275
As can be observed from figure 4, as LAI increases (and chlorophyll concentration increases as leaves
mature) , 7 re moved to longer wavelengths, and then reverted as senescence began. The A. re also presents a
tendency to reach a plateau but this temporary saturation, which disappears with the subsequent blue-shift of the
maximum inflection, remains for a longer period of time than for the NDVI. That is, the /Ge seems to present a
major inertia to recover the values corresponding to the first stage of the cycle. Therefore, the hysteresis is not as
evident as before. The mathematical coefficients obtained for the fits, according to model shown in expression (3)
(Y referring to >^ e ). are shown in table 3.
TABLE 3
Coefficients of the adjustment between Àr e and LAI data according to the model shown in expression (3).
pre-Lmax data
post-Lmax data
A
742
743
B
-52.0
-3051
C
0.99
1.98
r
0.99
0.97
Nevertheless, as can be observed from figure 5, the behaviour of the iGe as a function of the biomass for
the first stage of the cycle is almost similar than that shown by the NDVI. A high degree of correlation can be
observed again. Once more, the logarithmic model, as shown in expression 4 (Y referring to brc), has resulted to
be the better way to adjust the points (only for the pre-L max data since after this biomass remains more or less
constant). The parameters of the adjustment are: A=674, B=22.4, obtained with a correlation coefficient equal to
0.99.
4 - COMMENTS
The NDVI as well as the red-edge position were found to be useful to describe phenological events for a corn
canopy, since they reach significant correlations with biophysical parameters such as LAI and biomass.
Although data shown in this work are still under keen analysis, from these preliminary results and for
this kind of applications, the high resolution data do not seem to show any avaluable advantage in comparison
with those derived from the broad band analysis. Therefore, the NDVI can be still recommended as an appropriate
spectral parameter for monitoring crop growth, for its simple calculation and for the kidness of the results.
ACKNOWLEDGEMENTS
This work has been carried out within the context of the EFEDA field experiment, supported by the
Commission of the European Communities (Project EPOC-CT90-0030(LNBE)). The authors would like to
express their appreciation to Dr. M.T. Younis for his significant contribution to the field measurements
activities. Special thanks are given to the University of Castilla La Mancha EFEDA Group that kindly provided
tha LAI and biomass values.
REFERENCES
Anderson, G.L., and Hanson, J.D. (1992), Evaluating hand-held radiometer derived vegetation indices for
estimating above ground biomass, Geocarto International (1) 1992: 71-78.
Ashcroft, P.M., Catt. J.A., Curran, P.J., Munden, J., and Webster, R. (1990), The relation between reflected
radiation and yield on the Broadbalk winter wheat experiment, Int. J. Remote Sens. 11(10): 1821-1836.
Baret, F., Guyot, G., and Major, D.J. (1989), Crop biomass evaluation using radiometric measurements,
Photogrammetria (PRS), 43: 241-256.
Baret, F., Jacquemoud, S., and Guyot, G. (1992), Modeled analysis of the biophysical nature of spectral shifts
and comparison with information content of broad bands, Remote Sens. Environ. 41: 133-142.
Danson, F.M., Steven, M.D., Malthus, T.J. and Clark, J.A. (1992), High-spectral resolution data for
determining leaf water content, Int. J. Remote Sens. 13(3): 461-470.
Daughtry, C.S.T., Gallo, K.P., Goward, S.N., Prince, S.D., and Kustas, W.P. (1992), Spectral estimates of
absorbed radiation and phytomass production in com and soybean canopies, Remote Sens. Environ. 39: 141-
152.
Demetriades-Shah, T.H., Steven, M.D., and Clark, J.A. (1990), High resolution derivative spectra in remote
sensing, Remote Sens. Environ. 33: 55-64.
Gemmell, F.M., and Colls, J.J. (1992), The effects of sulphur dioxide on the spectral characteristics of leaves of
viciafaba L., Int. J. Remote Sens. 13(14): 2547-2563.
