Absorption and attenuation of visible and near-infrared light in water: dependence on temperature and salinity.

We have measured the absorption coefficient of pure and salt water at 15 wavelengths in the visible and near-infrared regions of the spectrum using WETLabs nine-wavelength absorption and attenuation meters and a three-wavelength absorption meter. The water temperature was varied between 15 and 30 degrees C, and the salinity was varied between 0 and 38 PSU to study the effects of these parameters on the absorption coefficient of liquid water. In the near-infrared portion of the spectrum the absorption coefficient of water was confirmed to be highly dependent on temperature. In the visible region the temperature dependence was found to be less than 0.001 m-1 degrees C except for a small region around 610 nm. The same results were found for the temperature dependence of a saltwater solution. After accounting for index-of-refraction effects, the salinity dependence at visible wavelengths is negligible. Salinity does appear to be important in determining the absorption coefficient of water in the near-infrared region. At 715 nm, for example, the salinity dependence was -0.00027 m-1 /PSU. Field measurements support the temperature and salinity dependencies found in the laboratory both in the near infrared and at shorter wavelengths. To make estimates of the temperature dependence in wavelength regions for which we did not make measurements we used a series of Gaussian curves that were fit to the absorption spectrum in the visible region of the spectrum. The spectral dependence on temperature was then estimated based on multiplying the Gaussians by a fitting factor.