Simulation Of Case-2 Water Bidirectional Reflection Distribution Function And Retrieval Of Chlorophyll-a Based On Broad Band Remote Sensing

Coastal waters and inland lakes are the most important areas for human life,and nearly 10% of the global population lives in coastal areas that cover less than 2% of the land.The remote sensing monitoring for chlorophyll-a of case-2 waters is of great significance for water source protection,ecological environment change monitoring,aquaculture planning and coastal infrastructure construction.Traditional ocean color satellites with medium and low spatial resolution(e.g.,MODIS,Sea Wi FS and MERIS)cannot satisfy the observation requirements of optical shallow coastal waters and small lakes.The developments of high spatial resolution remote sensing for water quality monitoring can make up for the lack of spatial resolution of traditional ocean color satellites and the limitation of cloud covers on single remote sensing data source.However,the high spatial resolution remote sensing sensors are mostly broad band(e.g.,Landsat and SPOT),which are limited by bandwidth and complex optical properties of case-2 waters.Therefore,the algorithm of chlorophyll-a retrieval by broad band remote sensing data is not mature,and new models are needed to provide theoretical basis for water quality monitoring in coastal waters and inland lakes.The case-2 waters affected by terrigenous material inputs with complex optical properties.Therefore,the research of retrieval algorithms for case-2 water has always been a hot and difficult problem.The retrieval of chlorophyll-a based on remote sensing data depends on the successful acquisition of water-leaving reflectance and the selection of algorithms for chlorophyll-a in different types of waters.The high concentration of inorganic suspended particulate matter derived from land changes the bidirectional reflection distribution function(BRDF)of waters which is the basis of standardized water leaving reflectance.In addition,the high concentration of inorganic suspended particles can mask the chlorophyll-a information in broad band remote sensing data.According to the optical properties of chlorophyll-a and inorganic suspended particles,the concentration of chlorophyll-a inversed by remote sensing will be overestimated without considering the errors caused by inorganic suspended particles.Therefore,the study of case-2 water BRDF characteristics and the sensitivity of broad band remote sensing data to inorganic suspended particulate matter are two difficult problems in the ocean color research.Considering the effects of inorganic suspended particles on water BRDF characteristics,Cloud Absorption Radiometer(CAR)was used for the first time to study BRDF of water.In addition,Lee’s BRDF model and Morel’s BRDF model are evaluated,and the suitable BRDF model for case-2 water is applied to the water leaving reflectance of MODIS products in Bohai Sea.The results show that Lee’s BRDF model is closer to the real observation data in back-scattering direction than Morel’s BRDF model in the case of simulating the BRDF of waters with inorganic suspended particles.After BRDF corrected for MODIS water leaving data by Lee’s model,the blue-green ratio(BGr)of water-leaving reflectance has a better correlation with concentration of chlorophyll-a,which improves the accuracy of MODIS chlorophyll-a production and provides theoretical basis for the normalization for water leaving reflectance in case-2 waters.Through the analysis of the measured spectrums and water constituent parameters from 85 sampling points of inland lakes(9 study areas)and Bohai Sea,the applicability of the algorithms for waters with different chlorophyll-a concentrations were discussed.By combining the algorithm BGr,the near-infrared three-band algorithm(3NR)and the semi-empirical improved algorithm based on the BGr in this paper,and setting the applicable conditions of the different algorithms,a broad band remote sensing chlorophyll-a retrieval model waters with complex optical properties is established.The applicable conditions of the new model are as follows:(1)If 3NR is greater than 0.03,the 3NR algorithm is used for the eutrophic water;(2)If BGr is greater than 0.81,the BGr algorithm is used for water with medium chlorophyll-a concentration;(3)In other cases,the improved blue-green band semi-empirical algorithm proposed in this study is used for water optical properties dominated by inorganic suspended particles.This model is not only effective in middle and high nutrient water,but also applicable to the case-2 water with low concentration of chlorophyll-a,and it extends the universality of the single model and provides theoretical reference for the retrieval of water color parameters by broad band remote sensing.Aiming at the problems that the retrieval of chlorophyll-a in optical shallow water affected by sunglint,bottom reflection and inorganic suspended particles,a study on chlorophyll-a inversion was carried out in Sanya Bay,and the biggest error sources of chlorophyll-a inversion in optical shallow waters were discussed.The results show that the bottom reflection has little influence on the retrieval of chlorophyll-a due to the effect of re-suspended inorganic particles when the sunglint was not obvious in the remote sensing images.In addition,inorganic suspended particulate matter is the biggest error source for chlorophyll-a retrieval in coastal waters and making the error of chlorophyll-a retrieval in Sanya Bay exceed 30%.The study of Sanya Bay reveals that inorganic suspended particles may be the biggest error source of coastal water,and provides a reference for the selection of chlorophyll inversion algorithm for coastal water.Compared with the traditional ocean color satellite with medium spatial resolution,the remote sensing application of ocean color parameter retrieval has the advantage of high spatial resolution in optical shallow water,estuaries and small lakes.It has application value for chlorophyll-a retrieval of water body dominated by inorganic suspended particles.