In: Wagner W., Székely, B. (eds.): ISPRS TC VII Symposium - 100 Years ISPRS, Vienna, Austria, July 5-7, 2010, IAPRS, Vol. XXXVIII, Part 7B 
(SWIR) part of the spectrum. Figure 1 also shows two water 
absorption features at approximately 970 nm and 1200 nm that 
are caused by the absorption by O-H bonds in liquid canopy 
water (Curran, 1989). Accurate measurements at these 
absorption features in the NIR are feasible with the increasing 
availability of hyperspectral images (Schaepman et al., 2009). 
Danson et al. (1992) showed that the first derivative of the 
reflectance spectrum corresponding to the slopes of the 
absorption features provides better correlations with leaf water 
content than those obtained from the direct correlation with 
reflectance. Rollin and Milton (1998) found moderate 
correlations between the first derivative at the left slope of both 
absorption features and CWC for a grassland site in the UK. 
devers et al. (2008) applied derivatives in a preliminary study 
at the field and airborne level. These studies showed that 
spectral derivatives at the slopes of the 970 nm and (to a lesser 
extent) 1200 nm absorption feature have good potential as 
predictors of CWC. 
Recently, devers et al. (2010) showed that the first derivative 
of the reflectance spectrum at wavelengths corresponding to the 
left slope of the minor water absorption band at 970 nm was 
highly correlated with CWC. PROSAIL model simulations 
showed that it was insensitive to differences in leaf and canopy 
structure, soil background and illumination and observation 
geometry. However, these wavelengths are located close to a 
water vapour absorption band at about 940 nm (Gao and Goetz, 
1990). In order to avoid interference with absorption by 
atmospheric water vapour, the potential of estimating CWC 
using the first derivative at the right slope of the 970 nm 
absorption feature is studied in this paper for a dataset acquired 
in 2008. Results are compared with PROSAIL simulations, 
using a new version of the PROSPECT model (Feret et al., 
2008). 
2. MATERIAL AND METHODS 
2.1 PROSAIL Radiative Transfer Model 
PROSAIL is a combination of the PROSPECT leaf RT model 
(Jacquemoud and Baret, 1990) and the SAIL canopy RT model 
(Verhoef, 1984), which has been used extensively over the past 
few years for a variety of applications (Jacquemoud et al., 
2009). At the leaf level, PROSAIL is using leaf chlorophyll 
content (C ab ), equivalent leaf water thickness (EWT), leaf 
structure parameter (N) and leaf dry matter (Cm) as inputs. At 
the canopy level, input parameters are LAI, leaf inclination 
angle distribution, soil brightness, ratio diffuse/direct 
irradiation, solar zenith angle, view zenith angle and sun-view 
azimuth angle. It also includes a parameter describing the hot 
spot effect (Kuusk, 1991). In a previous study, we used an older 
version of PROSPECT (version 3) simulating leaf reflectance 
and transmittance at a 5 nm spectral sampling interval. 
Recently, version 5 of PROSPECT has been released, 
performing simulations at a 1 nm spectral sampling interval and 
using updated values for the specific absorption coefficients of 
leaf constituents (Feret et al., 2008). 
To study the relationship between derivatives and CWC 
(calculated from LAI and EWT), the effects of the main leaf and 
plant inputs on this relationship were studied. C ab could be kept 
constant since it does not exhibit any effect beyond 800 nm. 
Since the specific absorption coefficient for dry matter is quite 
low and constant below 1300 nm (Fourty et al., 1996), a 
constant value for C m was used according to the findings of 
Jung et al. (2009) for a floodplain meadow. At the canopy level, 
the actual observation and solar angles of the experimental 
measurements (section 2.3) were used. Also spectral soil 
brightness values were obtained from the actual experiments. 
The other inputs for the PROSAIL simulations were varied 
according to the values given in Table 1. 
Since the absorption features of leaf constituents are 
implemented in the PROSAIL model by means of look-up 
tables and not as continuous functions, simulated spectra have 
to be smoothed for calculating derivatives. The simulated 
spectra were smoothed using an 8 nm wide moving Savitsky- 
Golay filter applying a fourth-degree polynomial fit within the 
window according to the results of Le Maire et al. (2004). 
Table 1. Nominal values and range of parameters used for the 
canopy simulations with the PROSAIL model. 
PROSAIL parameters 
Nominal values and range 
Equivalent water thickness 
0.01 - 0.10 g.cm' 2 (step of 
(EWT) 
0.01) 
Leaf dry matter (Cm) 
0.002 g.cm' 21 
Leaf structure parameter (N) 
1.0/1.8/2.5 
Chlorophyll concentration 
(Cab) 
40 pg.cm' 2 
Leaf area index 
0.5/ 1.0/ 1.5/2/3/4/5/6 
Leaf angle distribution 
Spherical / Planophile / 
Erectophile 
Hot-spot parameter 
0.0/0.1 
Soil reflectance 
Actual values 
Diffuse/direct radiation 
0 
Solar zenith angle 
35° 
View zenith angle 
0° 
Sun-view azimuth angle 
0° 
f Source: (Jung et al., 2009) 
2.2 Study Site 
The study site is an extensively grazed fen meadow acting as a 
buffer zone around a protected bog ecosystem, located in the 
Achterhoek area in the Netherlands and forming part of 
Europe’s Natura-2000 ecological network. Ground sampling 
took place from June 9 th - 11 th , 2008. 40 Plots of 3 by 3 m were 
randomly distributed over the site. In three comers of each plot 
subplots of 0.5 x 0.5 m were harvested by cutting all above 
ground vegetation. Vegetation fresh weight for every subplot 
was determined after harvesting. After drying for 24 hours at 
70°C, vegetation dry weight and CWC were determined. 
Subsequently, the average CWC per plot was calculated. 
2.3 Field Spectroradiometry 
The study site was measured with an ASD FieldSpec Pro FR 
spectroradiometer on June 9 th and 10 th , 2008. Nadir 
measurements were performed between 1 lh and 15h local time, 
resulting in a solar zenith angle varying between 30° and 40°. 
All subplots of all 40 plots were measured before harvesting the 
biomass. Measurement height above the plot was about 1.5 m 
and the instrument field of view was 25°. As a result, at the plot 
level a circular area of about 0.35 m 2 was measured and each 
measurement represents the average of 50 readings at the same 
spot. The sampling interval was 1 nm. Calibration was done by 
using a Spectralon white reference panel. 
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