Determination of the downwelling diffuse attenuation coefficient of lakewater with the sentinel-3A OLCI

The Ocean and Land Color Imager (OLCI) on the Sentinel-3A satellite, which was launched by the European Space Agency in 2016, is a new-generation water color sensor with a spatial resolution of 300 m and 21 bands in the range of 400-1020 nm. The OLCI is important to the expansion of remote sensing monitoring of inland waters using water color satellite data. In this study, we developed a dual band ratio algorithm for the downwelling diffuse attenuation coefficient at 490 nm (Kd(490)) for the waters of Lake Taihu, a large shallow lake in China, based on data measured during seven surveys conducted between 2008 and 2017 in combination with Sentinel-3A-OLCI data. The results show that: (1) Compared to the available Kd(490) estimation algorithms, the dual band ratio (681 nm/560 nm and 754 nm/560 nm) algorithm developed in this study had a higher estimation accuracy (N = 26, coefficient of determination (R2) = 0.81, root-mean-square error (RMSE) = 0.99m-1and mean absolute percentage error (MAPE) = 19.55%) and validation accuracy (N = 14, R2= 0.83, RMSE = 1.06 m-1and MAPE = 27.30%), making it more suitable for turbid inland waters; (2) A comparison of the OLCI Kd(490) product and a similar Moderate Resolution Imaging Spectroradiometer (MODIS) product reveals a high consistency between the OLCI and MODIS products in terms of the spatial distribution of Kd(490). However, the OLCI product has a smoother spatial distribution and finer textural characteristics than the MODIS product and contains notably higher-quality data; (3) The Kd(490) values for Lake Taihu exhibit notable spatial and temporal variations. Kd(490) is higher in seasons with relatively high wind speeds and in open waters that are prone to wind- and wave-induced sediment resuspension. Finally, the Sentinel-3A-OLCI has a higher spatial resolution and is equipped with a relatively wide dynamic range of spectral bands suitable for inland waters. The Sentinel-3B satellite will be launched soon and, together with the Sentinel-3A satellite, will form a two-satellite network with the ability to make observations twice every three days. This satellite network will have a wider range of application and play an important role in the monitoring of inland waters with complex optical properties

Algorithm to derive inherent optical properties from remote sensing reflectance in turbid and eutrophic lakes

Inherent optical properties play an important role in understanding the biogeochemical processes of lakes by providing proxies for a variety of biogeochemical quantities, including phytoplankton pigments. However, to date, it has been difficult to accurately derive the absorption coefficient of phytoplankton [aph(λ)] in turbid and eutrophic waters from remote sensing. A large dataset of remote sensing of reflectance [ Rrs (λ)] and absorption coefficients was measured for samples collected from lakes in the middle and lower reaches of the Yangtze River and Huai River basin (MLYHR), China. In the process of scattering correction of spectrophotometric measurements, the particulate absorption coefficients [ap(λ)] were first assumed to have no absorption in the near-infrared (NIR) wavelength. This assumption was corrected by estimating the particulate absorption coefficients at 750 nm [ap(750)] from the concentrations of chlorophyll-a (Chla) and suspended particulate matter, which was added to the ap(λ) as a baseline. The resulting mean spectral mass-specific absorption coefficient of the nonalgal particles (NAPs) was consistent with previous work. A novel iterative IOP inversion model was then designed to retrieve the total nonwater absorption coefficients [anw(λ)] and backscattering coefficients of particulates [bbp(λ)], aph(λ), and adg (λ) [absorption coefficients of NAP and colored dissolved organic matter (CDOM)] from Rrs (λ) in turbid inland lakes. The proposed algorithm performed better than previously published models in deriving anw(λ) and bbp(λ) in this region. The proposed algorithm performed well in estimating the aph(λ) for wavelengths \u3e 500 nm for the calibration dataset [N = 285, unbiased absolute percentage difference (UAPD) = 55.22%, root mean square error (RMSE) = 0.44 m−1] and for the validation dataset (N = 57, UAPD = 56.17%, RMSE = 0.71 m−1). This algorithm was then applied to Sentinel-3A Ocean and Land Color Instrument (OLCI) satellite data, and was validated with field data. This study provides an example of how to use local data to devise an algorithm to obtain IOPs, and in particular, a ph (λ), using satellite Rr s (λ) data in turbid inland waters

Inversion of inherent optical properties in optically complex waters using sentinel-3A/OLCI images: A case study using China\u27s three largest freshwater lakes

Inherent optical properties (IOPs) play an important role in underwater light field, and are difficult to estimate accurately using satellite data in optically complex waters. To study water quality in appropriate temporal and spatial scales, it is necessary to develop methods to obtain IOPs form space-based observation with quantified uncertainties. Field-measured IOP data (N = 405) were collected from 17 surveys between 2011 and 2017 in the three major largest freshwater lakes of China (Lake Chaohu, Lake Taihu, and Lake Hongze) in the lower reaches of the Yangtze River and Huai River (LYHR). Here we provide a case-study on how to use in-situ observation of IOPs to devise an improved algorithm for retrieval of IOPs. We then apply this algorithm to observation with Sentinel-3A OLCI (Ocean and Land Colour Instrument, corrected with our improved AC scheme), and use in-situ data to show that the algorithm performs better than the standard OLCI IOP product. We use the satellite derived products to study the spatial and seasonal distributions of IOPs and concentrations of optically active constituents in these three lakes, including chlorophyll-a (Chla) and suspended particulate matter (SPM), using all cloud-free OLCI images (115 scenes) over the lakes in the LYHR basin in 2017. Our study provides a strategy for using local and remote observations to obtain important water quality parameters necessary to manage resources such as reservoirs, lakes and coastal waters

Influence of Wind on Suspended Matter in the Water of the Albufera of Valencia (Spain)

Wind significantly influences suspended matter in lakes, especially in shallow lagoons. To know how wind affects the water in Albufera of Valencia, a shallow coastal lagoon, the measured variables of turbidity and transparency have been correlated with the estimates by processing Sentinel-2 satellite images with the Sen2Cor processor. Data from four years of study of winds show that most of them are light to gentle easterly breezes and moderate to fresh westerly breezes. The obtained results show significant correlations between the measured variables and those obtained from the satellite images for total suspended matter and water transparency, as well as with the average daily wind speed. There is no significant correlation between wind and chlorophyll a. Moderate to fresh breezes resuspend the fine sediment reaching concentration values from 100 to 300 mg/L according to satellite data. However, it is necessary to obtain field data for the values of moderate and fresh winds, as for now, there are no experimental data to verify the validity of the satellite estimates

Teadusuuringutest rakendusteni–optiliselt keerukate vete seire satelliitsensori MERIS/ENVISAT abil

Väitekirja elektrooniline versioon ei sisalda publikatsioone.Järved ja rannikuveed pakuvad olulisi ökosüsteemi teenuseid. Tagamaks veekeskkondade seire ja ökoloogilise seisundi hindamise on Euroopa Liidus loodud mitmeid direktiive ja regionaalseid konventsioone. Kuna vee kvaliteet võib olla muutlik nii sesoonselt kui ruumiliselt võimaldab kaugseire efektiivset seire meetodit, mille abil saab hinnata vee kvaliteedi hetkeolukorda, muutusi võrreldes varasema seisundiga ning seda ka veekogudes, mis ei ole kaetud tavaseireprogrammide raames. Käesolevas töös uuriti esimese spetsiaalselt optilistelt keerukate vete seireks loodud satelliitsensori MERIS/ENVISAT andmete kasutamisvõimalusi viie Põhja Euroopa järve ja kahe Läänemere rannikuala bio-optiliste andmete alusel. Olemasolevate MERIS standardalgoritmide õigsuse hinnang näitas, et need ei anna täpseid tulemusi veekogudes, kus on kõrge lahustunud orgaanilise aine ja klorofüll a hulk. Fütoplanktoni parameetrite (klorofüll a, sinivetikate biomass, fütoplanktoni biomass) hindamiseks kasutati punases ja lähisinfrapunases spektriosas töötavat spektraalset indeksit, mis kalibreeriti kohalikesse oludesse. Kuna indeks on rakendatav MERIS L1b andmetele, lubab see kvantitatiivselt hinnata vee kvaliteedi parameetreid sinivetika õitsengute korral, mille puhul MERIS standardalgoritmid ei tööta. Hindamaks kaugseire andmetest veealust valgusvälja, millest sõltub veealuste organismide elutegevus, loodi kaalufunktsioonidel põhinev kombineeritud kanalisuhte algoritm, mis selgemate vete puhul kasutab kanalite 490/709 suhet ning sogasemate puhul 560/70 ning hindab edukalt valguse difuusset nõrgenemiskoefitsienti, Kd(490), satelliidiandmetest. Secchi sügavuse hindamiseks andis parimaid tulemusi algoritm, mis võttis pikselhaaval sisendiks satellidiandmetest arvutatud diffusse ja summaarse nõrgenemiskoefitsiendi ning peegeldusteguri väärtused üle nähtava laineala. Töös arendatud algoritmid rakendati MERIS arhiivi 2002–2011 andmetele hindamaks erinevate järvede ökoloogilist seisundit nii nagu on nõutud EL veepoliitika raamdirektiivi poolt. Tulemused näitasid, et kaugseire andmeid saab kasutada täiendava infoallikana ökoloogilise seisundi hindamisel. Väljatöötatud algoritmid ja rakendused on kohandatavad 2016. aasta veebruaris tööd alustanud Sentinel-3/OLCI andmetele, mille abil on optiliselt keerukate vete seire kosmosest võimalik vähemalt aastani 2029.Lakes and coastal areas provide a wide range of essential ecosystem services. Various directives and regional conventions have been established to ensure the monitoring and assessment of the ecological status of the aquatic ecosystems. Since water quality can have rapid changes in temporal and spatial scale, remote sensing can provide a cost-effective approach to assess the current and derive historic water quality information also for waterbodies that have not been part of conventional monitoring programmes. This thesis presents research about applications for MERIS/ENVISAT data in order to monitor optically-complex aquatic environment such as inland and coastal waters on the basis of bio-optical data from five North European lakes and two Baltic Sea coastal sites. The validation of MERIS standard water quality products indicated their unsuitability for waters with high amounts of chlorophyll a and humic substances. To map the phytoplankton parameters (CHL, cyanobacterial biomass, phytoplankton biomass) a spectral index which operates on red and near-infrared bands was used and calibrated to local conditions. So, this index allows derivation of the water quality parameters quantitatively in case of highly scattering cyanobacterial blooms, which is not possible with standard algorithms. To estimate underwater light field via transparency, an empirical combined band ratio algorithm was developed which switches from various band ratios based on the water transparency and determines the diffuse attenuation coefficient Kd(490) with high accuracy. Additionally, Secchi depth can be also estimated reliably via satellite derived inputs of diffuse and total beam attenuation coefficients and reflectance over visible wavelengths. The developed algorithms were applied on the MERIS archive from 2002 to 2011 to estimate the ecological status in lakes as required by the EU Water Framework Directive which showed that remote sensing products could be used as an additional source of information for assessment and reporting purposes. Despite the study in this thesis is based on the MERIS/ENVISAT data, the developed algorithms and methods can be applied on new Sentinel3/OLCI data that will provide EO data over optically complex waters at least until 2029

Lähi-kaugseire meetodite arendamine veekogude seisundi hindamiseks

Väitekirja elektrooniline versioon ei sisalda publikatsioone.Veekogude kvaliteedi hindamine on inimkonnale oluline olnud juba tuhandeid aastaid ja viimastel aastakümnetel on rohkem tähelepanu hakatud pöörama ka veekogude ökoloogilisele seisundile. Euroopas on veekogude kvaliteedi hindamise aluseks kaks dokumenti: Euroopa Liidu Vee Raamdirektiiv ja Euroopa Liidu Merestrateegia Raamdirektiiv. Mõlemad dokumendid sätestavad, et aastaks 2020 tuleb Euroopa Liidu veekogudes saavutada „hea“ seisund. Nende eesmärkide täitmiseks tuleb regulaarselt veekogude seisundit seirata. Kuivõrd kõikidelt veekogudelt veeproovide võtmine ja laboris analüüsimine ei ole võimalik (liigne raha ja tööjõukulu) ning lisaks ei anna sellised proovid ülevaadet veekogu seisundi parameetrite ruumilise jaotuse kohta tuleb appi võtta optilised instrumendid. Lisaks välitöödel kasutatavale optikale on Copernicus programmi raames järgnevatel aastakümnetel kättesaadav ka mitu erinevat satelliiditulemit. Nende tulemite kasutamiseks peab aga pidevalt nende täpsust hindama ja leidma täpsemaid arvutusmeetmeid, mis sobiksid konkreetsete parameetrite hindamiseks. Töö käigus tõestati, et vee optilised omadused, nagu neeldumine ja hajumine, varieeruvad Läänemere rannikuosas rohkem, kui on variatsioon ranniku ja mere keskosa vahel. Lisaks absoluutväärtuste erinevusele tuvastati ka spektraalse kuju muutusi eri piirkondade vahel. Tõestati, et elektromagnetkiirguse lähisinfrapuna piirkonda saab rakendada veekogude seires (tavaliselt eeldatakse, et selles spektripiirkonnas on veest tulev signaal null) ja eriti on see kasulik ohtralt lahustunud orgaanikat sisaldavate järvede seires. Testiti ja pakuti välja sobivaid kaugseire algoritme Läänemere vee kvaliteedi parameetrite hindamiseks. Analüüsiti erinevate spektromeetrite tulemuste varieeruvust ja leiti, et mõõtmisprotokolli korrektsel jälgimisel on erinevate sensorite tulemused küll erinevad, ent seire teostamiseks piisavalt sarnased. Lõpetuseks uuriti, millised on erinevate käsispektromeetrite potentsiaalsed rakendused.Knowing the quality of different waterbodies has been essential for human kind for thousands of years. There are two main European Union’s documents guiding the status assessment of water bodies: Water Framework Directive and Marine Strategy Framework Directive. Both of these documents state that all waterbodies in the European Union have to achieve “good” status by the year 2020. In order to fulfil this requirement, water bodies have to be monitored in regular bases. It is impossible to collect laboratory samples from every waterbody as it would be too expensive and would require many workers and still wouldn’t provide information about the spatial distribution of water quality parameters within each waterbody. Optical instruments can provide data fast and over larger areas and therefore have to be included in the monitoring programs. In addition to devices used at the in situ measurements are several satellite products that are available through Copernicus program for the coming decades. These products must, however, be constantly validated with in situ measurements. Additionally, new calculation methods have to be developed to improve the results precision. During this thesis, the variability of optical properties (like absorption and scattering) was assessed in the Baltic Sea. It was studied how much this variability influences the reflectance signal that reaches water remote sensing instruments. The performance of different set-ups and protocols of field spectrometers to collect reflectance data was assessed. The possibility to use near-infrared part of the spectrum in water remote sensing was investigated. In extreme absorbing lakes this is the only part of radiation providing us information about the water properties, but it proved to be useful also in other waterbodies. The performance of many remote sensing algorithms in retrieving water quality parameters in the Baltic Sea was tested. The possible applications for hand-held spectrometers were investigated

Optiliste veetüüpide põhine lähenemine sise- ja rannikuvee veekvaliteedi hindamiseks

Väitekirja elektrooniline versioon ei sisalda publikatsiooneInimestele on meeldinud ajast aega elada seal, kus maa ja vesi kohtuvad. Mistõttu on järvede, jõgede ja rannikualade lähedal inimtegevuse mõju suurenenud, mis omakorda põhjustab veekogude seisundi muutumist ning loob vajaduse veekogude operatiivseks seireks. Enamasti põhinevad veekogude seireprogrammid veekogudes teostatud punktmõõtmistel. See meetod aga ei suuda kajastada kogu veekogu kiiresti muutuvaid omadusi ja reaalset seisundit. Seetõttu on oluline lisaks punktmõõtmistele rakendada veekeskkonna operatiivse jälgimise meetodeid, millest kaugseire on üks võimsamaid. Kaugseire pakub tõhusaid viise veekvaliteedi ruumiliste ja ajaliste erinevuste jälgimiseks. Euroopa Liidu ja Euroopa Kosmoseagentuuri Copernicus programmi raames loodud Sentinel-2 ja Sentinel-3 seeria satelliitide hea ruumilise, ajalise ja spektraalse lahutusega andmete tasuta kättesaadavus on loonud reaalse võimaluse sise- ja rannikuvete seires operatiivselt kasutada täiendavalt satelliitandmeid. Need andmed võimaldavad jälgida kogu veekogu ajalist ja ruumilist muutlikkust ning seirata ka raskesti ligipääsetavaid veekogusid. Sise- ja rannikuveed on optiliselt keerukad, sest vee optilised omadused on mõjutatud sõltumatult erinevate optiliselt aktiivsete ainete poolt. Seetõttu standardsed kaugseire algoritmid veekvaliteedi hindamiseks neis veekogudes tihti ei tööta. Doktoritöö tulemusena tutvustati optiliste veetüüpide põhist lähenemist sise- ja rannikuvete veekvaliteedi parameetrite hindamiseks kaugseireandmete põhjal. Eelnimetatud meetod võtab arvesse vee optilisi omadusi ega piiritle ennast konkreetse veekoguga, seetõttu on tulemused rakendatavad kõigil sarnaste optiliste omadustega veekogudel üle maailma.Humans have long enjoyed living where land and water meet. At the same time, the impact of human activities close to lakes, rivers, and coastal areas has increased, which has caused the deterioration of water bodies. Therefore, the state of a water body requires constant monitoring to assess the magnitude of the impact of human activity and to respond when needed. Traditional water monitoring programs are mainly based on in situ measurements; however, considering that water bodies are dynamic in nature, this method may not reflect the status of the whole water body. Therefore, in addition to traditional monitoring, it is important to implement methods that allow more operative monitoring of the aquatic environment. Remote sensing offers effective ways to observe spatial and temporal variations in water quality. The free availability of data with high spatial, temporal and spectral resolution from the Sentinel-2 and Sentinel-3 family satellites launched under the European Union and the European Space Agency Copernicus programme has created a real opportunity for satellite data being used operationally for additional water quality monitoring for inland and coastal waters. Such waters are optically complex, as they are independently influenced by different optically significant constituents. Therefore, standard remote sensing algorithms to estimate water quality often fail in these waters. As a result of the thesis, an optical water type guided approach to estimate water quality in inland and coastal waters using remote sensing data was presented. The method considers the optical properties of water but does not limit itself to a particular water body. So, results are applicable to all the water bodies with similar optical properties of water.https://www.ester.ee/record=b534022

Towards high fidelity mapping of global inland water quality using earth observation data

This body of work aims to contribute advancements towards developing globally applicable water quality retrieval models using Earth Observation data for freshwater systems. Eutrophication and increasing prevalence of potentially toxic algal blooms among global inland water bodies have become a major ecological concersn and require direct attention. There is now a growing necessity to develop pragmatic approaches that allow timely and effective extrapolation of local processes, to spatially resolved global products. This study provides one of the first assessments of the state-ofthe-art for trophic status (chlorophyll-a) retrievals for small water bodies using Sentinel-3 Ocean and Land Color Imager (OLCI). Multiple fieldwork campaigns were undertaken for the collection of common aquatic biogeophysical and bio-optical parameters that were used to validate current atmospheric correction and chlorophyll-a retrieval algorithms. The study highlighted the difficulties of obtaining robust retrieval estimates from a coarse spatial resolution sensor from highly variable eutrophic water bodies. Atmospheric correction remains a difficult challenge to operational freshwater monitoring, however, the study further validated previous work confirming applicability of simple, empirically derived retrieval algorithms using top-of-atmosphere data. The apparent scarcity of paired in-situ optical and biogeophysical data for productive inland waters also hinders our capability to develop and validate robust retrieval algorithms. Radiative transfer modeling was used to fill this gap through the development of a novel synthetic dataset of top-of-atmosphere and bottom-of-atmosphere reflectances, which attempts to encompass the immense natural optical variability present in inland waters. Novel aspects of the synthetic dataset include: 1) physics-based, two-layered, size and type specific phytoplankton IOPs for mixed eukaryotic/cyanobacteria 6 assemblages, 2) calculations of mixed assemblage chl-a fluorescence, 3) modeled phycocyanin concentration derived from assemblage based phycocyanin absorption, 4) and paired sensor-specific TOA reflectances which include optically extreme cases and contribution of green vegetation adjacency. The synthetic bottom-of-atmosphere reflectance spectra were compiled into 13 distinct optical water types similar to those discovered using in-situ data. Inspection showed similar relationships and ranges of concentrations and inherent optical properties of natural waters. This dataset was used to calculate typical surviving water-leaving signal at top-of-atmosphere, as well as first order calculations of the signal-to-noise-ratio (SNR) for the various optical water types, a first for productive inland waters, as well as conduct a sensitivity analysis of cyanobacteria detection from top-of-atmosphere. Finally, the synthetic dataset was used to train and test four state-of-the-art machine learning architectures for multi-parameter retrieval and cross-sensor capability. Initial results provide reliable estimates of water quality parameters and inherent optical properties over a highly dynamic range of water types, at various spectral and spatial sensor resolutions. It is hoped the results of this work incrementally improves inland water Earth observation on multiple aspects of the forward and inverse modelling process, and provides an improvement in our capabilities for routine, global monitoring of inland water quality

Field Intercomparison of Radiometers Used for Satellite Validation in the 400–900 nm Range

An intercomparison of radiance and irradiance ocean color radiometers (the second laboratory comparison exercise—LCE-2) was organized within the frame of the European Space Agency funded project Fiducial Reference Measurements for Satellite Ocean Color (FRM4SOC) May 8–13, 2017 at Tartu Observatory, Estonia. LCE-2 consisted of three sub-tasks: (1) SI-traceable radiometric calibration of all the participating radiance and irradiance radiometers at the Tartu Observatory just before the comparisons; (2) indoor, laboratory intercomparison using stable radiance and irradiance sources in a controlled environment; (3) outdoor, field intercomparison of natural radiation sources over a natural water surface. The aim of the experiment was to provide a link in the chain of traceability from field measurements of water reflectance to the uniform SI-traceable calibration, and after calibration to verify whether different instruments measuring the same object provide results consistent within the expected uncertainty limits. This paper describes the third phase of LCE-2: The results of the field experiment. The calibration of radiometers and laboratory comparison experiment are presented in a related paper of the same journal issue. Compared to the laboratory comparison, the field intercomparison has demonstrated substantially larger variability between freshly calibrated sensors, because the targets and environmental conditions during radiometric calibration were different, both spectrally and spatially. Major differences were found for radiance sensors measuring a sunlit water target at viewing zenith angle of 139° because of the different fields of view. Major differences were found for irradiance sensors because of imperfect cosine response of diffusers. Variability between individual radiometers did depend significantly also on the type of the sensor and on the specific measurement target. Uniform SI traceable radiometric calibration ensuring fairly good consistency for indoor, laboratory measurements is insufficient for outdoor, field measurements, mainly due to the different angular variability of illumination. More stringent specifications and individual testing of radiometers for all relevant systematic effects (temperature, nonlinearity, spectral stray light, etc.) are needed to reduce biases between instruments and better quantify measurement uncertainties

A semi-analytical algorithm for deriving the particle size distribution slope of turbid inland water based on OLCI data: A case study in Lake Hongze

The particle size distribution (PSD) slope (ξ) can indicate the predominant particle size, material composition, and inherent optical properties (IOPs) of inland waters. However, few semi-analytical methods have been proposed for deriving ξ from the surface remote sensing reflectance due to the variable optical state of inland waters. A semi-analytical algorithm was developed for inland waters having a wide range of turbidity and ξ in this study. Application of the proposed model to Ocean and Land Color Instrument (OLCI) imagery of the water body resulted in several important observations: (1) the proposed algorithm (754 nm and 779 nm combination) was capable of retrieving ξ with R2 being 0.72 (p < 0.01, n = 60), and MAPE and RMSE being 4.37% and 0.22 (n = 30) respectively; (2) the ξ in HZL was lower in summer than other seasons during the period considered, this variation was driven by the phenological cycle of algae and the runoff caused by rainfall; (3) the band optimization proposed in this study is important for calculating the particle backscattering slope (η) and deriving ξ because it is feasible for both algae dominant and sediment governed turbid inland lakes. These observations help improve our understanding of the relationship between IOPs and ξ, which are affected by different bio-optic processes and algal phenology in the lake environment