Dear Taufik, pls also condider other indices to evaluate these models. For example, SD (standard deviation), RMSE and the model robustness. Those extrems at the lower and higher ends of data are critical to the value of R2. If the maximum and minimum SD (Secchi Depth) values are removed in Figure 4, then the R2 value would therefore decrease.

Sure. Water absorption is typically observed in the NIR and SWIR bands. Check out LSWI from Xiao et al 2004, 2005 and others.

Radiometry and signal-to-noise ratio was significaly improved for the NIR band in the OLI sensor L-8.

Comments / questions:

1. What depths are you looking at --- X-axis scale about 1.5m?

2. Were the waters clear or quite turbid?

3. What SD concentrations are you dealing with (Y-axis units not clear to me)? You may want to use Y-axis range that is similar on all the plots.

4. At lower sediment concentrations the visible bands are important but you have to be careful with water penetration and seeing the bottom.

5. At higher sediment concentrations the NIR band becomes more important and the correlation will be better than the visible bands because often the spectral range in the visible bands becomes more flat.

I was involved in a project 15 to 20 years ago analyzing the sediment concentrations of the Colorado River at the bottom of the Grand Canyon using a spectral radiometer with Landsat TM bands (Landsat 7 not 8) with the following results:

Spectral radiometers with Landsat TM spectral bands were mounted on the river's edge and pointed at the water approximately five feet away from the shore line. Measurements were taken every 15 minutes during day light hours for a year and were correlated with suspended sediment concentrations (SSC) and turbidity results derived from water samples that were collected every few weeks. The results indicated that the visible bands were important during low to moderate SSC and the near-infrared band became important when the SSC levels were moderate to high to very high concentrations.

Band 1 and 2 of Landsat 8 and Band 1 in Landsat 7 are more powerful for bathymetric, suspended sedimentary and aerosol studies than B4

Dear it is regarding to water absorption is captured by NIR & SWIR bands.

My best egards