Remote Monitoring Of Vertical Distribution Of Inorganic Suspended Matter Concentration In Lake Hongze,China

Lake Hongze?HZL?is the fourth largest freshwater lake in China and the largest lake in the Huai River basin.HZL is not only an important source of drinking water for5 million people in Huai'an City,but also a damming reservoir with important regulation and storage functions in the East Line of South-to-North Water Transfer Project in China.Under the influence of water disturbance of Huai River,climate change,and human activities,HZL has become a typical turbid lake dominated by inorganic suspended sediment.The concentration of inorganic suspended matter?ISM?and its temporal and surface-vertical distribution affect the underwater light field,and further affect the lake ecosystem function via photosynthesis.In addition,HZL has become a"Hanging Lake"since the bottom of HZL is 4-8 m higher than the eastern Subei Plain.The three-dimensional fluctuation of ISM in HZL may affect the safety of the artificial dam,which was built in the eastern part of the lake in the Eastern Han Dynasty?about 200 AD?.Therefore,studies on the temporal and spatial distribution of the surface-vertical ISM comprehensively can not only enhance the understanding of the biogeochemical process in the water column,but also lay a data and theoretical foundation for the lake management,such as the supervise in water-sediment balance,the protection of lake environment and the construction of lake ecological security.At present,the ISM in the surface water layer is targeted by most quantitative studies with remote sensing,and the vertical distribution of the ISM is assumed homogeneous.But the reality is that the influence of the variation of ISM in different water layers on the optical properties of surface water cannot be ignored.Taking HZL as the case,this study focuses on its light transfer mechanism of different layers of the water column,analyzes the response relationship between underwater light field information and water color parameters according to the measured water quality and optical data in three field experiments in HZL,determines the boundary value of water radiation transfer simulation,and calibrates the key optical parameters describing light absorption and scattering.A forward radiative transfer calculation is performed to relate the inherent optical properties?IOPs?of the optically active components?OACs?in HZL to their corresponding apparent optical parameters?AOPs?and to their compositional characteristics?including composition,concentration,particle size?in different water layers of the lake.Furthermore,a look-up table?LUT?and a corresponding matching method suitable for HZL are established for remotely monitoring the concentration of inorganic suspended matter at different depths?ISM_z?and the column mass of inorganic suspended matter?CMISM?with Sentinel 3/OLCI?Ocean and Land Color Instrument?data.The main research contents and conclusions of this paper are as follows:?1?Vertical distribution of components,concentration,particle size and optical propertiesAccording to the in-situ data in HZL,the vertical distribution of OACs?components,concentration and/or particle size of the underwater color parameters?and the interaction mechanism between OACs and their IOPs and/or AOPs are explored.In terms of water composition,HZL is a typical lake dominated by inorganic suspended sediment.The average ratio of ISM to TSM is about 87%.The correlation coefficient between ISM and TSM is 0.99.The deeper the water layer,the more proportion of ISM in TSM.In terms of water component concentration,the overall concentration of chlorophyll a?Chla?and organic suspended matter?OSM?are low,and Chla and OSM decreased with the increase of water depth.While the TSM and ISM values are high,the TSM and ISM increase with the increase of water layer depth.In terms of particle size distribution,the suspended particulates in HZL are fine mineral particles mostly(D_v⁵⁰<17?m,D_A<7?m,such as clay,silicon fine sediment,etc.).The deeper the water layer,the more lager particle size of suspended particles.In the spectral range of 400-800 nm,most of the non-pure water absorption of samples in different water layers are dominated by non-pigment particles.The IOPs,such as absorption,backscattering,attenuation coefficients and AOPs in different water layers of HZL are influenced by the ISM,TSM and particle size distribution significantly.?2?LUT construction to describe ISM vertical variation and water color parametersBased on the above analysis of the optical characteristics of the water column of HZL,this study determines the boundary values of the water body radiation transmission simulation,constructs the parameterization of key optical quantities such as the absorption,scattering,and/or backscattering coefficients of the OACs in different water layers,and created a vertical parameter LUT suitable for the surface layer of suspended matter in HZL.The LUT is applicable to water bodies dominated by inorganic suspended sediments,and their Chla is at low and medium levels,ranging from 0 to 40mg/L;a _CDOM?440??absorption of colored dissolved organic matter at 440nm?is at low and medium levels,ranging from 0 to 2 m^-1;The range of ISM is the widest from 0 to96 mg/L;the backscattering probability and the backscattering slope intervals are 0.012to 0.04 and 0.4 to 2.4,respectively,covering the measured data range of the IOPs for most samples of HZL;at the same time,six vertical distribution modes of ISM,such as index,logarithm and linearity are set;the LUT has a total of 475,200 samples.By comparing and verifying the measured data and the corresponding results in the lookup table,it is found that the two data are very close:the MAPE?average absolute percentage error?value range for the remote sensing reflectance?34 bands?is 2.8%to 16.6%,and the average value is 7.19%.?3?Remote sensing estimation of surface-vertical ISMIn order to improve the match calculation efficiency while to ensure the matching accuracy,the R_rs???of OLCI are used to fuzzy estimate the surface Chla and ISM firstly.Further,a two-step matching approach is designed to derive the ISM_z and CMISM using the magnitude and shape of R_rs???under the consideration of the atmospheric correction accuracy.Finally,comparing the OLCI-derived surface ISM and CMISM with 23 quasi synchronous?±3 hours?in-situ sample data,the results show that the MAPE of the surface ISM and CMISM are 18.24%and 23.74%,respectively.When the above algorithms are applied to 95 high-quality images acquired in 2018,it is found that the matching similarity?P?of more than 90%of the pixels is higher than 90%.?4?The spatial and temporal distribution of the ISMz and CMISM in Hongze LakeThe spatial distribution of the ISM_z and CMISM of HZL is similar.The high value is mostly located in the eastern open water of HZL,especially at northeast waters?NE?and river channel?RC?.At the same time,Chengzi Lake?CZL?in the northern HZL and wetland?WL?in the western HZL are in the bay area characterized by sluggish flow and low values of ISM_z and CMISM.The seasonal patterns of ISM_z and CMISM are low in spring and high in autumn.The monthly variation features of surface ISM and CMISM are consistent with the quadratic polynomial distribution,and the fitting determination coefficients are higher than 0.44,showing a strong monthly variation feature of the annual distribution in most partial of HZL.