COASTAL OPTICAL MEASUREMENTS II

RETRIEVAL OF WATER CONSTITUENTS FROM MULTIPLE EARTH OBSERVATION SENSORS IN THE MEKONG DELTA AND ESTUARY

Heege, Thomas¹; Kiselev, Viacheslav¹; Gebhard, Steffen²; Huth, Juliane¹; Long, Trinh Thi³; Tri, Vo Khac<sup>3

1</sup>EOMAP GmbH & Co.KG Special Airport Oberpfaffenhofen, Building 319, Gilching, --, 82205, Germany; ²DLR Oberpfaffenhofen, Münchner Str. 20, Wessling, --, 82234, Germany; ³Southern Institute for Water Resources, 2A Nguyen Bieu St., Dist. 5, Ho Chi Minh City, --, --, Viet Nam

The development of a water management system for the Mekong river basin, performed in the frame of the WISDOM project, requires the application of effective operational techniques of remote sensing data processing for the instruments with different spatial and spectral resolution. The use of the physics based flexible modular image processing system MIP for this purpose ensures standardized product outputs for a variety of satellite sensors. The processing chain of this system is automatically adapted to the sensor parameters as well as to the regional speciic inherent optical properties (SIOP) of the aquatic system.

The algorithms of MIP are based on a coupled retrieval of atmospheric and water properties providing for the best fit of measured and model radiances in all spectral channels. The number of retrieved water species and final products depend on the spectral and radiometric resolution of the sensor. At least two parameters, namely suspended matter and atmospheric aerosol optical depth, are retrieved. For higher number of channels also the absorbing water constituents such as Colored Dissolved Organic Material (CDOM) and phytoplankton pigments (Chorophyll) can be obtained.

MIP system is used for processing of MODIS (250m, 500m and 1km resolution) as well as SPOT and Landsat ETM7 scences of Mekong delta. The SIOPs of this water basin were measured in a dedicated campaign on the Basac river of the delta in January 2008. The processing chain and inversion algorithm demonstrated robust performance. However, the number of retrievable parameters and the product quality depend, as it was expected, on sensor characteristics and observation conditions (e.g. sun glitter existence). The role of different impact factors in operational applications is discussed on specific examples.

SATELLITE RETRIEVAL OF CHLOROPHYLL CDOM AND NAP IN OPTICALLY COMPLEX WATERS USING A SEMI-ANALYTICAL INVERSION BASED ON SPECIFIC INHERENT OPTICAL PROPERTIES. A CASE STUDY FOR GREAT BARRIER REEF COASTAL WATERS

Brando, Vittorio E¹; Dekker, Arnold G¹; Schroeder, Thomas ¹; Park, Young Je ¹; Clementson, Lesley ²; Steven, Andy ³; Blondeau-Patissier, David<sup>1

1</sup>CSIRO Land & Water GPO Box 1666, Canberra, --, 2601, Australia; ²CSIRO Marine & Atmospheric Research, Hobart, --, 7001, Australia; ³CSIRO Land & Water, Brisbane, --, 4068, Australia

Valid and accurate satellite-derived information on water quality of the Great Barrier Reef (GBR) waters is required to effectively manage this world heritage listed area. Previous studies have shown that for this region the variability in optical properties leads to poor performance of standard algorithms.

To account for the seasonal and spatial variability of the spectral shapes and the magnitude of the Specific Inherent Optical Properties (SIOP), a generic remote sensing algorithm was developed based on the inversion of a semi-analytical model with a variable SIOP parameterisation.

In this study we present the regional SIOP parameterization and application of this algorithm to the coastal waters between Cairns (~16.8 ºS) and Townsville (~18.8 ºS). To characterise the variability in optical properties in these coastal waters and to estimate the key SIOP parameters for the model, two field campaigns were conducted during the austral dry (October 2007) and wet (April 2008) seasons. Wet season sampling followed monsoonal activity that resulted in significant riverine discharge to the GBR coastal waters.

Light absorption, scattering, backscattering, as well as particulate, pigment and dissolved matter concentrations were measured at 44 stations, spread over 28 different locations. Sixteen of these locations were visited both in the dry and wet season enabling a direct comparison. In both seasons we observed highest values (in concentration or IOP) of most optical properties at sites close to river mouths, decreasing with distance from the coast. Particulate and dissolved concentrations were higher during the wet season. Sites close to the rivers showed the greatest differences between seasons for several SIOP parameters.

The inversion algorithm was applied with the regional SIOP parameterization to MODIS data for the retrieval of the inherent optical properties and the concentrations of the optically active constituents. Our results show that the MODIS based retrieval accurately depicts the observed seasonal and spatial variability of the key optical properties for these complex coastal waters.

ESTIMATING SHELF SEAWATER COMPOSITION BY INVERSION OF AC9 INHERENT OPTICAL PROPERTY MEASUREMENTS

Brown, Ian Christopher¹; Cunningham, Alex¹; McKee, David<sup>1

1</sup>University of Strathclyde 107 Rottenrow, Glasgow, --, G4 0NG, United Kingdom

Shelf seas are complex, dynamical systems, within which important physical and biological processes occur. Water composition information may be used to quantify and map these processes both spatially and temporally. This information is routinely obtained through laboratory sample analyses which have poor spatial and temporal resolution compared with those of other physical variables such as temperature and salinity. Hence, there is a requirement for the development of supplementary methods of determining water composition providing comparable coverage to that of other physical variables. The high sampling rate and relative ease of deployment suggest that in situ optics would provide a suitable basis for such procedures. As in situ absorption and scattering coefficient measurements are routinely made using WETLabs AC-9 (or AC-S) dual-beam spectrophotometers and the magnitude and spectral variation of these inherent optical properties (IOPs) are determined by the concentration and composition of seawater constituents, IOP inversion is a prime candidate. Such inversion techniques, applied to in situ optical data obtained from moorings or towed-bodies, may provide useful insights into variability in water composition and shelf sea processes.

Proposed strategies for partitioning in situ absorption coefficient measurements between optically significant constituents include matrix inversion (Gallegos & Neale, 2002) and constrained, non-linear regression methods (Schofield et al, 2004). As an extension of these ideas, we have developed a sequential, mechanistic, specific IOP based approach to partition both in situ absorption and scattering coefficient measurements between constituents and subsequently retrieve constituent concentrations. A comparison of these inversion procedures is presented, demonstrating that our specific IOP based procedure performs comparably with the previously published matrix inversion algorithm, appropriately constrained. Both procedures were validated using synthetic IOPs and field IOP measurements. Our procedure was used to interpret vertical water column structure and identify complex patterns of phytoplankton and suspended sediment distribution in Scottish fjords.

VARIABILITY IN THE LIGHT ABSORPTION CHARACTERISTICS OF TWO CONTRASTING AUSTRALIAN COASTAL REGIONS.

Clementson, Lesley Ann¹; Stramski, Dariusz<sup>2

1</sup>CSIRO Marine and Atmospheric Research PO Box 1538, Hobart, Tasmania, --, 7001, Australia; ²Scripps Institution of Oceanography, San Diego, California, 92093-0238, United States

Spectral absorption coefficients and pigment concentration and composition from two diverse coastal marine environments in Australia have been analysed to determine the variability in absorption and its sources. Of the two regions, one is the west coast of Tasmania (41-43ºS) where the surface waters are dominated by high values of CDOM associated primarily with terrestrial sources. The other region is the coastal waters off south-west Western Australia (31-33ºS) where the absorption coefficients for all in-water components (phytoplankton, detritus, and CDOM) are low to the point that these waters are optically similar to many open ocean environments. Our analysis is focused on the green-to-blue ratio of the total absorption coefficient and its relationship to chlorophyll-a concentration. This absorption band ratio usually plays a dominant role in driving the variability in the blue-to-green ratio of remote sensing reflectance, R_^rs, with the variability in the blue-to-green ratio of backscatter playing a minor role. The reflectance band ratio is commonly used to retrieve bio-optical data products such as the chlorophyll concentration from ocean colour satellite imagery. Our results thus provide insights into potential performance of band-ratio algorithms for retrieving chlorophyll in these two coastal regions. We demonstrate the extent to which the total absorption band ratio (i.e., a proxy for the reflectance band ratio) varies at given chlorophyll concentrations within the investigated regions. We also discuss the sources of this variability in terms of phytoplankton, detritus, and CDOM absorption components as well as the composition of phytoplankton pigments. Our results can aid in the determination of regional bio-optical algorithms for the investigated coastal waters.

REMOTE SENSING OF APPARENT AND INHERENT OPTICAL PROPERTIES OF TASMANIAN COASTAL WATERS: APPLICATION TO MODIS DATA

Schroeder, Thomas¹; Brando, Vittorio¹; Cherukuru, Nagur¹; Clementson, Lesley²; Blondeau-Patissier, David¹; Dekker, Arnold¹; Fischer, Juergen<sup>3

1</sup>CSIRO Land & Water GPO Box 1666, Canberra, --, ACT 2601, Australia; ²CSIRO Marine Research, Castray Esplanade, Hobart, Tasmania, 7001, Australia; ³Free University Berlin, Carl-Heinrich Becker Weg 6-10, Berlin, -, D-12165, Germany

Global ocean colour algorithms significantly overestimate chlorophyll concentrations for the CDOM dominated coastal waters around Tasmania. In this work we describe the development, application and validation of the coupling of two physics-based inversion algorithms with the objective to improve the accuracy of chlorophyll estimates from remote sensing data in Tasmanian coastal waters. In this application, the proposed coupled algorithms derive apparent and inherent optical properties from spectral radiance measurement of the Moderate Resolution Imaging Spectrometer (MODIS) on-board of the NASA Earth Observation Systems (EOS) Terra and Aqua spacecrafts. The proposed coupled algorithms are generic in nature and can be applied to any other ocean colour sensor, like MERIS, SeaWiFS or OCM. The retrieval of optical properties is composed of two steps. First, an atmospheric correction algorithm based on inverse modelling of radiative transfer simulations and artificial neural network (ANN) inversion, derives the remote sensing reflectance at mean sea level on a pixel-by-pixel basis. Second, the inherent optical properties and the concentrations of the optically active constituents are retrieved from atmospherically corrected spectra by inverting a semi-analytical model with a variable specific inherent optical properties (SIOP) parameterization. The underlying SIOP sets of the semi-analytical model were derived from in-situ optical measurements in Tasmanian coastal waters. A validation of the algorithm outputs is performed with an independent data set of chlorophyll measurements. First results will be presented.