Assessment of water quality in Lake Garda (Italy) using Hyperion

 
Assessment of water quality in Lake Garda (Italy) using Hyperion
Remote Sensing of Environment 109 (2007) 183 – 195
                                                                                                                             www.elsevier.com/locate/rse

         Assessment of water quality in Lake Garda (Italy) using Hyperion
                            Claudia Giardino a,⁎, Vittorio E. Brando b , Arnold G. Dekker b ,
                                       Niklas Strömbeck c , Gabriele Candiani a
                                                   a
                                                     Optical Remote Sensing Group, CNR–IREA, Milano, Italy
                                     b
                                         Environmental Remote Sensing Group, CSIRO-Land and Water, Canberra, Australia
                                              c
                                                Department of Limnology, EBC, Uppsala University, Uppsala, Sweden
                             Received 22 August 2006; received in revised form 20 December 2006; accepted 23 December 2006

Abstract

    For testing the integration of the remote sensing related technologies into the water quality monitoring programs of Lake Garda (the largest
Italian lake), the spatial and spectral resolutions of Hyperion and the capability of physics-based approaches were considered highly suitable.
Hyperion data were acquired on 22nd July 2003 and water quality was assessed (i) defining a bio-optical model, (ii) converting the Hyperion at-
sensor radiances into subsurface irradiance reflectances, and (iii) adopting a bio-optical model inversion technique. The bio-optical model was
parameterised using specific inherent optical properties of the lake and light field variables derived from a radiative transfer numerical model. A
MODTRAN-based atmospheric correction code, complemented with an air/water interface correction was used to convert Hyperion at-sensor
radiances into subsurface irradiance reflectance values. These reflectance values were comparable to in situ reflectance spectra measured during
the Hyperion overpass, except at longer wavelengths (beyond 700 nm), where reflectance values were contaminated by severe atmospheric
adjacency effects. Chlorophyll-a and tripton concentrations were retrieved by inverting two Hyperion bands selected using a sensitivity analysis
applied to the bio-optical model. The sensitivity analysis indicated that the assessment of coloured dissolved organic matter was not achievable in
this study due to the limited coloured dissolved organic matter concentration range of the lake, resulting in reflectance differences below the
environmental measurement noise of Hyperion. The chlorophyll-a and tripton image-products were compared to in situ data collected during
the Hyperion overpass, both by traditional sampling techniques (8 points) and by continuous flow-through systems (32 km). For chlorophyll-a the
correlation coefficient between in situ point stations and Hyperion-inferred concentrations was 0.77 (data range from 1.30 to 2.16 mg m− 3). The
Hyperion-derived chlorophyll-a concentrations also match most of the flow-through transect data. For tripton, the validation was constrained by
variable re-suspension phenomena. The correlation coefficient between in situ point stations and Hyperion-derived concentrations increased from
0.48 to 0.75 (data range from 0.95 to 2.13 g m− 3) if the sampling data from the re-suspension zone was avoided. The comparison of Hyperion-
derived tripton concentrations and flow-through transect data exhibited a similar mismatch. The results of this research suggest further studies to
address compatibilities of validation methods for water body features with a high rate of change, and to reduce the contamination by atmospheric
adjacency effects on Hyperion data at longer wavelengths in Alpine environment. The transferability of the presented method to other sensors and
the ability to assess water quality independent from in situ water quality data, suggest that management relevant applications for Lake Garda (and
other subalpine lakes) could be supported by remote sensing.
© 2007 Elsevier Inc. All rights reserved.

Keywords: Hyperspectral satellite data; Lake waters; Bio-optical modelling; In situ data

1. Introduction                                                                    supply, irrigation, transportation), industrial (processing and
                                                                                   cooling, energy production, fishery) and recreational purposes.
   Lake water is an essential renewable resource for mankind                       Sustainable use of water resources requires the coupling of
and the environment and it is important for civil (drinking water                  surface waters assessment monitoring programs and decision
                                                                                   making and management tools. The Water Framework Di-
                                                                                   rective (WFD) of the European Commission (Directive 2000/
 ⁎ Corresponding author. Tel.:+39 0223699298; fax: +39 0223699300.                 60/EC, 2000) is the major reference in Europe to guide efforts
   E-mail address: giardino.c@irea.cnr.it (C. Giardino).                           for attaining a sustainable aquatic environment in the years to
0034-4257/$ - see front matter © 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.rse.2006.12.017
Assessment of water quality in Lake Garda (Italy) using Hyperion
184                                  C. Giardino et al. / Remote Sensing of Environment 109 (2007) 183–195

come. The WFD includes guidelines which define the cat-                       This study is part of ongoing research efforts aimed at de-
egories of quality and the required components and parameters.            veloping RS strategies towards the implementation of the WFD,
As some of these parameters can be determined by Remote                   ensuring systematic monitoring of water quality in Lake Garda,
Sensing (RS) with a reasonable accuracy, RS-related technol-              the largest Italian lake. Empirical and semi-empirical ap-
ogies may be integrated in the monitoring programs defined by             proaches were previously investigated to retrieve chlorophyll
the WFD, provided they can be demonstrated to independently               concentrations in the lake (Brivio et al., 2001; Candiani et al.,
assess Water Quality Parameters (WQPs).                                   2003; Giardino et al., 2005) but their results were scene
   Since the 1980s satellite RS represents an opportunity for             dependent. The aim of this study is to provide a RS-based
synoptic and multitemporal viewing of water quality. To estimate          measurement tool, transferable to different RS-instruments. It
WQPs from satellite data three different approaches can be used           would be useful for water management authorities of Lake
(Cracknell et al., 2001; Dekker et al., 1995). The (1) empirical          Garda for coarse scale regular monitoring (with high revisiting
approach is based on the development of bi-variate or multivariate        time spaceborne sensors), for intermediate/fine scale studies
regressions between RS data and measured WQPs. Digital                    (with high spatial resolution satellite and airborne sensors) and
numbers or radiance values at the sensor, as well as their band           for retrospective analysis with time-series imagery (as in
combinations, are correlated with in situ measurements of WQPs,           Dekker et al., 2005). As a test, hyperspectral Hyperion data
usually collected in coincidence of the sensor overpass. A                (with a 30 m pixel size and a choice of more than 200 spectral
summary of empirical approaches for lakes can be found in                 channels), analytical modelling, and in situ measurements
Lindell et al. (1999). The (2) semi-empirical approach may be             coincident with the satellite overpass for a validation or com-
used when spectral characteristics of the parameters of interest          parison of the image-derived products, were considered ap-
are known. This knowledge is included in the statistical analysis         propriate. The approach used in this study builds on the method
by focusing on well-chosen spectral areas and appropriate                 developed for Hyperion imagery of a sub-basin of a subtropical
wavebands used as correlates. An example of a semi-empirical              bay in Australia (Brando & Dekker, 2003). Based on a bio-
approach with different sensors is reported by Härmä et al. (2001)        optical model sensitivity analysis, Hyperion bands were
over Finnish lakes. In the (3) analytical approach, WQPs are              selected and concentrations of chlorophyll-a and tripton (the
related to the bulk Inherent Optical Properties (IOPs) via the            non-algal particles of the suspended particulate matter) were
Specific Inherent Optical Properties (SIOPs). The IOPs of the             retrieved. Point in situ data for an initial validation of the
water column are then related to the Apparent Optical Properties          products, followed by a comparison of concentrations retrieved
(AOPs) and hence to the Top of Atmosphere (TOA) radiance,                 from Hyperion using high spatial resolution flow-through
such as described by the radiative transfer theory (Mobley, 1994;         estimates of chlorophyll-a and tripton were used.
Vermote et al., 1997). The analytical method involves inverting all
above relations (WQPs → IOPs → AOPs → TOA radiances) to                   2. Materials and methods
determine the WQPs from RS data. An example of such approach,
using Landsat over lakes, can be found in Dekker et al. (2001) for        2.1. Study area and fieldwork activities
the total suspended matter retrieval.
   Quantitatively, the relationships developed to assess water                Approximately 500,000 lakes over 1 ha surface area exist in
quality in lakes within empirical and semi-empirical approaches           Europe. Most of the largest European lakes are located in the
are often scene dependent and only apply to the data from which           Nordic countries and in the Alpine regions (EEA, 1999). The
they are derived. Well-calibrated and validated physics-based             most important Italian lake district is located in the subalpine
approaches are instead applicable to every scene acquired over            region and represents more than 80% of the total Italian
the selected lake (presuming constant SIOPs), giving the                  lacustrine volume (Premazzi et al., 2003). Lake Garda, located
opportunity to assess water quality independently from ground             65 m a.s.l. around 45°40′ N and 10°41′ E at the eastern border
measurements of WQPs. The monitoring of spatially heteroge-               of the subalpine region, is the largest Italian lake. It has a surface
neous parameters, as re-suspension phenomena due to local                 area of 368 km2, a volume of 49 million m3 and a maximum and
variability in wind and circulation, or algal blooms at the surface       a mean depth of 350 m and 133 m, respectively. The average
also necessitates these (in situ independent) methods. Dekker             value of the Secchi disk depth is 4.5 m in summer and 16 m in
et al. (2002) investigated the capabilities of Landsat-TM and             winter. Chlorophyll-a (CHL-a) and suspended particulate matter
SPOT data for retrospective analysis in Dutch lakes. Both                 (SPM) concentrations range from 0.5 to 12 mg m− 3 and from
sensors were capable of describing larger concentration                   0.1 to 5.5 g m− 3, respectively. The coloured dissolved organic
gradients characterised by temporal changes that were not                 matter concentration (absorption coefficient at 440 nm, aCDOM
represented by point in situ data. Kutser (2004) used Hyperion            (440)) ranges from 0.017 to 0.36 m− 1. Average concentrations
data to map accumulation of aggregations of cyanobacteria in              of CHL-a, SPM and aCDOM(440) are around 2.7 mg m− 3, 2.5 g
the Gulf of Finland, an assessment unachievable by traditional            m− 3 and 0.09 m− 1, respectively (Premazzi et al., 2003; Zilioli,
in situ sampling due to spatial and temporal issues. Kutser               2002). According to the OECD guidelines (Vollenweider &
(2004) also showed that flow-through systems were only                    Kerekes, 1982), Lake Garda can be classified as an oligo-
suitable to map chlorophyll from a fixed depth, and therefore             mesotrophic basin.
inappropriate for assessing cyanobacteria blooms closer to or at              In collaboration with the local agencies in charge of
the water surface.                                                        limnological monitoring of Lake Garda, intensive fieldwork
C. Giardino et al. / Remote Sensing of Environment 109 (2007) 183–195                                185

activities were run in the whole basin, within several national                       particulate matter can be divided into phytoplankton and into
and international projects (Lindell et al., 1999; Zilioli, 2002,                      the non-algal component (i.e., tripton), TR was indirectly
2004). More than 30 days over 5 years of in situ measurements                         estimated from SPM concentrations. They were measured
were performed to achieve a comprehensive dataset of                                  on pre-combusted and pre-weighted Watman GF/F filters,
concentrations of WQPs, of IOPs and AOPs, leading to the                              dried at 95 °C overnight. The biomass of phytoplankton
parameterisation of a three-component bio-optical model                               was considered correlated to CHL-a and the formula
according to Strömbeck et al. (2003). In this study in situ data                      TR = SPM − 0.07CHL-a (with TR and SPM in g m− 3 and
collected on 22nd July 2003 to validate the atmospheric                               CHL-a in mg m− 3) was used to separate TR from SPM. Gons
correction of Hyperion data, to validate the image-derived                            et al. (1992) observed that for fresh water algae the part of
WQPs products, as well as to improve the bio-optical                                  SPM determined by the biomass of phytoplankton can vary
parameterisation are presented.                                                       between 0.02 and 0.1. The average value of 0.07 had been
                                                                                      successfully adopted in Dutch lakes (Hoogenboom et al.,
• Water samples of the first integrated meter of water column in                      1998), Finnish lakes (Kutser et al., 2001) as well as in coastal
  9 point stations (Fig. 1) were analysed for CHL-a and tripton                       waters (Brando & Dekker, 2003). Thus, it was supposed to be
  (TR) concentrations. The in situ samples were collected                             valid for Lake Garda too.
  within a 3-hour interval around the image acquisition. Water                      • Five water samples collected during the day were used to
  was filtered through Watman GF/F glass fiber filters and the                        measure the absorption spectra of phytoplankton aph(λ)
  material retained was analysed for CHL-a concentrations                             and tripton aTR(λ), according to the method showed in
  according to the analytical method ISO 10260-E (1992).                              Strömbeck and Pierson (2001). The absorption spectra of
  Phytoplankton was composed by 70% of Chlorophyta                                    particles ap(λ) retained onto the GF/F filters, were measured
  species and by 30% of almost equal parts of Cryptophyceae,                          using a laboratory spectrophotometer and the filter-pad
  Diatomeae and Cyanophyta species. Because the suspended                             technique (Tassan & Ferrari, 1995). The filters were then
                                                                                      treated with cold Methanol to extract pigments and the
                                                                                      absorption spectra of tripton aTR(λ) of these bleached filters
                                                                                      were measured. The absorption spectrum of phytoplankton
                                                                                      aph(λ) was derived by subtracting aTR(λ) from ap(λ) spectra.
                                                                                    • A 32-km-long transect (Fig. 1) of fluorescence and turbidity
                                                                                      data were collected using a flow-through system. The system
                                                                                      is composed of a hydraulic device, (essentially an intake
                                                                                      pipe) continuously pumping water from 0.5 m depth into a
                                                                                      Turner Design Scufa-II fluorometer/turbidimeter, and a GPS,
                                                                                      both logged by a Campbell data-logger. Logged values of
                                                                                      fluorescence (in mV) and turbidity, in Nephelometric
                                                                                      Turbidity Units (NTU), were corrected for delays caused
                                                                                      by the flow-through system. Flow-through data were con-
                                                                                      verted into chlorophyll-a and tripton using the concentrations
                                                                                      derived from laboratory analysis on water samples. Eight
                                                                                      laboratory-concentrations were regressed against the average
                                                                                      of logged values over the nearest 100 m to the GPS location
                                                                                      where the water samples were collected. By means of linear
                                                                                      regression analysis, the measured in vivo fluorescence was
                                                                                      transformed into chlorophyll-a concentrations (R2 = 0.55),
                                                                                      and turbidity into tripton concentrations (R2 = 0.68) (Fig. 2).
                                                                                      It was hence assumed that flow-through data were able to
                                                                                      describe both CHL-a and TR concentrations along the 32-
                                                                                      km-long transect although turbidity, because it includes
                                                                                      phytoplankton scattering, is more closely a measure of SPM.
                                                                                    • Spectroradiometric measurements of water radiance using
                                                                                      the PR-650 spectroradiometer were performed to calculate
                                                                                      the subsurface irradiance reflectance R(0−, λ) in three
                                                                                      pelagic stations (4, 6 and 7 in Fig. 1). R(0−, λ) values were
                                                                                      computed from remote sensing reflectances Rrs(0+, λ),
Fig. 1. Study area and location of fieldwork activities performed within a 3-hour     measured above-water according to the SeaWifs protocol
interval around the image acquisition. 1 to 9 are the stations where water was        (Fargion & Mueller, 2000). Effects of the lake surface
sampled for laboratory analysis for chlorophyll-a and tripton concentrations
(stations 4, 6 and 7 have also PR-650 radiometric measurements). The flow-            roughness on above-water Rrs(0+, λ) determinations were
through system was cruised throughout all the stations, for a length of about         corrected by a sky-radiance reflectance factor and by an
32 km. The 7.5-km-wide portion of lake imaged by Hyperion is outlined.                offset term, that does not impose a constrained normalisation
186                                                 C. Giardino et al. / Remote Sensing of Environment 109 (2007) 183–195

Fig. 2. Scatter plots of fluorescence vs. chlorophyll-a (using stations 1 to 8) and of turbidity vs. tripton (using stations 2 to 9) with the calibration lines (turbidity in
Station 1 and fluorescence in Station 9 were no available).

      at 750 nm (Toole et al., 2000). Literature data (Toole et al.,                     tion spectra of coloured dissolved organic matter. In this study
      2000) for clear sky and high wind speeds were used (on 22nd                        SCDOM and STR were equal to 0.021 and 0.012, respectively.
      July, 2003 the average wind speed on the lake was 6 m s− 1).                          The spectral total backscattering coefficient bb(λ) was
      Assuming an air/water interface parameter of 0.533 (Lee                            computed as:
      et al., 1994) and a Q-factor of 4.2 sr− 1 the roughness-cor-                                                    ⁎
      rected Rrs(0+, λ) were then transformed into R(0−, λ)                              bb ðkÞ ¼ bbw ðkÞ þ ½CHL  abbph ðkÞ
                                                                                                                       
      values. As reported in Strömbeck et al. (2003) the Q-factor                                         ⁎          k −ni
                                                                                                  þ ½TRbbTR ð550Þ                                                      ð3Þ
      was the average, between 400 and 750 nm, of a spectral Q-                                                     550
      factor computed using the HYDROLIGHT 4.2 model
      (Mobley, 1994; Mobley & Sundman, 2001).                                            where, bbw(λ) is the backscattering coefficient of pure water
                                                                                         (Morel, 1974; Dall'Olmo & Gitelson, 2006), b⁎bph (λ) is the
                                                                                         specific backscattering caused by phytoplankton, b⁎bTR(λ) is the
2.2. The bio-optical model
                                                                                         specific backscattering coefficient at 550 nm for 1 g m− 3 of
                                                                                         tripton, ni is an exponent describing the spectral dependency of
    The bio-optical model used in this study was similar to
                                                                                         tripton backscattering (mainly due to its inorganic components).
previously published three-components (i.e., chlorophyll-a,
                                                                                         The specific backscattering by phytoplankton was computed
tripton and coloured dissolved organic matter) Case-2 or lake
                                                                                         using an expression based on Gordon et al. (1988), Morel
water models, e.g., Pierson and Strömbeck (2001). The sub-
                                                                                         (1988), Ammenberg et al. (2002), and Roesler and Boss (2003):
surface irradiance reflectance R(0−, λ) was calculated as a
function of absorption and backscattering coefficients according                                                                            
                                                                                                         ⁎                   k −nph            bbph
to Walker (1994):                                                                        b⁎bph ðkÞ ¼                 ⁎
                                                                                                         bphð555Þ þ aphð555Þ             ⁎
                                                                                                                                      −kaphðkÞ
                                                                                                                                555              bph
                   1                  bb ð kÞ                                                                                                                           ð4Þ
Rð0−;kÞ ¼                     d                                                 ð1Þ
                    l d ðkÞ       að kÞ þ bb ð kÞ
                    P
               1þ   l u ðkÞ
                    P

                                                                                         where, b⁎ph (555) is the chlorophyll-a specific scattering, nph is
where, a(λ) is the spectral total absorption coefficient, bb(λ) is                       an exponent describing the spectral dependency of the phy-
the spectral total backscattering coefficient, and μ̄d(λ)/μ̄ u(λ) is                     toplankton beam attenuation, k is an empirical coefficient reg-
the ratio of the average cosine of the downwelling light to that of                      ulating the effect by phytoplankton absorption and bbph / bph is
the upwelling light (Mobley, 1994).                                                      the average spectral backscattering efficiency of phytoplankton.
    The spectral total absorption coefficient a(λ) was computed                             The parameterisation of the bio-optical model used in this
as:                                                                                      study is largely based on the data presented in Strömbeck et al.
aðkÞ ¼ aw ðkÞ þ ½CHL  aa⁎ph ðkÞ þ aCDOM ð440Þe−SCDOM ðk−440Þ
                                                                                         (2003), which have been acquired in Lake Garda on 10th and
                                                                                         11th October 2002. The dataset contains discrete measurements
       þ ½TRa⁎ ð440Þe−STR ðk−440Þ
                       TR                                                                of WQPs, total absorption a and total scattering b coefficients
                                                                                ð2Þ      at 9 wavelengths obtained with a WET Labs ac-9, and total
                                                                                         backscattering bb coefficients at 6 wavelengths obtained by a
where, aw(λ) is the pure water absorption (Pope & Fry, 1997;                             HOBILabs HydroScat-6. In particular, scattering b and back-
Smith & Baker, 1981), a⁎ph(λ) is the chlorophyll-specific                                scattering bb data of the lake, were used to parameterise Eq. (4)
phytoplankton absorption, SCDOM is the slope factor of the                               originally adopted for oceanic phytoplankton. Because the
absorption spectra of coloured dissolved organic matter,                                 average specific absorption coefficients of phytoplankton,
a⁎TR(440) is the absorption coefficient at 440 nm specific                                                      ⁎ (λ) derived from data collected on 22nd
                                                                                           ⁎ (λ) and tripton, aTR
                                                                                         aph
for 1 g m− 3 of tripton, STR is the slope factor of the absorp-                          July 2003 were comparable to data collected on 10th and 11th
C. Giardino et al. / Remote Sensing of Environment 109 (2007) 183–195                                                  187

Table 1                                                                               where, N is the number of bands, and X̄ i and X̂i are the
List of the bio-optical parameters and in situ water quality data presented in this   subsurface reflectance values from in situ data and forward
study with information about the day of acquisition and data source (APPA is the
Environmental Protection Agency of Trento, ARPAV is the Environmental
                                                                                      modelling, respectively. The number of bands N was 28; 22 of
Protection Agency of Veneto)                                                          these in the visible (VIS) range, from 480 to 690 nm, and the
Parameter                     Day of acquisition        Source
                                                                                      remaining in the near-infrared (NIR) range, from 700 to 750 nm.
                                                                                         The optical closure between in situ determinations of R(0−,
aw(λ)                         –                         Smith and Baker (1981),
                                                                                      λ) and the simulated values from forward modelling was
                                                        Pope and Fry (1997)
bbw(λ)                        –                         Morel (1974), Dall'Olmo       considered satisfactory in all stations (Fig. 4, Table 2). In
                                                        and Gitelson (2006)           particular, the convergence was good in the VIS range (average
a⁎ph (λ)                      10th–11th October 2002    HelsinkiUniversity and        RMSE of the three stations 0.006, relative RMSE 12%) while
                                                        Luode Consulting Oy           beyond 700 nm a larger divergence was observed (average
                              22nd July 2003            APPA
                                                                                      RMSE 0.012, relative RMSE 55%).
b⁎bph (λ)                     10th–11th October 2002    HelsinkiUniversity and
                                                        Luode Consulting Oy
a⁎TR(440), STR                10th–11th October 2002    HelsinkiUniversity and        2.3. Hyperion data and pre-processing analyses
                                                        Luode Consulting Oy
                              22nd July 2003            APPA                             On 22nd June 2003, image data from an area of 7.5 by 42 km
b⁎bTR (λ)                     10th–11th October 2002    HelsinkiUniversity and
                                                                                      was acquired by Hyperion with a near-nadir viewing. At the
                                                        Luode Consulting Oy
SCDOM                         10th–11th October 2002    APPA                          time of the overpass Sun zenith and azimuth angles were 32°
μ̄u(λ), μ̄d(λ)                –                         HYDROLIGHT 4.2                and 136°, respectively. For this study 28 Hyperion spectral
CHL-a, TR                     22nd July 2003            ARPAV                         bands ranging from 480 nm to 750 nm were selected to be
Fluorescence, turbidity       22nd July 2003            CNR–IREA                      relevant for WQPs estimation and reliable for the sensor
                                                                                      calibration (Green et al., 2003). Following the approach by
October 2002, they were averaged and integrated in the existing                       Brando and Dekker (2003), the image was convolved using a
dataset. Apparently, the natural modifications of algal and
tripton compositions occurred between the two periods had a
negligible effect on their absorption spectral properties.
Together with the μ̄ d(λ)/μ̄ u(λ) ratio (average value from
450 nm to 750 nm equal to 0.327), that was derived running
HYDROLIGHT 4.2 with inputs typical of Lake Garda (e.g.,
IOPs, averages values of wind speed and visibility ranges,
summertime Sun zenith angles at 11 ± 1 h UTC) (Strömbeck
et al., 2003), these SIOPs uniquely characterise the parameter-
isation of Eq. (1) for Lake Garda waters. Table 1 summarises the
bio-optical model parameters with the day of acquisition and the
data provider, Fig. 3 shows the SIOPs used in this study.
    The performance of the above parameterisation was
evaluated using the 22nd July 2003 dataset. It consisted of the
PR-650-derived measurements of R(0−, λ), collected in the
stations 4, 6 and 7 (Fig. 1), in which concentrations of CHL-a
and TR were also known. These concentrations, together with
the long-term (i.e., 0.09 m− 1) average value of aCDOM(440) of
Lake Garda (as aCDOM(440) concentrations were not measured
in this campaign), were given as input to the bio-optical model
to simulate R(0−, λ) spectra, assuming that SIOPs were the
same of October 2002. The optical closure between in situ
determinations of R(0−, λ) and the simulated values from
forward modelling was quantified with the Root Mean Square
Error (RMSE) and the relative RMSE (in %):
             vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
             uN                     2
             uP P
             u         X     − X̂
             ti¼1          i       i                                                  Fig. 3. SIOPs of Lake Garda: spectra of absorption (upper graph) and
RMSE ¼                                                                        ð5Þ     backscattering (lower graph) coefficients used in the bio-optical model. aw is the
                         N −1                                                         absorption coefficient of pure water, a⁎ph is the chlorophyll-specific absorption
                                                                                      coefficient of phytoplankton, aTR⁎ is the specific absorption coefficient of tripton,
                            RMSE                                                            ⁎
                                                                                      and aCDOM    is the specific absorption coefficient of coloured dissolved organic
Relative RMSE ¼                    d 100                                      ð6Þ     matter. bbw is the backscattering coefficient of pure water, bb⁎ph is the
                              P
                              N P
                            1
                            N   Xi                                                    chlorophyll-specific backscattering coefficient of phytoplankton, and bb⁎TR is
                               i¼1                                                    the specific backscattering coefficient of tripton.
188                                           C. Giardino et al. / Remote Sensing of Environment 109 (2007) 183–195

                                                                                      of NEΔR(0−, λ)E is about 0.001 for Hyperion data acquired
                                                                                      over water. The filtered image was atmospherically corrected to
                                                                                      R(0−, λ) using the MODTRAN-based c-WOMBAT-c procedure
                                                                                      (Brando & Dekker, 2003). The procedure consists of: (i) a three
                                                                                      step atmospheric inversion from at-sensor-radiance to apparent
                                                                                      reflectance and (ii) a two step inversion of the air–water
                                                                                      interface from apparent reflectance to subsurface irradiance
                                                                                      reflectance. c-WOMBAT-c was run with actual measurements
                                                                                      of visibility range (15 km) derived from sun-photometer ob-
                                                                                      servations performed synchronously to the sensor overpass
                                                                                      (Vermote et al., 1997). A Q-factor of 4.2 sr− 1, an air/water
                                                                                      interface parameter of 0.533, a nadir-viewing geometry and a
                                                                                      maritime extinction for aerosols were also given as inputs to the
                                                                                      atmospheric correction code.
                                                                                          The atmospherically corrected image was geo-located and
                                                                                      image-derived R(0−, λ) values were compared to in situ data
                                                                                      measured during the Hyperion overpass in the pelagic stations
                                                                                      4, 6 and 7 (Fig. 1). The optical closure (Fig. 4, Table 2) between
                                                                                      in situ and Hyperion spectra was on average good in the VIS
                                                                                      range (from 480 to 690 nm, the average reflectance RMSE of
                                                                                      the three stations was 0.007 and the relative RMSE 14%) and
                                                                                      inferior in the NIR bands (beyond 700 nm, the average
                                                                                      reflectance RMSE was 0.011 and the relative RMSE 77%).
                                                                                      More in detail, in Station 7 the near-infrared wavelength re-
                                                                                      flectance values from image data were over-estimated compared
                                                                                      to in situ values (beyond 700 nm the RMSE was 0.015 and the
                                                                                      relative RMSE 159%). The most likely cause of such over-
                                                                                      estimation in the atmospherically corrected image data was the
                                                                                      contamination of Hyperion radiances by adjacency effects, due
                                                                                      to multiple reflections of radiation coming from the neighbour-
                                                                                      ing environment, specifically in the northern part of Lake
Fig. 4. In situ, forward-modelled and Hyperion-derived subsurface irradiance          Garda. Here adjacency effects were probably caused by the
reflectance R(0−, λ) spectra in stations 4, 6 and 7 (Fig. 1): “In situ” spectra are   vegetation growing over the steep sides, laterally delimiting the
derived from above-water measurements of Rrs(0+, λ); “Model” spectra are
                                                                                      northern narrow part of the lake, where Station 7 is located
calculated from forward bio-optical modelling using CHL-a (in mg m− 3) and TR
(in g m− 3) concentrations measured in situ (shown in each graph) and assuming        (Fig. 1). The contribution of reflections from the vegetated
aCDOM(440) = 0.09 m− 1 in all stations; “Hyp” spectra are computed from               environment on the water surface increases the signal measured
Hyperion imagery using the c-WOMBAT-c atmospheric correction code. All the            by the sensor at longer wavelengths (Floricioiu & Rott, 2005)
data were collected on 22nd July 2003. Average reflectance RMSEs (see also            and c-WOMBAT-c can fail in removing these quantities.
Table 2) are 0.006 (relative RMSE 13%) between 480 and 690 nm, and 0.011
(relative RMSE 66%) beyond 700 nm.
                                                                                      2.4. Band selection and model inversion

5 × 5 low pass filter to reduce the environmental noise-                                 A direct inversion of the bio-optical model was applied to
equivalent reflectance differences NEΔR(0−, λ)E. According                            the Hyperion image using a linear Matrix Inversion Method
to Brando and Dekker (2003) and Wettle et al. (2004), the value                       (MIM), as in Brando and Dekker (2003). They ran MIM on a
Table 2
List of RMSE and relative RMSE (in %) measuring the optical closure of in situ determinations of R(0−, λ) vs. forward-modelled and Hyperion-derived R(0−, λ)
values
                 In situ vs. forward-modelled                                           In situ vs. Hyperion-derived
                 St. 4             St. 6             St. 7              AvMod           St. 4            St. 6         St. 7            AvHyp             AvAll
VIS              0.006             0.007             0.005              0.006           0.005            0.006         0.010            0.007             0.006
                 12%               13%               10%                12%             10%              10%           21%              14%               13%
NIR              0.015             0.020             0.002              0.012           0.009            0.009         0.015            0.011             0.011
                 72%               73%               22%                55%             42%              31%           159%             77%               66%
RMSE and relative RMSE for each station are reported together with their average value (in bold); AvMod is the average for stations 4, 6 and 7 of RMSEs computed
from in situ and forward-modelled R(0−, λ) values, AvHyp is the average for stations 4, 6 and 7 of RMSEs computed from in situ and Hyperion-derived R(0−, λ)
values. AvAll is the average of AvMod and AvHyp RMSEs and measures how R(0−, λ) values from Hyperion, forward bio-optical modelling and in situ optical data
converge. RMSEs and relative RMSEs are separately computed for VIS (22 bands from 480 to 690 nm) and NIR (6 bands from 700 to 750 nm) ranges.
C. Giardino et al. / Remote Sensing of Environment 109 (2007) 183–195                                   189

Hyperion image of a coastal site to retrieve chlorophyll, tripton                     model changing aCDOM(440) from 0.03 to 0.30 m− 1 and fixing
and aCDOM(440) by inverting three bands (490 nm, 670 nm and                           CHL-a and TR to 1 mg m− 3 and to 1 g m− 3, respectively. The
700–740 nm). The bands were chosen through some iterations                            first derivative was rescaled by a 0.01 factor to appreciate the
starting from wavelengths closest to spectral feature typical for                     sensitivity of the model in discriminating aCDOM(440) at con-
each of the WQPs and avoiding the shortest wavelengths (blue)                         centration ranges of the lake of around 0.09 m− 1. The first
where Hyperion was noisy. In this study, the large number                             derivative spectra relative to variations of CHL-a were computed
of Hyperion bands was exploited by performing a sensitivity                           by 10 forward runs of the bio-optical model incrementing CHL-a
analysis of the bio-optical model on aCDOM(440), CHL-a                                by 1 mg m− 3 within a range between 1 and 1 mg m− 3, and fixing
and TR independently, based on the first derivative approach                          TR to 1 g m− 3 and aCDOM(440) to 0.09 m− 1. Similarly, the first
(Hoogenboom et al., 1998).                                                            derivative spectra relative to variations of TR were computed by
   The first derivative spectra relative to variations of aCDOM                       10 forward runs of the bio-optical model incrementing TR by 1 g
(440) were obtained from 10 forward runs of the bio-optical                           m− 3 within a range between 1 and 10 g m− 3, and fixing CHL-a
                                                                                      to 1 mg m− 3 and aCDOM(440) to 0.09 m− 1.
                                                                                          Fig. 5 presents the first derivative spectra of R(0−, λ) vs.
                                                                                      each of the WQPs. The maximum variation of the first
                                                                                      derivative of R(0−, λ) for aCDOM(440) occurred at shortest
                                                                                      wavelengths (Fig. 5a), in a region where Hyperion data are too
                                                                                      noisy and ill calibrated (Green et al., 2003). Moving towards the
                                                                                      region where Hyperion provides calibrated data (i.e., N 480 nm),
                                                                                      the variation of the first derivative dR(0−, λ) / daCDOM(440)
                                                                                      falls within the Hyperion NEΔR(0−, λ)E of 0.001. This implies
                                                                                      that estimates of aCDOM(440) at the concentration range of the
                                                                                      lake was not achievable and therefore fixed to the long-term
                                                                                      average value for Lake Garda (i.e., 0.09 m− 1). This, a priori
                                                                                      determination of an unmeasurable signal due to small aCDOM
                                                                                      (440) variations illustrates the usefulness of the first derivative
                                                                                      approach. Based on sensitivity analysis for CHL-a and TR, the
                                                                                      Hyperion bands at 490 nm and 550 nm were selected for the
                                                                                      inversion. The 490 nm band was chosen because of the location
                                                                                      of the maximum variation of the first derivative of R(0−, λ) for
                                                                                      CHL-a concentration (Fig. 5b). The 550 nm band was chosen
                                                                                      because it is the hinge point of the first derivative of R(0−, λ)
                                                                                      for CHL-a concentration (Fig. 5b), as well as being in the region
                                                                                      where the maximum variation of the first derivative of R(0−, λ)
                                                                                      for TR concentration occurred (Fig. 5c). However, since Hy-
                                                                                      perion spectra show band to band spikes or dips (Cairns et al.,
                                                                                      2003), a selection based on single bands could match some
                                                                                      spikes (Fig. 4). The binning of bands 480–500 nm and of bands
                                                                                      550–560 nm were thus used instead of the two single channels,
                                                                                      centred at 490 nm and 550 nm, respectively.
                                                                                          To implement the MIM algorithm to retrieve CHL-a and TR
                                                                                      concentrations, Eq. (1) (in which Eqs. (2) and (3) were
                                                                                      substituted) was rewritten to a set of 2 equations (for 2 wave-
                                                                                      lengths), where each equation has the form:
                                                                                                                                               
                                                                                                 ⁎           ⁎            1þP
                                                                                                                            l d ðki Þ=P
                                                                                                                                      l u ð ki Þ
                                                                                      ½CHL  a aph ðki Þ þ bbph ðki Þ 1−
                                                                                                                            Rð0−; ki Þ
Fig. 5. Sensitivity analysis for the band selection of the MIM image inversion.
(a): first derivative spectra of R(0−, λ) vs. aCDOM(440), rescaled by a 0.01 factor                                                        
(aCDOM(440) concentration was changed from 0.03 to 0.30 m− 1, the other two                     ⁎ ðk Þ þ b⁎ ðk Þ 1− 1 þ l d ðki Þ= l u ðki Þ
                                                                                                                        P         P
WQPs were kept constant to CHL-a = 1 mg m− 3 and TR = 1 g m− 3); (b): first             þ ½TR aTR  i     bTR i
                                                                                                                        Rð0−; ki Þ
derivative spectra of R(0−, λ) vs. CHL-a (CHL-a concentration was changed
from 1 to 10 mg m− 3, the other two WQPs were kept constant to aCDOM(440)
= 0.09 m− 1 and TR = 1 g m− 3); (c): first derivate spectra of R(0−, λ) vs. TR (TR                                                        
                                                                                                                    1þP
                                                                                                                      l d ðki Þ=P
                                                                                                                                l u ð ki Þ
concentration was changed from 1 to 10 g m− 3, the other two WQPs were kept                 ¼ aw ðki Þ−bbw ðki Þ 1−
constant to aCDOM(440) = 0.09 m− 1 and CHL-a = 1 mg m− 3). The NEΔR(0−,                                               Rð0−; ki Þ
λ)E of Hyperion is overlaid on each of the graphs as dot lines. The box indicates
the wavelength range of Hyperion bands that may be used. Note the different y
axis ranges of each plot.                                                               −aCDOM ð440Þe−SCDOM ðki −440Þ                                 ð7Þ
190                                         C. Giardino et al. / Remote Sensing of Environment 109 (2007) 183–195

where, R(0−, λi) is the Hyperion-derived subsurface irradiance                      scattering waters (i.e., light-blue colours in the pseudo true
reflectance, i is the band used in the inversion and aCDOM(440)                     colour Hyperion image). The two MIM-retrieved Hyperion
is fixed to 0.09 m− 1.                                                              WQP maps describe the chlorophyll-a and tripton concentra-
                                                                                    tions. In these product maps, pixels where bottom depth was
3. Results and discussion                                                           less than 10 m, were masked because the bio-optical model we
                                                                                    used (Eq. (1)) is applicable in optically deep waters only. Both
   Fig. 4 and Table 2 show how subsurface irradiance re-                            maps show ranges of CHL-a and TR between 0 and 5 mg m− 3
flectance values from atmospherically corrected image data, in                      and 0 and 5 g m− 3, respectively. The patterns in TR map seem
situ optical data and bio-optical modelling converge. A                             correlated to the patterns in pseudo true colour Hyperion image:
reasonable optical closure was achieved in the range from 480                       TR concentrations are higher in the light-blue waters, whilst
to 690 nm (average reflectance RMSE 0.006 and relative RMSE                         they are lower in the dark-blue (less-scattering) waters. The fact
13%, Table 2). Below 480 nm Hyperion data were not reliable                         that the CHL-a map is uncorrelated to the TR map indicates
for calibration (Green et al., 2003) while beyond 700 nm                            successful decomposing of the R(0−, λ) signal and independent
Hyperion reflectances did not seem very well corrected for                          assessment of CHL-a and TR concentrations following this
adjacency effects and the optical closure was inferior (average                     method.
reflectance RMSE 0.011 and relative RMSE 66%, Table 2). At                             Validation of CHL-a and TR Hyperion-derived maps was
the two wavelengths (490 and 550 nm) where bands used for                           performed using in situ point stations. Fig. 7 shows the two
MIM were located and a good convergence to the same                                 scatter plots depicting the Hyperion-derived CHL-a and TR
subsurface irradiance reflectance was obtained (the reflectance                     estimations vs. in situ concentrations measured in 8 pelagic
differences were on average less than 0.005, or 7% as relative                      stations (all the stations in Fig. 1, except Station 1 which is
value).                                                                             located in shallow waters where Hyperion data were masked to
   Fig. 6 presents the pseudo true colour Hyperion image and                        avoid bathymetric effects). Hyperion data was averaged on a 3
the two WQP maps retrieved applying the MIM to the image.                           by 3 pixel region of interest centred on the location of in situ
The pseudo true colour Hyperion image qualitatively describes                       sampling stations. The Hyperion-derived CHL-a was in good
the diversity of waters within the lake. The turquoise–cyan                         agreement with in situ point data, showing a correlation
colours in the southern part of the lake are due to bottom effects                  coefficient (r) of 0.77, a determination coefficient (R2) of
of bright substrates. Most of the deep waters are dark-blue but                     0.59, a RMSE of 0.36 mg m− 3 (relative RMSE 20%), a bias of
lighter blue waters come up about the middle of the scene, as                       0.12 mg m− 3 (relative bias 6%), and being close to the 1:1 line
well as in the south, on the western side of the peninsula. On                      (Fig. 7). The RMSE and the relative RMSE were computed with
22nd July 2003, re-suspension of sediments was caused by the                        Eqs. (5) and (6), where N is now equal to 8 (i.e., the number of
strong wind action resulting in variable patterns of more highly                    stations), and X̄ i and X̂ i are the in situ observed and the

Fig. 6. The pseudo true colour Hyperion image (with locations of point in situ stations) and the two MIM-retrieved products obtained from Hyperion data acquired on
22nd July 2003. In the product maps shallow waters are masked.
C. Giardino et al. / Remote Sensing of Environment 109 (2007) 183–195                                                     191

Fig. 7. Scatter plots of Hyperion-derived products and in situ concentrations measured in point stations: on left for chlorophyll-a, on right for tripton. Dot lines indicate
the 1:1 relation. Both graphs do not include data from Stations 1 because it is located in the shallow waters. The statistic in the tripton graph does not include the black
symbol (i.e., Station 5).

Hyperion-derived concentrations of WQPs, respectively. The                               (a). Overall, Hyperion-based estimations are in agreement with
Hyperion-derived TR did not match in situ point data (r = 0.48,                          transect measurements. The first part of the transect (0–4 km of
R2 = 0.23 and RMSE 1.1 g m− 3), due to divergence between                                the length) is not shown since it is not included in the image
Hyperion and in situ point data observed in Station 5. Removing                          footprint. In Section I of the transect (4–6 km of the length),
this measurement from the dataset the regression analysis                                only calibrated transect data are plotted because Hyperion data
performed better (r = 0.75, R2 = 0.57, RMSE 0.55 g m− 3,                                 were masked to avoid bathymetric effects. Section II (6–14 km
relative RMSE 31%, bias 0.27 g m− 3, and relative bias 15%)                              of the length) shows a good agreement in range and spatial
and data become closer to the 1:1 line (Fig. 7). Station 5 is                            behaviour between the Hyperion-derived CHL-a and the
located in the region of the light-blue, more scattering waters.                         transect fluorescence-derived CHL-a data, even if the peak
Due to the strong wind blowing on the day of the image                                   occurrences in concentration are sometimes shifted in phase. In
acquisition the light-blue water pattern was characterised by a                          Section III (14–20 km of the length), when the transect data
high rate of change (both in time and space). The water at                               were acquired almost at the same time of Hyperion data,
Station 5 was probably sampled when these more scattering                                imagery-derived CHL-a match flow-through-derived CHL-a
waters (imaged by Hyperion at 9:50 UTC) had already moved                                data. Unfortunately this is the section where many flow-through
elsewhere (assuming that the 6 m s− 1 north–south direction                              data were missed through filtering of wave-related anomalies.
wind resulted in a 0.06 m s− 1 water current, within 10 min a one                        In Section IV of the transect (20–22 km of the length),
Hyperion pixel displacement may occur).                                                  Hyperion-derived CHL-a values were lower than flow-through-
    The ability to monitor water quality in highly dynamic                               derived concentrations but presented a similar ascending
systems could be hindered by the spatial or temporal density of                          gradient. Section V (22–32 km of the length), shows a rea-
point sampling offered by traditional sampling techniques                                sonable agreement in range and spatial behaviour between
rendering them inappropriate to validate RS-derived products.                            imagery-derived CHL-a and the flow-through-derived CHL-a
Lindfors et al. (2005) suggested that validation of remotely                             transect data with a descending gradient towards the transect
sensed data products and locations of point measurements                                 end. The last kilometre depicts Hyperion data only because the
needed for monitoring work should be based on continuously                               fluorescence measurements were not available.
measured flow-through values. They discussed IOPs, salinity                                 Fig. 8(b) describes the spatial variation for tripton. As for the
and temperature in Lake Vänaren (Sweden) and in the Gulf of                              chlorophyll-a, the first 4 km of the transect were outside the
Finland. In this study, to qualitatively evaluate the spatial                            image footprint. In Section I of the transect, only calibrated
variation of the WQP retrievals, the flow-through calibrated                             transect data are plotted because Hyperion data were masked to
transects of CHL-a and TR were resampled according to the                                avoid bathymetric effects. In Section II, estimations derived
30 m size of Hyperion pixels. Flow-through data were first                               from Hyperion and flow-through transect data are comparable
cleaned of anomalies, e.g., spikes due to bubbles, saturation and                        in values, except for the 3 g m− 3 peak assessed by Hyperion at
lack of data (caused by wave-related difficulties in pumping                             the 7th kilometre. This peak also exists in the transect flow-
water from subsurface into the onboard instrumentation). Fig. 8                          through data but it is located at the 5th kilometre, in Section I.
illustrates the comparison between two indirect estimates of                             Hyperion-derived concentrations appeared shifted in phase with
WQPs: the Hyperion-derived concentrations and the flow-                                  respect to flow-through data. As for Fig. 8(a), Section III is the
through-derived transect in situ data. In Fig. 8, the location of                        region where collections of flow-through data were closer to the
transect in situ data with respect to the time of the Hyperion                           Hyperion overpass. Within this section, both Hyperion-derived
overpass is indicated to show the temporal mismatch between                              TR concentrations and flow-through-derived data show a close
the acquisition of transect in situ data and image data.                                 agreement exhibiting a steep ascending gradient and compara-
    The spatial trend of CHL-a concentrations, derived from                              ble concentration ranges. Unfortunately between the 18th and
Hyperion data and from flow-through data is plotted in Fig. 8                            the 20th kilometres many flow-through data were missed due to
192                                           C. Giardino et al. / Remote Sensing of Environment 109 (2007) 183–195

Fig. 8. Comparison of Hyperion-derived products and in situ concentrations estimated from the flow-through data along the horizontal transect: (a) chlorophyll-a, (b)
tripton. Hyperion and flow-through transect data are extracted from a 30 m per 30 m pixel grid (see text for labels I to IV). The approximate location of transect in situ
data with respect to the time of Hyperion overpass is also indicated.

wave-related anomalies. In Section IV of the transect, flow-                           first part of Section V, lower concentrations in the last part of
through data exhibit a flat trending, contrary to the image data                       Section V. The flow-through system gave the TR concentrations
that presents a peak with two times higher concentrations. This                        (actually geo-coded to Hyperion pixels) 1–1.5 h later with
is the region where the transect track crosses the light-blue                          respect to Hyperion, when the front had probably moved
waters. As observed before, when point in situ data at Station 5                       elsewhere and the distribution of tripton was changed. In
were compared to image-derived TR estimations (Fig. 7), it                             general, Fig. 8(b) shows that Hyperion-derived tripton con-
seems that the light-blue water front, quickly changing its                            centrations were not comparable to flow-through-derived
position in time and space, was not described by in situ                               values, expect for few kilometres in Section III (from 15th to
observations. A re-suspension of tripton due to the strong wind                        19th kilometre) where Hyperion and transect acquisitions match
action, which was synoptically imaged at 9:50 UTC by                                   in time. These results suggest that Hyperion-derived tripton
Hyperion, had been easily missed by in situ observations                               concentrations, in occasion of events subjected to local vari-
collected at about 0.3–0.5 h later. In Section V, both the flow-                       ability in wind, re-suspension and circulation, are difficult to
through data and the Hyperion retrieved TR concentrations                              compare to in situ data due to the incompatibilities of methods
present a descending gradient but with different concentration                         used for tripton assessments. Even fast monitoring methods like
ranges and slopes. Moreover, the peak of 4 g m− 3 of TR                                flow-through measurements are time consuming (∼ 3 h for a 32-
observed by Hyperion (at the 25th kilometre), did not occur in                         km-long transect) and they could become inappropriate to
the transect in situ data. Hyperion synoptically assessed the                          describe natural events with a high rate of change as may occur
tripton distribution at 9.50 UTC: higher concentrations in the                         in wind driven currents in lakes.
C. Giardino et al. / Remote Sensing of Environment 109 (2007) 183–195                                            193

4. Conclusions                                                           may produce large errors in obtaining R(0−, λ) and conse-
                                                                         quently in retrieving concentrations of water quality parameters.
    This work presents a procedure to map CHL-a and TR                      This study aimed to use Hyperion imagery as a bench-mark
concentrations in Lake Garda from hyperspectral satellite data           for moving towards operational use of RS-related technologies
based on forward and inverse bio-optical modelling. The per-             that, integrated with traditional survey programmes, could
formance of the analytical inversion approach was measured by            provide useful information to implement the European WFD.
the optical closure between forward-modelled reflectance, in             Within the WFD it is possible, for each water body, to monitor
situ reflectance and atmospherically corrected Hyperion                  only the water quality elements most sensitive to a certain risk or
reflectance. The closure was sufficient for the purposes of this         pressure. For Lake Garda this could be the deviation from a
study but further investigations on atmospheric adjacency ef-            trophic level assessed with two causal elements (i.e., phospho-
fects, focused on surface reflected vegetation spectra from steep        rous and nitrogen) and with one response parameter, the
slopes, are recommended to obtain a better closure at longer             chlorophyll-a concentration. The Hyperion data processing
wavelengths. The bio-optical model sensitivity analysis indi-            presented in this study will be transferred to the assessment of
cated the optimal bands to run the inversion as well as the              lake water quality (mainly chlorophyll-a) using more operational
inability to detect aCDOM(440) in this study. The matrix in-             instruments (being a part of a technology validation/demonstra-
version method was applied to run the inversion on the spatially         tion mission, Hyperion cannot be considered suitable for a long-
and spectrally convolved Hyperion image. The MIM algorithm               term monitoring). Large swath MODIS and MERIS sensors
provided ranges of CHL-a concentrations comparable to in situ            (both having the spectral bands used by MIM) offer almost-daily
data collected the day of the satellite overpass. Results for            imagery of northern Italy and the method presented could be
tripton were less satisfactory but an improvement was found if           extended to the other large (relative to the spatial resolution of
data from a re-suspension zone were avoided. A further eval-             the remote scanner that is) lakes of the subalpine region, where
uation of image-products was based on high spatial resolution            visibilities’ ranges are provided by airports or Aeronet stations.
transect in situ data: about 32 km (some transect in situ data           An onerous activity needs however to be completed mainly to
were missed because of the wave-related anomalies) of flow-              asses the lakes SIOPs or to evaluate how they differ from the
through-derived measurements of CHL-a and TR were                        Lake Garda ones. To start, Premazzi et al. (2003) discussed that
qualitatively compared to concentrations retrieved from Hyper-           the in the subalpine region composition of the phytoplankton
ion. For chlorophyll-a the Hyperion-derived concentrations               communities would register marked similarities from one lake to
were on average comparable to transect in situ data. The com-            another, as regards density, biomass and species.
parison was more difficult for tripton since some incompatibil-
ities of methods happened. On the day of the Hyperion overpass           Acknowledgements
a strong wind occurred over the lake resulting in re-suspension
of sediment (tripton). Further investigations are therefore nec-            This work is in memory of Eugenio Zilioli who passed away
essary, mainly addressing the compatibilities of methods for             in 2004 and who made a considerable effort in Europe to
monitoring water body features with high rate of wind or wave            establish remote sensing of lakes as a tool for environmental
driven change. Matthews et al. (2001) already observed how               monitoring. Hyperion data were acquired by the Helge Axson
continuous fluorometers towed behind boats may offer an                  Johnsons Foundation, Sweden. IOP data were collected by A.
increased capability to monitor chlorophyll-a with respect to            Lindfors and K. Rasmus at the Dep. of Geophysics, Helsinki
traditional sampling technique in highly dynamic coastal zone            University and Luode Consulting Oy, Helsinki. This study was
but, the linear track estimates, may be themselves inadequate to         funded by the Italian Space Agency (Ninfa Project), and by
describe the wide-scale heterogeneous phenomenon as synop-               ESA and Regione Lombardia with financial support grants to N.
tically retrieved by RS. The results also indicate that fast             Strömbeck and to G. Candiani, respectively. The CNR/CSIRO
processing of hyperspectral images is feasible: once the pre-            Agreement (2004–06 Program) and the Scientific Office at the
processing was done the Hyperion image processing took only              Embassy of Italy in Canberra supported the collaboration
180 s on a standard desktop PC. Another advantage of the                 among our institutes. This work would not be possible without
method is that each good set of in situ AOPs, IOPs and SIOPs             the assistance and contributions provided during these years by
measurements added to the spectral library of the lake will              L. Alberotanza from CNR-ISMAR, G. Zibordi from JRC, and
improve the algorithm performance (thus at a certain moment              by C. Defrancesco and G. Franzini from the Environmental
no further in situ measurements will be required as all source           Protection Agencies of Trento and Veneto, respectively. We are
materials, e.g., inflowing waters, re-suspended material and             grateful to T. Kutser from Estonian Marine Institute for his
algal populations are characterised properly). In such a context,        valuable suggestions. Constructive comments from the anon-
next studies could also benefit from more information about              ymous reviewers, including ones on an earlier version of the
variability in the SIOPs over time and space. The presented              manuscript, were greatly appreciated.
procedure is also transferable to other lakes, for which the
optical characterisation of the water body is known and in-              References
formation about atmospheric properties during the satellite              Ammenberg, P., Flink, P., Lindell, T., Pierson, D., & Strombeck, N. (2002). Bio-
overpass is accessible. In particular, accurate visibility ranges          optical modelling combined with remote sensing to assess water quality.
are required since Keller (2001) observed how incorrect values             International Journal of Remote Sensing, 23, 1621−1638.
194                                              C. Giardino et al. / Remote Sensing of Environment 109 (2007) 183–195

Brando, V. E., & Dekker, A. G. (2003). Satellite hyperspectral remote sensing             Keller, P. A. (2001). Comparison of two inversion techniques of a semi-
    for estimating estuarine and coastal water quality. IEEE Transaction on                   analytical model for the determination of lake water constituents using
    Geoscience and Remote Sensing, 41, 1378−1387.                                             imaging spectrometry data. Science of the Total Environment, 268,
Brivio, P. A., Giardino, C., & Zilioli, E. (2001). Determination of chlorophyll               189−196.
    concentration changes in Lake Garda using an image-based radiative transfer           Kutser, T. (2004). Quantitative detection of chlorophyll in cyanobacterial
    code for Landsat TM images. International Journal of Remote Sensing, 22,                  blooms by satellite remote sensing. Limnology and Oceanography, 49,
    487−502.                                                                                  2179−2189.
Candiani, G., Giardino, C., Strömbeck, N., & Zilioli, E. (2003). Imaging spec-            Kutser, T., Herlevi, A., Kallio, K., & Arst, H. (2001). A hyperspectral model for
    trometry to estimate chlorophyll-a in Lake Garda. 3rd EARSeL Workshop on                  interpretation of passive optical remote sensing data from turbid lakes.
    Imaging Spectroscopy, Herrsching, Germany, 13–16 May (pp. 389−394).                       Science of the Total Environment, 268, 47−58.
Cairns, B., Carlson, B. E., Ruoxian, Ying, Lacis, A. A., & Oinas, V. (2003).              Lee, Z., Carder, K. L., Hawes, S. K., Steward, R. G., Peacock, T. G., & Davis,
    Atmospheric correction and its application to an analysis of Hyperion data.               C. O. (1994). Model for the interpretation of hyperspectral remote-sensing
    IEEE Transaction on Geoscience and Remote Sensing, 41, 1232−1245.                         reflectance. Applied Optics, 33, 5721−5731.
Cracknell, A. P., Newcombe, S. K., Black, A. F., & Kirby, N. E. (2001). The               Lindell, T., Pierson, D., Premazzi, G., & Zilioli, E. (Eds.). (1999). Manual for
    ADMAP (algal bloom detection, monitoring and prediction) concerted                        monitoring European lakes using remote sensing techniquesEUR Report,
    action. International Journal of Remote Sensing, 22, 205−247.                             vol. 18665. Luxembourg: Office for Official Publications of the European
Dall'Olmo, G., & Gitelson, A. A. (2006). Effect of bio-optical parameter                      Communities (EN).
    variability and uncertainties in reflectance measurements on the remote               Lindfors, A., Rasmus, K., & Strömbeck, N. (2005). Point or pointless-quality of
    estimation of chlorophyll-a concentration in turbid productive waters:                    ground data. International Journal of Remote Sensing, 26, 415−423.
    Modeling results. Applied Optics, 45, 3577−3592.                                      Matthews, A. M., Duncan, A. G., & Davison, R. G. (2001). An assessment of
Dekker, A. G., Malthus, T. J., & Hoogenboom, H. J. (1995). The remote sensing                 validation techniques fore estimating chlorophyll-a concentration from
    of inland water quality. In F. M. Danson & S.E. Plummer (Eds.), Advances                  airborne multispectral imagery. International Journal of Remote Sensing,
    in environmental remote sensing (pp. 123−142). John Wiley & Sons Ltd.                     22, 429−447.
Dekker, A. G., Vos, R. J., & Peters, S. W. M. (2001). Comparison of remote                Mobley, C. D. (1994). Light and water — radiative transfer in natural waters San
    sensing data, model results and in situ data for total suspended matter (TSM)             Diego: Academic Press.
    in the southern Frisian lakes. Science of the Total Environment, 268,                 Mobley, C. D., & Sundman, L. K. (2001). HYDROLIGHT 4.2, users’ guide
    197−214.                                                                                  Redmond: Sequoia Scientific.
Dekker, A. G., Vos, R. J., & Peters, S. W. M. (2002). Analytical algorithms for           Morel, A. (1974). Optical properties of pure water and pure seawater. In E.
    lake water TSM estimation for retrospective analyses of TM and SPOT                       Jerlov & Steeman Nielsen (Eds.), Optical aspects of oceanography:
    sensor data. International Journal of Remote Sensing, 23, 15−35.                          Academic.
Dekker, A. G., Brando, V. E., & Anstee, J. M. (2005). Retrospective seagrass              Morel, A. (1988). Optical modeling of the upper ocean in relation to its
    change detection in a shallow coastal tidal Australian lake. Remote Sensing               biogenous matter content (Case I waters). Journal of Geophysical Research,
    of Environment, 97, 415−433.                                                              93, 10749−10768.
Directive 2000/60/EC (2000, December 22). Water Framework Directive of the                Pierson, D. C., & Strömbeck, N. (2001). Estimation of radiance reflectance and
    European Parliament and of the Council of 23 October 2000 establishing a                  the concentrations of optically active substances in Lake Malaren, Sweden,
    framework for community action in the field of water policy. Official                     based on direct and inverse solutions of a simple model. Science of the Total
    Journal L, 327.                                                                           Environment, 268, 171−188.
EEA (1999). Lakes and reservoirs in the EEA area. Topic report No 1/1999                  Pope, R. M., & Fry, E. S. (1997). Absorption spectrum (380–700 nm) of pure
    Copenhagen: European Environment Agency (EEA).                                            water. II. Integrating cavity measurements. Applied Optics, 36, 8710−8723.
Fargion, G. S., & Mueller, J. L. (2000). Ocean optics protocols for satellite ocean       Premazzi, G., Dal Miglio, A., Cardoso, A. C., & Chiaudani, G. (2003). Lake
    color sensor validation, revision 2. NASA/TM-2000–209966, Greenbelt,                      management in Italy: The implications of the water framework directive.
    Maryland, USA.                                                                            Lakes and Reservoirs, Research and Management, 8, 41−59.
Floricioiu, D., & Rott, H., (2005). Atmospheric correction of MERIS data over             Roesler, C. S., & Boss, E. (2003). Spectral beam attenuation coefficient
    perialpine regions, MERIS-(A)ATSR Workshop, Frascati, Italy, 26–30                        retrieved from ocean color inversion. Geophysical Research Letters, 30.
    September 2005, CD-ROM ISBN 92–9092–908–1 ESA.                                            doi:10.1029/2002GL016366
Giardino, C., Candiani, G., & Zilioli, E. (2005). Detecting chlorophyll-a in Lake         Smith, R. C., & Baker, K. S. (1981). Optical properties of the clearest natural
    Garda (Italy) using TOA MERIS radiances. Photogrammetric Engineering                      waters (200–800 nm). Applied Optics, 20, 177−184.
    & Remote Sensing, 71, 1045−1052.                                                      Strömbeck, N., & Pierson, E. (2001). The effects of variability in the inherent
Gons, H. J., Burger-Wiersma, T., Otten, J. H., & Rijkeboer, M. (1992). Coupling               optical properties on estimations of chlorophyll a by remote sensing in
    of phytoplankton and detritus in a shallow, eutrophic lake (Lake Loosdrecht,              Swedish freshwater. Science of the Total Environment, 268, 123−137.
    The Netherlands). Hydrobiologia, 233, 51−59.                                          Strömbeck, N., Candiani, G., Giardino, C., & Zilioli, E., (2003). Water
Gordon, H., Brown, O., Evans, R., Brown, J., Smith, R., Baker, K., et al. (1988).             quality monitoring of Lake Garda using multi-temporal MERIS data.
    A semianalytical radiance model of ocean color. Journal of Geophysical                    MERIS Users Workshop, Frascati, Italy, 10–13 November 2003, CD-ROM
    Research, 93, 10909−10924.                                                                ISBN 92–9092–860–3 ESA.
Green, R. O., Pavri, B. E., & Chrien, T. G. (2003). On-orbit radiometric and              Tassan, S., & Ferrari, G. (1995). An alternative approach to absorption
    spectral characteristics of EO-1 Hyperion derived with an underflight of                  measurements of aquatic particles retained on filters. Limnology and
    AVIRIS and in situ measurements at Salar de Arizaro, Argentina. IEEE                      Oceanography, 40, 1358−1368.
    Transaction on Geoscience and Remote Sensing, 41, 1194−1203.                          Toole, D. A., Siegel, D. A., Menzies, D. W., Neumann, M. J., & Smith, R. C.
Härmä, P., Vepsäläinen, J., Hannonen, T., Pyhälahti, T., Kämäri, J., Kallio, K., et al.       (2000). Remote-sensing reflectance determinations in the coastal ocean
    (2001). Detection of water quality using simulated satellite data and semi-               environment: impact of instrumental characteristics and environmental
    empirical algorithms in Finland. Science of the Total Environment, 268,                   variability. Applied Optics, 39, 456−469.
    107−121.                                                                              Vermote, E., Tanré, D., Deuzé, J. L., Herman, M., & Morcrette, J. J. (1997).
Hoogenboom, H. J., Dekker, A. G., & Althuis, I. J. A. (1998). Simulation of                   Second simulation of satellite signal in the solar spectrum (6S). 6S user
    AVIRIS sensitivity for detecting chlorophyll-a over coastal and inland                    guide (v. 2) and 6S code (v. 4.1), July 1997.
    waters. Remote Sensing of Environment, 65, 333−340.                                   Vollenweider, R. A., & Kerekes, J. J. (1982). Eutrophication of waters:
ISO 10260-E (1992). Water quality measurement of biochemical parameters                       monitoring assessment and control Paris: Organisation for Economic Co-
    spectrophotometric determination of chlorophyll-a concentration Geneva,                   operation and Development (OECD).
    Switzerland: ISO (E).                                                                 Walker, R. E. (1994). Marine light field statistics New York: Wiley.
You can also read