1. Study area & research’s purposes
Table of contents
Study area & research’s purposes
Image processing (focused on adjacency)
Validation (for one lake)
Future work

2. Study area & research’s purposes
This study is part of ongoing research efforts aimed to developing Earth Observation (EO) strategies towards the implementation of the Water Framework Directive (WFD), ensuring systematic monitoring of water quality in the largest lakes of the Subalpine ecoregion
Within the WFD it is possible, for each water body, to monitor only the water quality elements most sensitive to a certain risk or pressure
For Subalpine lakes this could be the deviation from a trophic level assessed with two causal elements (i.e., phosphorous and nitrogen) and with one response parameter, the chlorophyll-a concentration
Mapping chl-a from EO data

3. Image processing: MERIS FR Level 2b (products)
07/05/2005
22/07/2003
ESA ITT
ENVI-DTEX-EOPS-SW
A limitation in the current standard MERIS L2 products for lake pixels is the applied atmospheric correction.
A problem for the atmospheric correction over lake pixels is the adjacency effect
The bio-optical model used in the L2 processing is very general and does not take into account the broad variety of the inherent optical properties of the different water types typically met in lake waters

4. Image processing: MERIS FR Level 1b (ToA radiances)
Image-processing chain: from Top of Atmosphere (ToA) radiances to chlorophyll concentrations
The ‘coastal Waters and Ocean MODTRAN-4 Based ATmospheric correction’ (‘c-WOMBAT-c’)
Atmospheric inversion procedure from at-sensor radiance to apparent reflectance and then to remote sensing reflectance (Brando and Dekker, 2003)
To parameterise the input MODTRAN file required by c-WOMBAT-c the MODO interface (ReSe Applications Schlaepfer ) was used
MODTRAN inputs: visibility, continental model for aerosol, SunZ, SunA, etc…

5. Image processing: inversion in the atmosphere
Adjacency effects are corrected for using a background radiance image generated by convolving the original radiance image (ToA radiance) with a spatial weighting function (De Haan et al., 1997; Adler-Golden et al., 1998).
19  19 pixel  5.5 km
11  11 pixel  3.3 km
3  3 pixel  900m
ToA L [Wm-2sr-1µm-1]
ToA L [Wm-2sr-1µm-1]
ToA L [Wm-2sr-1µm-1]
Wavelength [nm]
Wavelength [nm]
Wavelength [nm]

6. Image processing: inversion in the atmosphere
Spectra from pixel in the narrower sub-basin
Results of c-WOMBAT-c using different inputs for the correction of adjacency effects
Rapp [-]
Wavelength [nm]
A kernel of 15  15 pixels  4.5 km (corresponding to the average extension of narrow portions of the lakes) was used as input file for the atmospheric correction
Rapp [-]
Spectra from pixel in the wider sub-basin
Wavelength [nm]

7. Validation: Lake Garda
Comparison of MERIS-derived chlorophyll-a (in mg m-3) and in situ measurements performed in 2003, 2004 and 2005 by the local agency (APPA-Trento)
Monthly-based averages
Monthly-based averages for 2005
2003
2004
2005
MERIS
MERIS
2005
In situ
In situ

8. Next steps
Better understanding of the atmospheric correction procedure focused on the selection of kernel size to correct the adjacency effects in the narrowest portions of the investigated lakes
Computing the effect of adjacency effect correction on water quality retrieval
Further testing with c-WOMBAT-c (the program is available, ask to Vittorio Brando:
Assessing of inherent optical properties of each lake and/or to evaluate how they differ from the Lake Garda ones
Increasing of in situ dataset for validation