Hi,

It may be a naive and improper question for this forum.
I have in situ Chl-a data collected at very shallow station, 0.5 m or 1 m . I believe Chl-a algorithm will not work for this station, due to bottom reflectance.

Then, how deep is about the shallowest bottom depth (physically) that allowing Chl-a algorithm works well?

Second question, what does "optically deep water" mean?

Thank you.

Ajis

Ajis,

Both band-ratio and semi-analytic chlorophyll algorithms have the assumption that the
reflectance signal is entirely modulated by consituents within the water column.  Bottom
reflectance will alter the signal, and result in an incorrect chlorophyll estimate.

There is no simple answer as to what depth is sufficient, but a good limit is that the water
is 'optically' deeper than the physical depth.  If the attenuation of the water column is high,
than the incident irradiance will be attenuated a shallower depths than if the water column
attenuation is low.   'Brown' or 'green' water is much more optically deep than 'blue' water.
For example, clear open ocean water can have a diffiuse attenuation coefficient at 490nm of
0.025 m^-1, so the "first optical depth" of such a water body is at 40m.  For coastal waters,
Kd490 can be signficantly higher- which means that the first optical depth will be shallower.

How shallow depends upon what is in the water.

Sean 

Thank you Sean.
Ajis