1177 
at full closure of the crop; G is the dry soil characteristic with roughness information incorporated; D represents 
the extinction by the canopy layer. B is the sensitivity of backscatter to soil moisture. Under the assumptions 
given above, the Cloud model equation can be inverted and rewritten as: 
LAI = -cos0/D' * ln((y-C)/-K) (4) 
Similarly as with the CLAIR model we find an exponential relationship between remote sensing measurement 
and LAI. Again, the accuracy of LAI estimation will decrease with increasing LAI value. Since we are dealing 
again with a non-linear function, a first order approximation of the standard deviation of the LAI can be derived 
as: 
o[LAI] = cos9/(K.D') * exp(D'*LAI/cos9) * 0(7) (5) 
Because it was shown before that optical remote sensing data can be used for estimating LAI with quite a 
high accuracy, ground measurements of LAI can be replaced by LAI estimates from optical remote sensing data. 
Since no LAI measurements were performed during MAC Europe 1991 in Flevoland, (optical) AVIRIS data were 
used for calibrating the Cloud model. An intersection was made of all sugar beet fields in the AVIRIS image 
with all sugar beet fields in the AIRSAR image of the beginning of July 1991. Data extraction resulted in a total 
of 37 sugar beet fields for 2 polarizations (HH,VV) and 3 frequencies of the AIRSAR (C-, L- and P-band; resp. 
5.3 GHz, 1.3 GHz and 0.3 GHz). Calibration results showed that L-Band HH and C-Band VV-polarization were 
useful to invert. They represent also the configuration of the recently launched radar satellites ERS-1 and JERS-1. 
To calibrate the Cloud model for L-band HH and for C-band VV, a random calibration set of 20 fields was selected 
from the available fields (table 1). Figure 2 plots the estimated LAI using the Cloud model against the "measured" 
LAI (from AVIRIS) for the calibration set of MAC Europe. In addition, the lines exhibiting deviations + /- two 
standard deviations from the "measured" LAI are shown. 
Table 1. Calibration results of the Cloud model in equation (4) for sugar beet using data from MAC Europe 1991 
on the Flevoland test site. 
L-band HH 
C-band VV 
D' parameter 
0.8967 
0.3660 
C parameter 
0.1369 
0.6821 
K parameter 
0.1767 
0.4394 
R-square 
0.6250 
0.6665 
residual mean square 
0.000505 
0.003012 
0(7] 
0.022 
0.055 
(a) (b) 
Figure 2. Relationship between estimated LAI using the Cloud model and measured LAI for sugar beet in L-band 
HH-polarization (a) and C-band W-polarization (b). Flevoland test site, MAC Europe 1991 campaign.