Light attenuation is measured with a light meter (also called light extinction meter, or PAR-meter). Measured attenuation is tuned to the part oft the light spectrum to which phytoplankton and aquatic plants are sensitive (PAR: Photosynthetic Active Radiation 400-700nm).
The attenuation coefficient (K) is a measure for the extinction (attenuation) or weakening of diffuse light directed downward in the water. The attenuation coefficient accounts for incident light from all angles in the vertical (downward) direction. Light attenuates under water because it is absorbed and reflected by particles (algae, humus, suspended matter) and water itself. In principle, the reflected light is still available for the growth of algae or aquatic plants. .
The attenuation of light is calculated on the basis of the concentration of so-called optically active substances, in addition to water itself these are:
- humic acids
- Inorganic suspended matter
- detritus (organic fraction of suspended matter)
- algae (as chlorophyll-a concentration).
The simplest way to calculate the attenuation coefficient is by the specific contribution of the above substances empirically through regression analysis.
K = k1*[chlorophyll-a] + k2*[detritus] + k3*[suspended matter] + k4*[humic acids] + kwater
Where k1-4 are the specific attenuation coefficients (attenuation per unit of concentration) for specific optically active substances. Sometimes the contribution of water and humic substances is lumped in to what is then called "background" attenuation.This approach is useful, but a strong simplification of reality. The approach is based on the Beer-Lambert law which which is valid only for monochromatic (one wavelength) light. As visible light or PAR (that part of the spectrum for growth of algae and aquatic plants is of interest) ranges from 400-800nm it cannot be handled by only one attenuation coefficient.
The equation further assumes a linear relationship between attenuation and concentrations of various substances, but the attenuation coefficient is also dependent on the composition and angle of irradiation of the incident light. Here the attenuation is calculated by means of the spectral light module UITZICHT (Buiteveld, 1990, Buiteveld 1995) and which is included in the processes of the library water quality model DELWAQ Deltares.
For each of these substances and water VIEW calculates an absorption coefficient and a reflection coefficient which both are wavelength dependent in the considered visible light wave length spectrum.
The coefficients of this web application are taken from Buiteveld (1995) and can not be changed from the User Interface.