Abstract

Methods

Results

Conclusion

Literature Cited

Acknowledgements

This study presents descriptive and empirical results of optical parameters associated to TotalSuspended Sediments (TSS) in a tropical open bay located at the west coast of Puerto Rico.Spatial analysis indicated that absolute values of TSS, remote sensing reflectance (Rrs) and totalbackscattering (bb) increases with proximity to the shoreline. A good relationship between bbpand TSS was established (R2=0.7) in all wavelengths. The regression analyses between Rrs andTSS indicate that best wavelengths to estimate TSS are between 589 to 645 nm. It was foundthat a red to green ratio improved the correlation results. Rrs at 645 nm measured with thefield spectoradiometer showed the highest correlation coefficient with MODIS reflectance.Sensors with better spatial and spectral resolution are needed in order to generate operationalproducts of TSS in these highly variable optical waters.

Fiigure 2. Magnitude and variability of TSS valuesincreases with proximity to the shoreline (Fig. 2).Overall mean values from inshore, middle andoffshore stations were 7.5, 3.6 and 2.7 mg/l,respectively. This spatial association is not onlyexplained with river discharge, but also with theeffect of re-suspension in shallow waters.

Spatial variability of TSS

Rrs parameter (X) Regression relationship; n=72; TSS = R2

Rrs(442) 1357.9 * (X) - 1.512 0.17

Rrs(470) 1331.6 * (X) - 3.4123 0.24

Rrs(510) 1153.2 * (X) - 4.2096 0.45

Rrs(589) 579.58 * (X) + 1.6079 0.73

Rrs(620) 599.3 * (X) + 2.7821 0.73

Rrs(645) 602.63 * (X) + 3.1481 0.73

Rrs(665) 634.34 * (X) + 3.5357 0.71

Rrs(675) 641.76 * (X) + 3.6363 0.71

Rrs(859) 3675.4 * (X) + 5.9588 0.46

Rrs(560)/Rrs(620) 16.941* (X) -1.232 0.75

Rrs(412)/Rrs(670) 7.7701 * (X) -0.5628 0.77

Rrs(665)/Rrs(555) 23.943 * (X) - 0.7366 0.81

Rrs(655)/Rrs(545) 20.353 * (X) - 0.3937 0.84

Field Work

Figure 1. (a) Study area showing the areas of riverdischarge and the location of monitored stations.(b) An optical package was used in all the stations.(c) Water samples were collected in each stationand in river mouths. (d) Spectoradiometer GER-1500- Gives measurements of Rrs within the rangeof ~370-1000 nm.

(a)

(b)

(d)

(c)

Dates Season No. of stations

Fiel Data missing

Good quality images available

January 12-14, 2004 Dry 12 2

February 12, 2004 Dry 9 TSS Depth; Rrs 1

August 19, 2004 Dry 10 TSS Depth 0

July 19, 2005 Rainy 6 TSS Depth 0

August 17, 2005 Rainy 5 TSS Depth 1

September 20, 2005 Rainy 6 0

October 19, 2005 Rainy 6 1

December 6, 2005 Dry 6 1

March 8, 2006 Dry 5 Rrs 1

April 21, 2006 Dry 5 0

September 26, 2006 Rainy 6 0

October 26, 2006 Rainy 6 0

Cruise Details

Total suspended sediments (mg/l)

Remote sensing reflectance (sr^-1)

Particle Backscattering (m^-1)

442, 470, 510, 589, 620, 675 nm

Correlation Analyses

Spatial variability of bbp and Rrs Rrs vs. bbp

Vilmaliz Rodríguez-Guzmán and Fernando Gilbes-SantaellaGeological and Environmental Remote Sensing Laboratory, Department of Geology, University of Puerto Rico at Mayagüez

Empirical relationships between in situ optical measurements and TSS

TSS Spectral Response

This Project is Sponsored by NOAA-CREST Grant no. NA06OAR4810162

Special thanks to Patrick Reyes. Ramón López and Marrcos Rosado for helping in getting andprocessing part the data and giving helpful recommendations. Our appreciation to all thepeople that collaborate in the field and laboratory work.

Results of this study provided a baseline for better understanding spatial and spectralvariability of remote sensing parameters and their relationship with TSS. Regressionanalyses suggest that this quality parameter dominates the water-leaving signal between589- 645 nm. The high spatial variability in optical parameters of the study site incombination with the relative low spatial resolution of MODIS demonstrated that a betterocean color sensor is required for coastal studies in tropical open bays. Results also showthat use of band ratios will improve the development of algorithms and their application inthese waters.

Coastal Remote Sensing is one of NOAA-CREST trust research areas. In this study we derived empirical relationships between TSS, optical backscattering (bb), in situ remote sensing reflectance (Rrs) and MODIS data in order to provide a

baseline for the development of site specific algorithms to estimate TSS in tropical and highly variable coastal environments.

General Objective

MODIS Reflectance vs. in situ measurements

Figure 5. Relationship between Rrs and bb suggests that in long wavelengths (>510 nm) backscattering dominates the signal, while in shorter wavelengths (< 510 nm) absorption appears to dominate over backscattering (Fig. 4)

The single band regression analysis supports the use of wavelengthsat 589 nm or 620 to estimate TSS, because both showed the highersquare correlation coefficients comparing wavelengths of HS-6channels. Better relationships between red to green reflectanceratios suggest that suspended sediments dominate water opticalproperties, over the effect of phytoplankton and CDOM substances(Binding et al., 2003). This presents an advantage in thedevelopment of algorithms to estimate TSS from satellite deriveddata in Mayagüez Bay because the effect of other water constituentsin the water-leaving signal is reduced.

Figure 3. Measurements of bb in offshore stations presented typical behavior of clear waterswhere higher values were observed in the blue region continuously decreasing with longerwavelengths (Morel et al., 2007). High backscattering values at 589 nm and 620 nm observedin inshore stations correspond to target wavelengths to estimate TSS because they reach idealconditions where there is a dominance of TSS in the backscattering signal and the chlorophyllfluorescence effect is reduced.

Figure 4. Main variations associated to proximity to the shoreline are: (1) an increase inmagnitude, which is attributed to higher scattering by water constituents, and (2) a shiftingfrom 490 nm to 575 nm of the higher peak due to higher absorption by chlorophyll andCDOM combined with higher scattering of TSS in the red region (Froidefond et al., 2002).

Figure 3 Figure 4 Figure 5

Figure 8 shows the variations inmagnitude and spectral shape asaffected by the concentration ofTSS. The highest difference in Rrsmagnitude due to TSS was observedfrom 550 nm to 700 nm.

Figure 7

Figure 6

Figure 9. The best regressionwith MODIS data was foundwith Rrs, with a fairly goodcorrelation coefficient of 0.69.However this analysisindicated that it is a challengeto define any relationshipbetween MODIS reflectanceand in situ measurements ofoptically active componentsin this area. This difficultycould be diminished by theincorporation of moreobservations in the dataset.

Figure 2

Morel, A., Gentili, B., Claustre, H., Babin, M., Bricaud, A., Ras, J., & Tièche, F. (2007). Optical properties ofthe “clearest” natural waters. Limnoogy and Oceanograghy, 52 (1), 217-229.

Binding, C.E., Bowers, D. G. & Mithelson-Jacob, E. G. (2003). An algorithm for the retrieval of suspendedsediment concentrations in the Irish Sea from SeaWiFS ocean colour satellite imagery. International Journalof Remote Sensing, 24 (19), 3791–3806.

Froidefond J.M, Gardelb L., Guiralb D., Parrab M., Ternonb J.F. (2002). Spectral remote sensing reflectances ofcoastal waters in French Guiana under the Amazon influence. Remote Sensing of Environment, 80, 225-232.