| Since the 1980s,the sediment transport of the Yangtze River has decreased dramatically,which may change the sediment flux into the sea and cause erosion of the Yangtze River Subaqueous Delta.Therefore,it would have a significant impact on the evolution of the Yangtze River Subaqueous Delta,the estuarine ecological environment system and the channel safety.However,due to the complex hydrodynamic environment of the Yangtze River Estuary,effectively evaluating the variation of suspended sediment and analyze the influencing factors is a difficult and hot topic in this field.In this study,the monthly products of MODIS\SST and MODIS\((9(9)(490)were used as data sources,and the performance of DMD,DMD combined with QR decomposition and DINEOF methods on long-term dynamic data reconstruction were analyzed and compared.SST data was used to constrain the spatial distribution of the Changjiang dilution water(CDW).The time series and spatial changes of SSC represented by((9(9)(490)of typical observation stations are analyzed and studied,with a view to obtaining the change tendencys of suspended sediment from the Yangtze River into the sea from 2003 to 2019.The results demonstrated that the DMD algorithm combined with QR decomposition can effectively reconstruct SST and((9(9)(490)data.In the SST data reconstruction experiment,the first and third DMD mode may reflect the influence of the Yangtze River flushing water and circulation,and the second and fourth DMD modes may represent the influence of seabed topography.The monthly average SST of the Yangtze River Estuary has no obvious inter-annual variability during the investigation period,but illustrates obvious seasonal variability.The seasonal variability is characterized by the highest temperature in summer,the lowest temperature in winter and medium in autumn and spring.In terms of spatial distribution,the SST gradually increases from the coast to the open ocean,showing a"northwest-southeast"distribution pattern except for summer.In summer,affected by terrestrial heat sources,the coastal area shows the highest value of the month.In the((9(9)(490)data reconstruction experiment,the first,second,third,and fourth DMD mode may reflect the influence of seabed topography,Yangtze River and its circulation,mixing of CDW and saline seawater,and terrestrial inputs,respectively.The monthly average((9(9)(490)of the Yangtze River estuary has complex variability during the investigation period.The seasonal variability shows the signature of the lowest in summer,highest in winter,and medium in spring and autumn,it firstly decreases and then increases within a year.The spatial distribution is characterized as a tongue-like diminishing eastwards distribution from the coast to the open sea,and the boundary between high and low values roughly coincides with the depth contours in the region.The high value of((9(9)(490)in summer is higher than that in other seasons,and the spread is the most extensive in winter.Using the typical observation stations of the SST reconstruction experiment to clarify the influence range of the Yangtze River dilution water,and the analysis of the station of((9(9)(490)illustrated that the change in the amount of suspended sediment that the Yangtze River entered the sea from 2003 to 2019 displays neither singly increasing nor decreasing trend,and also not completely synchronized with the changes in the sediment volume of the Yangtze River,which is characterized by the combination of the three conditions.One is that the amount of suspended sediment entering the sea and the amount of sediment transported are basically the same,which indicates that the sedimentation rate and erosion rate of the Yangtze River Delta are in balance.The second case is that the amount of sediment transported increases while the amount of suspended sediment transported to the sea decreases,or the increase in sediment volume is smaller than the increase in the amount of suspended sediment into the sea,or the decrease in sediment volume is smaller than the decrease in the amount of suspended sediment into the sea.This may indicate that the sediment carried by the Yangtze River runoff is in the range of the Yangtze River dilution water.Thus the amount of suspended sediment entering the sea has decreased.The third situation is that the amount of sediment transported decreases and the amount of suspended sediment entering the sea increases,or the amount of sediment transported to the Yangtze River decreases more than the amount of suspended sediment entering the sea,or the decrease in the amount of sediment or the increase in the amount of sediment transported is smaller than the increase in the amount of suspended sediment entering the sea.This case may indicate that the suspended sediment entering the sea is not only the sediment carried by the Yangtze River dilution water,but may include the suspended sediment from compensatory erosion in the delta. |
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