Ms. Bouchra Nechad Profile

Bouchra Nechad

at Institut Royal des Sciences Naturelles de Belgique

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Proceedings Article | 4 November 2010 Paper

A model of diffuse attenuation of downwelling irradiance for ecosystem models

Bouchra Nechad, Kevin Ruddick

Proceedings Volume 7858, 78580D (2010) https://doi.org/10.1117/12.872979

KEYWORDS: Sun, Signal attenuation, Absorption, Ecosystems, Backscatter, Water, Data modeling, Clouds, Optical properties, Reflectivity

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Estimation of the underwater attenuation of light is important to ecosystem modellers, who require information on Photosynthetically Available Radiation (PAR), and on the euphotic depth for calculation of primary production. Characterisation of these processes can be achieved by determining the diffuse attenuation coefficient of PAR, KPAR . A review of bio-optical models of the spectral diffuse attenuation coefficient for downwelling irradiance, K_d , is presented and stresses the necessity for a better knowledge and parameterization of these coefficients. In the second part of this work, radiative transfer simulations were carried out to model K_dZ1% the spectral diffuse attenuation of downwelling irradiance averaged over the euphotic depth Z_1% (depth where the downwelling irradiance is 1% of its surface value). This model takes into account the effects of varying sun zenith angle and cloud cover and needs absorption and backscattering coefficients (the inherent optical properties, IOPs) as input. It provides average and maximum relative errors of 1% and 5% respectively, for sun zenith angles [0°-50°] and of 1.7% and 12% respectively at higher sun zenith angles. A relationship was established between K_dZ1% at a single wavelength (590nm) and KPAR at Z_PAR1% (where PAR is 1% of its value at the surface) which allows for a direct expression of KPAR_ZPAR1% in terms of inherent optical properties, sun angle and cloudiness. This model provides estimates of KPAR within 25% (respectively 40%) relative errors respectively with a mean relative error less than 7% (respectively 9%) for sun zenith angles ranging from 0° to 50° (respectively higher than 50°). A similar method is applied to derive a model for the diffuse attenuation of photosynthetically usable radiation, KPUR_ZPUR1% , with similar performance.

Proceedings Article | 10 September 2009 Paper

Calibration and validation of a generic multisensor algorithm for mapping of turbidity in coastal waters

B. Nechad, K. Ruddick, G. Neukermans

Proceedings Volume 7473, 74730H (2009) https://doi.org/10.1117/12.830700

KEYWORDS: Calibration, Reflectivity, Backscatter, Water, Absorption, Ocean optics, Near infrared, Algorithm development, Remote sensing, Scattering

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Turbidity, as defined in the standard ISO7027, is a parameter that is routinely measured in many national and regional water quality monitoring programmes. The definition of turbidity according to ISO and as related to satellite data products is discussed. While satellite data products are beginning to become available for the closely related parameter, Total Suspended Matter (TSM), the direct estimation of turbidity as a satellite data product has not yet been addressed. In situ measurements of TSM and of turbidity, obtained in the Southern North Sea (SNS), show high correlation (correlation coefficient of 98.6%). A generic multisensor algorithm for TSM as function of reflectance has been previously developed. The methodology is extended here to the estimation of turbidity from water-leaving reflectance. A set of 49 seaborne measurements of reflectance in the spectral range 600-850nm and turbidity in the SNS are used to calibrate the algorithm. The algorithm is also calibrated for the specific bands of MERIS. Validation of these models is carried out using an independent set of seaborne measurements of turbidity and reflectance and shows low relative errors in turbidity retrieval at 681nm (less than 35%). This wavelength is recommended, provided no significant fluorescence affects this range.

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