| As the largest freshwater lake in China,Poyang Lake plays an important role in the ecosystem and has received widespread attention.Nevertheless,the inundated area of the lake has shrunk sharply and drought occurs quite frequently in recent years,which is mainly caused for many reasons such as economic development,human activities and climate change.The suspended sediments concentration(SSC)has been increased significantly and the water quality has been severely deteriorated under the influence of extensive sand dredging,having a great impact on the natural environment,living and production around the lake.As a result,the study in the hydrodynamic characteristics with high variability and transport process in Poyang Lake,analyzing the influence of sand dredging activities causing SSC increase and its variability quantitatively,has great significance for water environment improvement,protection and management for the lake.In this research,Delft3D,the numerical model,is employed to simulate the long-term hydrodynamic process in Poyang Lake from 2011 to 2015 continuously,using historical meteorological and hydrological data along with bottom topological data in the lake.Taking historical in-situ data and results derived from Moderate Resolution Imaging Spectrometer(MODIS)satellite images as validation data,there is a good consistence between the simulating and validating results of water level,outlet discharge and inundated area,showing that Delft3D is applicable to simulate the variable hydrodynamic process with high precision in Poyang Lake.With multiple simulating results,the analysis on hydrological characteristics and hydrological responses to drought and dry climate condition is conducted.Based on the hydrodynamic model,the two-dimensional sediment transport model of Poyang Lake is developed considering with sand dredging vessels detected from Landsat satellite images.Validations are conducted,using in-situ data and SSC retrieved from MODIS images,which show a great precision increase compared with the simulation without sand dredging activities and a more consistent spatial pattern of SSC between the simulated and retrieved results of MODIS images.Finally,several contrastive simulations are implemented to analyze the spatiotemporal variation characteristics of SSC under circumstance of different intensities of sand dredging.(1)Delft3D is able to simulate the fast-changing nature of hydrological condition of Poyang Lake,showing different hydrological characteristics and variations in dry and wet seasons correctly.The simulated water level has a good consistence with the field observations of main hydrological stations in the lake,with the coefficient of determination(R2)all greater than 0.88,the root-mean-square error(RMSE)of Xingzi,Duchang,Kangshan are 0.26 m,0.33 m,0.63 m,and the relative error(RE)are 1.80%,1.21%,3.34%,showing a lower accuracy in the dry season than wet season.The outlet discharge simulated is highly correlated with in-situ data in Hukou,with the R2 of 0.869,the RMSE of 1557.83 m3/s and the RE of 18.70%.The inundated areas extracted from MODIS are in good accordance with simulated results,with the R2 of 0.829,the RMSE of 283.47km2 and the RE of 15.90%.(2)With the combination of in-situ data,MODIS retrieved results and simulated hydrological results,the long-term variability of hydrodynamic characteristics are explored.The largest and smallest inundated areas derived from hydrological model and MODIS have similar morphology and consistent spatial pattern,both reflecting dramatic changes of water distribution in dry and wet seasons,with a maximum ratio of the largest inundated area to the smallest reaching 4.58.The analysis draws a conclusion that some areas are flooded in most of the time of the year,including the main channel from south to north and several scattered lakelets such as Qinglan Lake in the southeastern,Dahuchi Lake near the Xiu River and inlet of Ganjiang River and Jiaotan Lake in the eastern,while other bottom areas get exposed above water on account of dry and wet season changes especially under dry weather conditions which cause decline of water level,shrink of inundated area and drought occurrence ahead of time.(3)Integrating with the validated hydrodynamic model,the two-dimensional sediment transport model is employed with consideration of sand dredging vessels detected from Landsat images.With the aid of in-situ data and satellite images,the validation shows much better consistence of the spatial pattern of SSC between simulated results and MODIS retrieved,with the R2 of 0.869 and the RMSE of 15.5mg/L between in-situ and simulated data,which shows much higher accuracy compared with the simulation without dredging information.(4)To make further explorations of SSC variability under influence of different sand dredging intensities,corresponding scenario simulations are designed.The results indicate that three main sand dredging areas show less effects on the SSC in the southern main lake,river inlets and water area in the west of Songmen Mountain but more in downstream areas in the northern including the main channel in the west of Tangyin and the north of Songmen Mountain,water-way in the northern and lake outlet,with a total affected area of 730 km2,which is one of the main factors causing high turbidity in the northern of Poyang Lake.In the same sand dredging intensity conditions,the recovery time for SSC of a computational grid to decrease to 50%of SSC without dredging is much less than from 50%to 5%.With the increase of sand dredging intensity,the recovery time for SSC to decrease to the concentration without disturbance by sand dredging increases remarkably in affected water areas,presenting characteristics that the recovery time changes from short to long when the water flows from main channel to outlet,south to north,with a steady increase from open water to nearshore water in the eastern of lake and the longest recovery time of SSC occuring in the lakelet named Huamiao Lake in the northeast of main channel near Tangyin. |
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