281 
180 
Fig. 3. The relationship between Chi concentration and the sum 
of reflectance (SUM), calculated as the area limited by the peak 
at 690 nm and a base line positioned from 670 throughout 750 nm. 
The line represents eq. 4. 
To a certain degree algorithms (3) and (4) avoided the 
problems of variation in peak position and shape. Both accounted 
not only for Chi fluorescence, but also for the Chi absorption at 
670 nm, and allowed the enhancement of sensitivity of the 
measurements. SM caused changes in the slope of the spectral 
reflectance measured from 600 to 750 nm. This tilts the base 
line for fluorescence, but does not prevent acceptably accurate 
determination of the line height and sum of reflectance 
above the base line. 
High Chi concentration period . To develop a model relating 
remotely sensed data to Chi concentration, unique spectral 
features are required. The red maximum, near 700 nm, is the only 
feature expedient for our goal. It is mandatory to include in 
the index for Chi determination the reflectance which indicates 
the minimal sensitivity to variation in Chi concentration 
(Gitelson at al., 1993). We found it near 670 nm (Fig. lb). 
Consequently the index of maximal to minimal sensitivity to Chi 
concentration was constructed. This index is in form 
R(max)/R(670), where R(max) is magnitude of the peak, and was 
compared to measured in-situ Chi. Linear regression was computed 
for the March set with r -0.95 (Fig. 4a): 
Chi = -32.35 + (43.08+1.6)*R(max)/R(670), mg.m -3 (5) 
Because of the semi-empirical nature of the model, it had to 
be validated by an independent data set. We performed validation 
by applying Eq. 5 to calculate the Chi concentration using 
reflectance measurements obtained in April. The resulting 
predicted values were compared to the in-situ measured Chi in 
Agril. The correlation between predicted and measured sets was 
r =0.96, andan estimation error of Chi concentration of less 
than 10.9 mg.m 3 was achieved. 
Another approach for Chi estimation was the calculation of 
the reflectance height at the wavelength corresponding to peak 
maximum above the base line, spanned between 670 and 850 nm. The 
reflectance in this region of spectrum depends almost entirely on 
Chi absorption and scattering (Gitelson, 1992 and 1993; Dekker,