Dear B. Manikandan,

There are few methods to calculate PAR underwater from PAR subsurface values. There are all based on the exponential decrease of light within th water column, which is controled by the vertical attenuation coefficient KdPAR. You can get such KdPAR from papers of Morel and also the one by Lee

Morel, A. (1988). Optical modeling of the upper ocean in relation to its biogenous matter content (case 1 water), Journal of Geophysical Research, 93, 10,749-10,768.

Lee Zhongping (2009) KPAR: An optical property associated with ambiguous values. Journal of Lake Sciences.

best

hubert

Hi ,

You will also need to convert the above-water PAR to the PAR right below surface, see Mobley and Boss (2012).

Also make sure your surface PAR is instantaneous PAR or daily PAR, which are very different. See Lee (2009) and Wei and Lee (2013).

Best wishes.

J.W.