Remote Sensing Monitoring And Analysis Of Spatial And Temporal Changes Of CDOM In Poyang Lake

Poyang Lake is the largest freshwater lake in China.It has obvious seasonal high dynamic water regime changes.It plays an important role in regulating the water level of the Yangtze River and maintaining the ecological balance of the surrounding areas.In recent years,the lake environment has been damaged to varying degrees.Especially in water quality,the emergence of eutrophication and excessive pollutant content endanger the survival and development of residents and animals and plants in the lake area.It is urgent to protect the“Clear water in a lake”of Poyang Lake.Through remote sensing,the water quality of Poyang Lake can be monitored effectively in the time scale and space range,and the changes of water components can be analyzed to provide technical support for water quality protection in the lake area.CDOM?coloured dissolved organic matter?is one of the most important components in water.It affects the material cycle and biological activities of water environment at all times.Studying the optical properties,concentration distribution and source of CDOM in water is helpful to understand the water quality of inland lakes,and is of great significance for monitoring and analyzing water color remote sensing.Based on the data obtained from two measurements in high and low water periods,the optical characteristics,concentration distribution and source of CDOM were analyzed,and the inversion model of CDOM concentration in Poyang Lake waters was established according to the measured data.The results of CDOM inversion from 1989 to 2017 are obtained,and the temporal and spatial variation mechanism and influencing factors of CDOM are analyzed.Based on the calculation and analysis of two measured data at 440 nm wavelength,the concentration of CDOM measured in high water period is between0.4047m^-1 and 0.7381m^-1,and in low water period is between 0.3201m^-1 and0.5159m^-1.On the time scale,the mean concentration of CDOM in dry season is lower than that in flood season;on the spatial scale,the concentration distribution of CDOM decreases from coastal to central lake,from estuary to main channel,showing strong terrigenous origin.The S value of CDOM spectral absorption slope was obtained by fitting.It was found that there was a negative correlation between a?440?and S value.The S values measured by the two experiments are within the known range of inland lakes,and the mean values are similar,but the spatial distribution is different.The S value of the main lake area is relatively stable,and the S value of the coastal waters changes obviously.The composition of CDOM in this waters is greatly affected by land sources.By analyzing the relationship between CDOM concentration and suspended sediment,it is found that there is a weak correlation between CDOM concentration and suspended sediment,while there is a good negative correlation between CDOM concentration and total phosphorus.The turbidity of water body is greatly affected by sand mining activities during the flood season,which affects the concentration of CDOM measured at some stations.The CDOM inversion model is established for Poyang Lake.The correlation coefficient is-0.7489,the average relative error is 7.2%,and the root mean square error is 0.0702m-1.The model has good inversion accuracy.Using Landsat series satellite remote sensing images,the concentration of CDOM in Poyang Lake in recent30 years was inverted,and its temporal and spatial variations were analyzed.It is found that the concentration distribution of CDOM varies with the change of Lake location.The high value areas generally occur near the shore of the lake area and the inflow of the five major rivers.The concentration of CDOM in the eastern and Western Lake areas is also higher than that in the Northern Lake areas,while that in the southern Lake areas is higher and that in the central lake areas is lower.The change of water regime is also an important factor affecting the concentration of CDOM.The inversion results show that the concentration of CDOM in dry season is lower than that in high season.Based on the analysis of the sources of CDOM by natural and human factors and combined with the existing studies,it is concluded that CDOM in Poyang Lake waters is mainly affected by terrestrial inputs,but less by phytoplankton degradation and sediment release.Among them,the dynamic change of water level and sediment content have great influence on the change of CDOM in Poyang Lake,and wetland coverage and water quality are also one of the influencing factors.