Abstract:We analyzed in detail the status quo of lake water color remote sensing from some aspects of satellite sensor, atmospheric correction, optical properties measurement, bio-optical model, radiative transfer model for the waters, and water quality parameter retrieval approach. It has a great difficulty to have practical application of water color remote sensing at a regional scale at present, depending on the complex components of lake water and on the inconsistency between satellite sensor and its actual demand from water quality monitoring. The progress in some key problems for lake water color remote sensing is still small, and there is a long way to go in applications of lake water color remote sensing. However, to be greatly pleasure, the satellite sensor and water color parameter retrieval approach are developing and making progresses, and the application in the future is hopeful.

Abstract:In studies of aquatic environments, an optical property, KPAR, which represents the diffuse attenuation coefficient of Photosynthetic Available Radiation (PAR), is used frequently. Because water's diffuse attenuation coefficient is highly spectral dependent and PAR is spectrally narrowing to wavelengths with less attenuation coefficients with increasing depth, this KPAR is significantly depth dependent in the upper water column even for well-mixed waters. In this article, with an aim for more reliable attenuation products and more accurate description of PAR profiles, the ambiguity associated with KPAR is highlighted, and the proper representation of the vertical variation of KPAR is advocated.

Abstract:The euphotic zone is defined as the layer where net phytoplankton production is positive. Therefore, determination of the euphotic depth is useful for estimating primary production. The TM data on 2007-01-07 and 2006-08-01, accompanied by real-time water quality parameters including concentrations of tripton and chlorophyll-a (Chl.a) in Lake Taihu, were used to develop quantitative retrieval models of tripton and Chl.a. Euphotic depth was calculated using the model developed in Lake Taihu based on the concentrations of tripton and Chl.a. The euphotic depth ranged from 0.27 to 2.28m (mean 0.56±0.22m) in winter depth from 0.21 to 2.03m (mean 0.98±0.24m) in the summer. In the winter, the euphotic depths showed spatial distributions which were ordered as: the south of Lake Taihu < the west coast of Lake Taihu < the lake center < Xukou Bay < Gonghu Bay < Meiliang Bay < East Lake Taihu < Zhushan Bay. In summer, the order changed as follows: the west coast of Lake Taihu

Abstract:In the present paper, we report an algorithm method to retrieve the oceanic constituent concentrations (CHL, SPM, and CDOM) in Case II waters. The method is derived from the radiative transfer simulations and is subsequently applied in the Artificial Neural Network (ANN) techniques. Information on absorption and total scattering of pure sea water, as well as absorption of phytoplankton and associated particles are taken from measurements or parameterisations in published literatures, and information on absorption of coloured dissolved organic matter and nonagal particles, as well as scattering of marine particles were derived from the COASTLOOC data set. Additionally, a new model on the backwards scattering probability model is used, of which probability is a function of the organic particulate matter and the total particulate matter (SPM) ratio and wavelength. Such defined inherent optical properties are input as a radiative transfer code in order to generate a synthetic data set of hemispherical reflectance spectra, subsequently used for the training of various ANNs to find the best approximation of the functional relationship between ocean colour and oceanic constituent concentrations. The performance of the ANN-based retrieval schemes is assessed by applying it to the hemispherical reflectance spectra contained in the COASTLOOC data set and PMNS data set, and comparing the retrieved oceanic constituent concentrations to those actual measurements. The results show that the ANN-based algorithms have good performance in retrieval of oceanic constituents for ocean colour measurements in Case II waters.

Abstract:Lake Taihu, which has been in serious eutrophic pollution status, was selected to be the study area. In Lake Taihu, two-time experiments of airborne hyperspectral remote sensing were carried out, covering seven airborne strips over Lake Taihu in both winter and summer. Besides of the two times of experiments, the in-situ inherent and apparent optical properties of Lake Taihu water were measured and analyzed for additional four times. The specific inherent optical property database of Lake Taihu was built. Based on the database and bio-optical model, analytical approaches were developed to retrieve chlorophyll, total suspended matter, and yellow substance. To validate these analytical approaches, airborne hyperspectral remote sensor WHI image and spaceborne hyperspectral remote sensor CHRIS image were used to retrieve water quality parameters, and the results were good

Abstract:The advantage of neural network method for estimating water quality parameters of complex water body has been approved. Using in-situ measurement data of chlorophyll-a concentration, imageries of MODIS 250m and retrieval model of water temperature, we develop two single-hidden-layer BP neural network models for estimating chlorophyll-a in Lake Taihu: Model NN1 without temperature input and Model NN2 with temperature input. The training method is used by Levenberg-Marquardt algorithm, and the early-stage determinationin the modeling is used to improve generalization. The results show that: the estimation precision of the two models is high, in which the estimation precision of neural network input with temperature has been improved although the test is not significant.

Abstract:Based on the in-situ data collected around Hong Kong sea area and its vicinity from March to May in 2001, spectral characteristics of water color components are analyzed. For establishing retrieval algorithm, the empirical mode between chlorophyll-a concentrations (Chl.a) and phytoplankton pigment absorption coefficients a_ph(675) is developed with a significant correlation. Relationship between total suspended matter concentrations (TSM) and de-pigment absorption coefficients a_d(440) is calculated. Underwater data which contain apparent optical property spectra are used to extrapolate water remote sensing reflectance (r_rs). A semi-analytical algorithm is developed to retrieve Chl.a which is compared with in-situ measurements lastly. The mean relative error is 45%, the correlation coefficient is 0.78, and the root mean standard error is 0.933. Results show that the algorithm is suitable for the study area with low concentrations of TSM and chlorophyll-a.

Abstract:Coefficients of absorption and backscattering are inherent optical properties (IOPs) of water, which play extraordinarily important roles in the detection of water constituent concentrations. However, it is very complicate and time-consuming to measure the IOPs of waters. In this study, a novel algorithm was proposed to derive the IOPs of water constituents through training samples with measured constituents' concentrations and reflectance spectra. Simulated data based on bio-optical model was used to assess the proposed algorithm. Results demonstrated that when the spectra of samples satisfied the analytical model and the training samples were independent each other, the performance of the algorithm can be accepted, which indicates that the algorithm proposed in this paper is theoretically reasonable.

Abstract:Identification of algae-bloom and aquatic macrophytes plays a significant role in inland water quality monitoring by remote sensing, which can be used to reflect the water quality status indirectly, and then the result of water can be used to retrieve water quality parameters. However, the mostly used multi-spectral remote sensing data cannot accurately identify algae-bloom and water grass. Only hyperspectral remote sensing data, as the data can be distinguished the subtle differences of the spectral characteristics between algae-bloom and water grass, can be used to identify algae-bloom and water grass with high accuracy. Unfortunately, there have been few of profound researches on the identification of algae-bloom and water from hyperspectral remote sensing data. Lake Taihu is selected to be the study area. Two experiments were carried out in Lake Taihu in July and October of 2006. Reflectance spectra of the floating vegetation, submerged vegetation, algae-bloom, and water were measured. Based on the analysis of the measured spectra, four spectral indexes were defined to build up formulas for identification of the four items. Reflectance spectra measured in October 2006 were used to determine the threshold values in the identification formulas, and reflectance spectra measured in July 2006 were used to validate the identification formulas. The identification results were very good.

Abstract:Chlorophyll-a is one of the main water color elements which affect inland water optical properties, and it is the important parameter reflecting water eutrophic level. In the recent years, many methods and models are developed to retrieve Chl.a concentration. These methods or models all uses immobile bands. However, characteristic bands of chlorophyll-a move with the concentration changes. Especially, phenomenon of red-shift effect occurs obviously at 701nm reflective peak. As a result, using immobile bands to retrieve Chl.a concentration must bring some errors. In order to obtain higher precision, author in this article analyzed water spectra with different Chl.a concentrations and brings forward a new method of using dynamic apex to retrieve the Chl.a concentrations.

Abstract:Remote sensing reflectance (Rrs) and concurrent chlorophyll-a (Chl) concentration were measured from August 2007 to September 2008 in two pools (Furong Pool and XMU Reservoir) in Xiamen, Fujian. During most of the time, the measured Rrs spectra for Chl ranging between 6 to 322mg/m³ show a prominent secondary peak in the far-red region of the spectrum. This is not from fluorescence alone, but appears to be caused by the red edge. Following the estimation of MODIS Fluorescence line height (FLH), we defined a red edge height (REH) during 705 and 710nm. It was found that REH was highly correlated with Chl. The relation functions are REH=0.016lg(chl)-7.8×10^-6(R2=0.79, n=41) in the Furong Pool, and REH=0.0001chl+0.0002 (R2=0.74, n=32) in the XMU Reservoir. Our results suggest that the red edge may provide a good tool for researching and management in order to study and monitor the algal blooms in waters where there are dense suspensions of algal cells.

Abstract:lower than 1 with the average value of 0.62±0.14, but was larger than 1 at sample 12 with the average value of 1.12±0.13. The r(B/C) can be preliminary used to distinguish the composition and source of CDOM. Significantly positive correlations were found between CDOM absorption coefficient and the fluorescence intensities of peaks A, C, D, but no significant correlation found with peak B

Abstract:Researches on optical properties of Chromophoric Dissolved Organic Matter (CDOM) has played an important role in quantitative water-color remote sensing retrieval. The optical properties of CDOM in coastal waters of Lake Taihu were investigated from June 10 to June 18 in 2007. The results show that the specific absorption coefficient of CDOM varied significantly and was higher apparently in Gonghu Bay than in other areas. The specific absorption coefficient at 440nm (ag(440)) also showed a similar trend, and it reached the highest value in Gonghu Bay. The ag(440) in waters of Meiliang Bay, Zhenhu Bay and Guangfu Bay also showed an increased trend. On the contrary, the spectral slope coefficient (Sg) in Gonghu Bay was lower than others, and showed a better correlation with ag(440), while Sg in other bays was higher and showed a poor correlation with ag(440). The results demonstrated that it is important to learn the optical properties of CDOM in Lake Taihu, which would then allow higher precision in remote sensing applications

Abstract:Chromophoric dissolved organic matter (CDOM) is the colored fraction of the dissolved organic carbon (DOC) pool. According to investigations and filed measurements in Lake Honghu, Lake Donghu, Lake Liangzi in September and October of 2007, optical characteristics and spatial distribution of CDOM in these lakes are analyzed. Results show that CDOM absorption coefficient in Lake Honghu is the highest, with the mean value of the CDOM absorption coefficient at 400nm (a_g(400)) is 1.45m^-1, while the lowest is in Lake Liangzi, with the mean value of the CDOM absorption coefficient at 400nm is 0.68m^-1. A significantly positive correlation between CDOM absorption coefficient and suspended sediment concentration is found in Lake Honghu. In Lake Liangzi, CDOM absorption coefficient is positive correlated with chlorophyll concentration, which means CDOM in the lake is mainly contributed by Phytoplankton degradation. The slope (S) in the exponential correlation function does not vary much in Lake Honghu and Lake Donghu. Fulivc acid is the dominated constituent of CDOM in Lake Liangzi, and the absorption coefficient of CDOM vary extremely. There is a negative correlation relationship (-0.9) between S and a_g(400) in Lake Liangzi.

Abstract:The Yunnan Plateau lakes with different lake types, located in Yunnan Province in southwestern China, are one of the five densest lake areas in China. The lakes, which are generally distributed at elevations from 1200m to 4000m above sea level, receive high intensity UV-B radiation. In this study, the spectral absorption properties of chromophoric dissolved organic matter(CDOM) and particulate matter are studied based on a wide investigation including 34 lakes in the Yunnan Plateau. The study will be helpful in the understanding of the ultraviolet radiation attenuation and the effects of increased UV-B radiation on lake ecosystems. A wide range of variability in CDOM absorption was found among the Yunnan Plateau lakes. CDOM absorption coefficient generally corresponded to the nutrient level with a significantly positive linear correlation between a_CDOM(320) and total nitrogen concentration. Exponential model with constant parameters of background items could give the most approximate estimation of CDOM absorption spectra. A marked phytoplankton absorption peak around 675nm was only observed in Qiluhu, Tinghu and Xinyunhu with very high chlorophyll a concentration. High contribution of CDOM to the total absorption was found during the wavelength range lower than 600 nm especially in the ultraviolet range lower than 400nm. The contribution of CDOM to the total absorption was significantly higher in UVR range (350-400nm) than in PAR range (400-700nm) (ANOVA, P<0.001). For the clear lake water with secchi disc: SD≥1.0, the contribution of CDOM absorption to the diffuse attenuation of ultraviolet radiation would increase, and CDOM absorption, with a large probability, would determine the attenuation depth of ultraviolet radiation.

Abstract:For improving the reliability and accuracy of lake boundary integration information from remote sensing images andreducing human error, this paper put forwards a weighted average algorithm for integrating of the border information extracted byusing the multi-temporal remote sensing images, and a processing approach of error interzone correction which can integrate thevarious temporal information effectively and improve the reliability of extracting the lake border information. According to thisfusion algorithm the area of Lake Hulun is 1928.35km². Using the geostatistics theory to validate the errors, the area is 1929.85km².The results illuminated that the ratio of nugget to sill is less than 25%, and the spatial correlation of the corrected data is superior tothat of the integrated data. Furthermore the spatial variation range has been reduced effectively, and the extent of fitting thetheoretical model is better in the corrected data than the original data.

Abstract:The optical property of inland water is very complex, and inversion of water components concentration of inland water body is hotspot and difficulty problem in water quality remote sensing. The remote sensing reflectance and water qualities of 67 stations were acquired in the Lake Taihu in October, 2004. Basing on the characteristic spectral analysis, it was obvious that the water body spectral character in Lake Taihu in autumn was dominated by the total suspended matter (TSM). The properties of the chlorophyll fluorescence peak mainly reflect the variation of the concentration of TSM. Hereby, we established the relation between the properties of the chlorophyll fluorescence peak and the concentration of TSM based on in-situ remote sensing reflectance with 1nm bandwith and water quality measurements. There were good response relationships between them. Making a concrete analysis ofthe correlation relationship between the absolute height of fluoresce peak, the height above the reference baseline, the normalized height referred to the maximum reflectance about 560nm in green band and 810nm in infrared band, the integral area (including the total integral area, the integral area between the spectral curve and the reference baseline, the integral area below the reference baseline and the wavelength axis) and the concentration of TSM. Their correlation coefficients were 0.8822, 0.7483, 0.8901, 0.8547, 0.8927, 0.8877 and 0.8632 respectively, and the average relative error were 27.25%, 41.03%, 27.11%, 25.75%, 24.91%, 25.47% and 27.54% respectively. The inversion accuracy of the total integral area method is obvious better than the reference baseline height method. No outstanding correlation relationship between chlorophyll concentration and fluorescence peak position were found based on reflectance spectra. The results show that chlorophyll fluorescence properties are effective in detecting TSM concentration in turbid inland water body, which provide a new optional sensitive band to retrieval TSM concentration in complex waters.

Abstract:The Shitoukoumen Reservoir, an urban water resource of Changchun City, was chose as a represent case of waters in Northeast of China, and then its spectral absorption properties of particulates were analyzed using the field data on June 13th and September 23rd 2008, respectively. Results showed that the content of total suspended matters (TSM) was higher but content of chlorophyll-a (Chl.a) was lower in June than that in September. The spectral curves of TSM were similar to that of tripton. Tripton contributed much more than phytoplankton in the absorption by TSM. In September, the contribution coming from phytoplankton slightly increased, which resulted in an obvious absorption peak around 440nm in the spectral absorption curves of TSM. Absorption by tripton at 440nm had a good relationship with TSM but not with Chl.a. The absorption of phytoplankton in Shitoukoumen Reservoir was generally low. Chl.a held the majority of the pigment composition and had good exponential relationships with its absorption at 440nm and 675nm, respectively. The average values of phytoplankton specific absorption coefficient at 440nm and 675nm in June were 0.0483m²/mg and 0.0263m²/mg respectively, while the data in September were 0.0337m²/mg and 0.0187m²/mg respectively.

Abstract:Suspended sediment concentration (SSC) is one of important indicators for describing water quality, and obtaining its spatial and temporal distribution information is necessary for understanding, managing and protecting lake ecosystems. This study aimed to developed the Moderate-resolution Imaging Spectroradiometer (MODIS) image-based SSC retrieval model for Lake Poyang, used the model to predict the SSC of Lake Poyang during the flood period from 2000 to 2007, analyzed its spatial and temporal change characteristics and further discussed the causes to induce these changes. The results revealed that: the red band of MODIS Terra was significantly related to the SSC (R2= 0.92, s.e.=12.02mg/L, F=154.30, P<0.001), and it may be applied to retrieve the SSC of Lake Poyang during the flood period. The SSC of Lake Poyang showed clear spatial and temporal distribution characteristics from 2000 to 2007, while the SSC had no clear change in the south, increased in the north and fluctuated largely in the centre of the lake. The dredging activities in the northern Lake Poyang was the main cause inducing the increased SSC in this region, and mixed with water backflow from the Yangtze River to Lake Poyang which resulted in the fluctuation of SSC in the centre. Non-significant changeof sediments from Rivers Fuhe, Xinjiang and Raohe caused the non-significant change of SSC in the south lake

Abstract:Ocean color product validation is vital to improve the algorithms and to secure subsequent applications. China coastal region is one of the most highly turbid waters in the world. The precision of NASA's standard MODIS ocean color products are poor for this region. However, the validations have not been made due to the shortage of high quality in-situ data. In this paper, in-situ data collected in the Yellow Sea and the East China Sea in the spring and autumn of 2003 are used to validate the main parameters of MODIS Aqua standard ocean color product, i.e., the normalized water-leaving radiance L_wN(λ) (converted to remote sensing reflectance R_rs(λ)) and chlorophyll-a concentration (Chl.a). The resolution of 3×3 pixel box and ±0.5h are used as the sampling window, and spatial homogeneity criteria are used to screen data. Based on the analysis of match-ups (in low and middle turbid waters), primary conclusions are obtained: (1) MODIS-derived R_rs(λ) can assure certain precisions but can not satisfy the expected error of 10%, and has obviously poor results in 412nm bands. There is good linear relationship between satellite images and modeled images. The overall root mean square error is 0.0030sr^-1 and the median of absolute values over the relative error is about 25%, respectively. Derived R_rs(λ) is similar with in-situ ones in spectrum shape, and is mostly underestimated especially in high value region. Besides, R_rs(λ) at 412 and 667nm bands are mostly underestimated with even negative values in low value region. The reasons for this phenomenon are due to the influences of water-leaving radiance in the near-infrared bands and improper aerosol models used in the atmospheric correction process. (2) MODIS derived Chl.a is very poor. The satellite derived values are systematically overestimated. And there is no obvious linear relationship between satellite derived and in-situ Chl.a, which the root mean square error is about 1.56mg/m³, the median absolute relative error about 103.25%, and the median satellite to in-situ ratio about 2.03. The major reason for this phenomenon is the OC3M model (coefficients) unsuitability.