A model of diffuse attenuation of downwelling irradiance for ecosystem models
Nechad, B.; Ruddick, K. (2010). A model of diffuse attenuation of downwelling irradiance for ecosystem models, in: Frouin, R.J. et al. (Ed.) Remote Sensing of the Coastal Ocean, Land, and Atmosphere Environment. Incheon, South Korea, October 1314, 2010. Proceedings of SPIE, the International Society for Optical Engineering, 7858. dx.doi.org/10.1117/12.872979
In: Frouin, R.J. et al. (Ed.) (2010). Remote Sensing of the Coastal Ocean, Land, and Atmosphere Environment. Incheon, South Korea, October 1314, 2010. Proceedings of SPIE, the International Society for Optical Engineering, 7858. SPIE: Bellingham. ISBN 9780819483881. 308 pp.
In: Proceedings of SPIE, the International Society for Optical Engineering. SPIE: Bellingham, WA. ISSN 0277786X; eISSN 1996756X
 
Author keywords 
diffuse attenuation coefficient of downwelling irradiance; average 
Project  Top  Auteurs 
 BELCOLOUR2 : Optical remote sensing of marine, coastal and inland waters

Abstract 
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 biooptical 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. 
