Ghezegegn, S.G.

Laanen, M.

Peters, S.

Salama, M.S.

Stefan, S.

English

University of Twente Research Information

Deriving inherent optical properties and associated Inversion: A case study in the River VantaaGhezehegn, Semhar1; Stefan, Simis2; Peters, Steef1; Laanen, Marnix1; Salama, Suhyb31Waterinsight, NETHERLANDS; 2Finnish environmental insitute,SYKE, FINLAND; 3University of Twente, NETHERLANDSIn coastal waters, mainly influenced by sediment load from river outlets, the light propagation in the water column isinfluenced by suspended particles. Empirical and semi-analytic algorithms have been developed to obtain estimates ofinherent optical properties (IOPs) from remotely sensed reflectance data. One of the advantages of semi-analyticalmodels over empirical ones is their capability to retrieve several parameters simultaneously and their global (generic)application. In previous studies the absorption of colored dissolved matters (CDOM) and non algal particulates (NAP)are considered as one because of their similar spectral slope. In addition, the spectral slope for backscatteringparticulate matter (bspm) is predicted using a power-law function fitted in the near infrared and extrapolated to 440nm. These values were assumed constant in order to reduce the inversion uncertainty and errors. Hence wronglyassumed spectral shapes will lead to significant errors in the derived inherent optical properties (IOPs). In this paperthe semi-analytical model called Garver-Siegel-Maritorena model (GSM) is modified to derive the IOPs and theirspectral dependencies. The research questions include: Does including the spectral variation of backscattering ofparticulate matter (bspm) by taking into account particulate absorption (adapted from Doxaran et al, 2009) improvesthe derived IOPs? Is there a way to de-convolve the absorption spectrum of NAP from that of CDOM? Using a state ofthe art hand held spectrometer WISP-3, hyper spectral reflectance data were collected in parallel with extensivelaboratory analysis for validating the model. The inversion of the GSM model is adapted to derive seven parameters invisible bands covering the wavelengths from 400nm to 850nm. The model was able to derive a linear relationshipbetween measured and derived IOPs with R2 value 0.847, 0.944, 0.952 for absorption of NAP, CDOM and coefficientof bspm respectively. But it fails to estimate Chl-a absorption (R2 = 0.43) probably because the spectrum isdominated by suspended particulate matter (SPM). The RMSE values were 1.91, 0.37, 1.11, 2.06 for absorption ofChl-a, NAP, CDOM and bspm respectively. The inversion uncertainty of the derived IOPs directly proportional withturbidity. The SPM concentration varies from 8-200 mg/l causing high variability in derived IOPs such as Chl-a(0.1-0.6m-1), and bspm (4.6-19.1m-1).The modified GSM algorithm performs well in turbid waters with higher CDOM.The WISP-3 provides hyperspectral reflectance data which can be used for validation of future application of thealgorithm on satellite images such as MODIS.http://www.livingplanet2013.org/abstracts/850980.htm2 of 2 5/27/2014 16:40

Deriving inherent optical properties and associated Inversion: A case study in the River Vantaa

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https://ris.utwente.nl/ws/files/28154009/Ghezegegn2013deriving.pdf

Deriving inherent optical properties and associated Inversion: A case study in the River Vantaa

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