Subsurface Irradiance Reflectance Research Articles

New model for subsurface irradiance reflectance in clear and turbid waters.

Modeling of subsurface irradiance reflectance fields especially in turbid coastal, harbor and lagoon waters has important applications in ecology, engineering and optical remote sensing. The present study aims at exploring many possible causes of variation in the proportionality factor f and analyzing its effect on the subsurface irradiance reflectance in different waters. A new model is then developed to estimate this optical property as a function of the absorption coefficient (a), backscattering coefficient (bb), incident illumination condition, and other wavelength-depth dependent factors. Implementation of this new model is examined for five types of waters with varying turbidity and chlorophyll. Model results are verified with in situ measurements data and compared with the results from existing models. Formulas already proposed for estimating R in the previous studies and generally expressed by R = 0.33(bb/(a + bb)) or R = f (bb/(a + bb)) where f = 0.975-0.629 μ(0) (μ(0) is the incident photons just below the sea surface) work fairly well in clear oceanic waters, but yield large errors in turbid coastal and lagoon waters due to the use of a constant value ~0.33 or the dimensionless parameter f which does not account for certain processes in the model (e.g., multiple scattering, depth-dependent changes in the diffuse components of solar radiation, and spectral variation in f). By contrast, the new model estimates the reflectances having good agreement with in situ data from just below the water surface and throughout the water column. The improved performance of the present model is because it includes a parameterization of the proportionality factor f which varies with wavelength and depends on the sun angle, inherent optical properties, and diffuse attenuation coefficients. Knowledge related to interrelationships between inherent optical properties and apparent optical properties can be used to study the variability of the subsurface reflectance in homogeneous and stratified coastal waters with respect to many possible causes of its variations.

Unattended processing of shipborne hyperspectral reflectance measurements

Hyperspectral remote-sensing reflectance (Rrs) from above-surface (ir)radiance measurements is derived using a new, automated method that is suitable for use on moving platforms. The sensors are mounted on a rotating platform that compensates for changing solar and ship azimuth angles, optimizing the sensor azimuth for minimal contribution of sky radiance to measured water-leaving radiance. This sea-surface reflectance (ρs) lies in the order of 2.5–8% of sky radiance, and is determined through spectral optimization, minimizing the propagation of atmospheric absorption features to Rrs. Up to 15 of these gas absorption features are frequently recognized in (ir)radiance spectra under clear and overcast skies. Rrs was satisfactorily reproduced for a wide range of simulated Case 2 waters and clear sky conditions. A set of 13,784 in situ measurements collected with optimized viewing angles on the high-absorption, low-scattering Baltic Sea was collected in April and July 2010–2011. The processing procedure yielded a 22% retrieval rate of ρs for the field data. The shape of the subsurface irradiance reflectance measurements (R(0−)) measured at anchor stations was well reproduced in above-surface Rrs in those cases where the algorithm converged on a solution for ρs, except under unstable or weak illumination conditions. Clear-sky conditions resulted in the best correspondence of Rrs and R(0−) and gave the highest (>50%) retrieval rates of ρs. Two indices, derived from the available sensor data, are given to describe illumination conditions, and are shown to predict the ability of the algorithm to retrieve Rrs.

Water Quality Monitoring for Lake Constance with a Physically Based Algorithm for MERIS Data.

A physically based algorithm is used for automatic processing of MERIS level 1B full resolution data. The algorithm is originally used with input variables for optimization with different sensors (i.e. channel recalibration and weighting), aquatic regions (i.e. specific inherent optical properties) or atmospheric conditions (i.e. aerosol models). For operational use, however, a lake-specific parameterization is required, representing an approximation of the spatio-temporal variation in atmospheric and hydrooptic conditions, and accounting for sensor properties. The algorithm performs atmospheric correction with a LUT for at-sensor radiance, and a downhill simplex inversion of chl-a, sm and y from subsurface irradiance reflectance. These outputs are enhanced by a selective filter, which makes use of the retrieval residuals. Regular chl-a sampling measurements by the Lake's protection authority coinciding with MERIS acquisitions were used for parameterization, training and validation.

Assessment of water quality in Lake Garda (Italy) using Hyperion

For testing the integration of the remote sensing related technologies into the water quality monitoring programs of Lake Garda (the largest Italian lake), the spatial and spectral resolutions of Hyperion and the capability of physics-based approaches were considered highly suitable. Hyperion data were acquired on 22nd July 2003 and water quality was assessed ( i) defining a bio-optical model, ( ii) converting the Hyperion at-sensor radiances into subsurface irradiance reflectances, and ( iii) adopting a bio-optical model inversion technique. The bio-optical model was parameterised using specific inherent optical properties of the lake and light field variables derived from a radiative transfer numerical model. A MODTRAN-based atmospheric correction code, complemented with an air/water interface correction was used to convert Hyperion at-sensor radiances into subsurface irradiance reflectance values. These reflectance values were comparable to in situ reflectance spectra measured during the Hyperion overpass, except at longer wavelengths (beyond 700 nm), where reflectance values were contaminated by severe atmospheric adjacency effects. Chlorophyll-a and tripton concentrations were retrieved by inverting two Hyperion bands selected using a sensitivity analysis applied to the bio-optical model. The sensitivity analysis indicated that the assessment of coloured dissolved organic matter was not achievable in this study due to the limited coloured dissolved organic matter concentration range of the lake, resulting in reflectance differences below the environmental measurement noise of Hyperion. The chlorophyll-a and tripton image-products were compared to in situ data collected during the Hyperion overpass, both by traditional sampling techniques (8 points) and by continuous flow-through systems (32 km). For chlorophyll-a the correlation coefficient between in situ point stations and Hyperion-inferred concentrations was 0.77 (data range from 1.30 to 2.16 mg m − 3 ). The Hyperion-derived chlorophyll-a concentrations also match most of the flow-through transect data. For tripton, the validation was constrained by variable re-suspension phenomena. The correlation coefficient between in situ point stations and Hyperion-derived concentrations increased from 0.48 to 0.75 (data range from 0.95 to 2.13 g m − 3 ) if the sampling data from the re-suspension zone was avoided. The comparison of Hyperion-derived tripton concentrations and flow-through transect data exhibited a similar mismatch. The results of this research suggest further studies to address compatibilities of validation methods for water body features with a high rate of change, and to reduce the contamination by atmospheric adjacency effects on Hyperion data at longer wavelengths in Alpine environment. The transferability of the presented method to other sensors and the ability to assess water quality independent from in situ water quality data, suggest that management relevant applications for Lake Garda (and other subalpine lakes) could be supported by remote sensing.

Some Notes on Water Color in Keweenaw Bay (Lake Superior)

Spectral subsurface irradiance reflectance—intrinsic water color—was derived from above-water radiance measurements using a hand-held spectroradiometer along a transect on Keweenaw Bay, Lake Superior. The reflectance spectra were typical of oligotrophic lacustrine waters. The reflectance peak wavelength shifted from 484 nm at stations farthest offshore to 540 nm near the head of the bay. This change coincided with a decrease in Secchi-disk depth from 16 to 8 m, and an increase in concentrations of chlorophyll a and total suspended matter from about 0.45 to 0.60 mg m −3 and from 0.3 to 0.5 g m −3, respectively. The concentration of chromophoric dissolved organic matter (gilvin), expressed as the absorption of filtrate at 440 nm, varied between 0.11 and 0.2 m −1. Like almost all inland waters, Keweenaw Bay should be classified as a Case 2 water due to the concentrations of gilvin and inanimate particles relative to plankton biomass. A model for chlorophyll-a estimation from spectral reflectance that adequately predicted concentrations in mesotrophic to highly eutrophic Case 2 waters elsewhere gave negative values when applied to the Keweenaw Bay transect. Evidently, there is a need of algorithm development for oligotrophic lacustrine waters. Estimates improved using a modified blue to green band ratio algorithm previously applied for remote sensing of oceanic waters. In optimization of semi-empirical algorithms for estimation of plankton biomass in Lake Superior, absorption by gilvin as well as by inanimate particles merits special consideration.

A chlorophyll-retrieval algorithm for satellite imagery (Medium Resolution Imaging Spectrometer) of inland and coastal waters

A chlorophyll (Chl) a retrieval algorithm, originally developed for spectral subsurface irradiance reflectance determined from above-water shipboard measurements, was adapted for use with satellite imagery to be acquired by the MERIS (Medium Resolution Imaging Spectrometer) instrument. This MERIS algorithm was calibrated for Chl a concentrations in the range 3-185 mg m(-3) using spectral reflectance calculated from shipboard measurements on the IJssel Lagoon (The Netherlands). Next, the algorithm was validated for various inland and coastal waters covering this concentration range. Despite the lower spectral resolution of MERIS as compared to the shipboard spectroradiometer, the standard error of estimate is expected to be similar, i.e. similar to9 mg m(-3) of Chl a in mesotrophic and eutrophic lakes, rivers, estuaries and coastal waters.

Inherent and apparent optical measurements in the Hudson/Raritan Estuary

During an August, 1999 field campaign, measurements were made to establish hydrologic optical properties of the Hudson/Raritan Estuary (New York-New Jersey): 1) concurrent above-and below-surface spectral irradiance; 2) sampling for laboratory determination of inherent optical properties; and 3) concentrations of optically-important water quality parameters. We used a bio optical model based on Gordon et al. (1975) to predict the subsurface irradiance reflectance from optically important water constituents. This model was then validated with the measured reflectance spectra from the field spectroradiometers. Modeling of reflectance is a prerequisite for processing remote sensing data to desired thematic maps. These are key input to the geographic information system (GIS) used to manage the water quality condition of the estuary.

The relation between inherent optical properties and reflectance spectra in turbid inland waters

The relationship between reflectance, absorption and backscattering is essential for the development of analytical and multitemporal algorithms for remote sensing of inland waters. In this study emphasis has been put on the wavelength dependency of the optical properties. The spectra of the subsurface irradiance reflectance R(O−), absorption a, scattering b and backscattering bb were determined for 18 waters covering four inland water types. These watertypes are: shallow eutrophic lakes, shallow mesotrophic lakes, deep lakes and river and canals. The linear backscatter albedo model R(0−) = r 1 b b(a + bb) was investigated for each water sample. The estimated r 1 ranges from 0.12 to 0.56 with an average of 0.29, with a relative error of 1% to 5%. The variation in r 1 between the water bodies was large. The spectral dependence of r 1 , was larger for eutrophic lakes than for the other three water types. The linear backscatter albedo model provides a good basis for remote sensing algorithms for water quality...

Subsurface irradiance reflectance spectra of inland waters differing in morphometry and hydrology

A database has been established for relating subsurface irradiance reflectance, i.e. water ‘colour’, to the optical properties and water quality parameters of more than 120 Dutch inland water bodies. The concentrations of total chlorophyll-a (TChl-a = chlorophyll-a plus phaeopigment), total suspended matter and the Secchi-disc depth varied between 0.6 and 468 mg m 3 , 0.8 and 98 g m 3 , and 0.15 and 5.50 m, respectively. The water bodies represented very different water types based on morphometry and hydrology. The depth ranged from 0.4 to 40 m, and the surface area from 0.25 ha (fens in the moorland Peel) to 1200 km 2 (Lake IJsselmeer). The mean specific phytoplankton absorption coefficient at 676 nm was 0.013 0.003 m 2 (mg TChl-a) 1 . The absorption by tripton and dissolved humic substances at 440 nm varied between 0.1 and 16.4 m 1 , and 0.1 and 65.5 m 1 , respectively. By using the spectral position and magnitude of the subsurface irradiance reflectance peak alone, it was possible to distinguish groups of water bodies according to a classical typology based on morphometry and hydrology.

Optical teledetection of the vertical attenuation coefficient for downward quantum irradiance of photosynthetically available radiation in turbid inland waters

Depth profiles of downward quantum irradiance of photosynthetically available radiation in situ and spectral subsurface irradiance reflectance, obtained from water-leaving radiance, were determined in different inland water types. These included the large, shallow and eutrophic IJssel lagoon in the Netherlands, eighteen Dutch lakes differing in depth and trophic state, and the large, shallow and eutrophic Lake Tai in China. The attenuation coefficient for downward irradiance ranged from 0.7 to 5.4 m-1. An empirical relationship with the backscattering coefficient in the near-infrared waveband, derived from the reflectance, and three spectral reflectance band ratios explained 91% of the variation. The potential assessment of the spatial distribution of phytoplankton production through shipboard optical teledetection of the concentration of chlorophyll-a and the attenuation coefficient for downward irradiance is discussed.

An algorithm for the retrieval of sediment content in turbid coastal waters from CZCS data

Abstract An algorithm for the retrieval of the sediment content of turbid coastal waters from Coastal Zone Colour Scanner (CZCS) data has been developed theoretically and tested against experimental values. The theoretical approach consisted of numerical simulations and sensitivity analyses, focused on the search for a retrieval variable with low sensitivity to chlorophyll-like pigments, and to uncertainties in the procedure for the determination of the atmospheric correction applied to the remotely measured radiances. The variableX, = [R(550) - R(670)]° [R{520)/R(550)]6where R(λ) is the subsurface irradiance reflectance at wavelength X, has been found to meet the above requirements, provided 0–5asλ 1·5 and – 2·5≤b≤–0·7. The algorithm log(sed.) = A + Blog(Ars) has been applied to the analysis of a set of CZCS scenes of the Adriatic Sea, with sediment and chlorophyll concentration ranges from 1 to 13gm-3 and from 0–1 to l–5mgm~3, respectively. The theoretically predicted low sensitivity to the atmospheric ...

Use of chromaticity in remote measurements of water quality

Optical cross sections (absorption and scattering per unit concentration of chlorophyll a, suspended mineral, and dissolved organic carbon) are utilized to generate subsurface irradiance reflectance spectra and upwelling irradiance spectra for a variety of water masses. Standard CIE chromaticity analyses are applied to the upwelling irradiance spectra. The results of these analyses serve as a basis for a discussion of severe restrictions in the application of chromaticity to water quality prediction.

Nonzero subsurface irradiance reflectance at 670 nm from Lake Ontario water masses

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text R. P. Bukata, J. H. Jerome, J. E. Bruton, and S. C. Jain, "Nonzero subsurface irradiance reflectance at 670 nm from Lake Ontario water masses," Appl. Opt. 19, 2487-2488 (1980) Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article