about 30 m,and the maximum effective river width in the braided river region of the middle reaches of the YZR was up to 5935.73 m,and more than 50%of the river in the whole course of the YZR was narrower than 150 m.Only 1.99%of the channels are wider than 2000 m.(3)The high spatial resolution ICESAT-2 altimetry data were preprocessed to obtain the water level information of different reaches of the YZR,and then the water surface slope of the YZR at different times was generated.The variation characteristics of the reach with multiple times of obtaining the results of water surface slope were analyzed.The results showed that the YZR at the horseshoe big bend,The average surface slope is 1935.17 cm/km.In the catchment area of the upper reaches of the YZR,the minimum surface slope is about 0.08 cm/km,and there are a large number of villages and farmland gathering places near the middle reaches of the YZR.The terrain is flat and the water level changes little,so the surface slope is gentle.It is also found that the narrower river of the YZR shows a wider range of slope than the wider river.This shows that in general,water surface slope decreases with the increase of width,but for each reach,the change of water surface slope is positively correlated with the change of river width in that reach. |
