Symposium on Remote Sensing for Resources Development and Environmental Management / Enschede / August 1986
An evaluation of different green vegetation indices
for wheat yield forecasting
A.Giovacchini
Aquater S.p.A., San Lorenzo in Campo, Pesaro, Italy
ABSTRACT: In this study different spectral vegetation indices have been compared in order to select the more
efficient index to forecast the final grain yield of the winter wheat. The study pointed out that at specific
phenological stages, the maximum correlation between vegetation index values and the final grain yield can be
obtained using indices belonging to different families. The maximum correlation value is yielded at the
heading phase adopting vegetation indices based on the ratio concept.
1 INTRODUCTION
Crop production forecasting, if carried out with a
sufficient reliability, assumes great economic and
strategic interest. Remote Sensing technology could
contribute significantly to increase forecasting
reliability both through a better estimation of the
cultivated surface and by directly or indirectly
measuring the biophysical parameters influencing the
crop yield.
Generally the relationships between crop and
remote sensing data have been analyzed using
different types of vegetation indices. These
spectral vegetation indices derive from the
combination of one more spectral bands referring to
the visible and near-infrared parts of the
electromagnetic.
Extensive scientific literature exists on the
relationships between biological crop characteri
stics and vegetation index values. Some authors
relate spectral vegetation indices to wheat dry-
matter accumulation (Tucker. G.J. et al. 1981;
Aasc.J.K. et al. 1981). A strong correlation between
spectral vegetation index values and final grain
yield of wheat and corn crops, has been proven for
specific phenological stages (Tucker G.J. et al.
1980; Daughtry C.S.T. et al. 1984). Some authors
have analyzed the possibility to estimate the leaf
area index of crops by using different spectral
vegetation indices (Goel. N.S. 1984; Badhwar C.D.
1984).
This study describes the relationships between the
final grain yield of the winter wheat and some
vegetation indices at several phenological stages.
The objective of this analysis is two fold:
- to verify if a unique vegetation index can be
used to forecast the final grain yield of the winter
wheat;
- to select the most appropriate vegetation index
in relation to the different phenological stages of
the crop. 2
2 MATERIAL AND METHODS
The experiment was carried out taking into account
30 fields of durum wheat (cultivar Creso). The crop
fields did not belong to a specific experimental
design, but were part of an observational study
carried out on field crops located in the Cesano
Valley (Central Italy).
During the growing season of the crops, seven
measurement surveys were carried out from March to
July. By using an Exotech radiometre (mod 100),
spectral data was collected in the four bands of the
MSS Landsat sensor. Each wheat field had been
previously stratified on the basis of the vegetation
cover density by using color aerialphotographs at
1:5.000 scale. Three radiometric measurements were
carried out in each strata placing the instrument at
3 m above the ground. In order to collect more
representative spectral data, the radiometer was
moved four times in each plot to obtain data
referring to sample units of 2x2 square meters. The
radiometer was calibrated on a standard panel before
collection of the spectral data. At the same time,
the following data referring to biophysical plant
characteristics was collected: plant height, humid
and dry biomass, % of weeds, surficial soil
2
moisture, n° of stalks per m ,• vegetation condition,
% of green vegetation cover. In addition, at harvest
the grain yield of each wheat field was assessed.
3 RESULTS AND DISCUSSION
For each measurement campaign spectral data was
processed to calculate the vegetation indices
reported on Tab. 1. For each vegetation index, the
table reports the spectral bands used indicating the
correspondent MSS Landsat bands.
Both for each vegetation index and collection
date, corresponding to a specific phenological
stage, the coefficient of correlation between
vegetation index values and final grain yield, were
calculated. Tab. II shows these coefficients of
correlation.
As shown in Tab. II, all the vegetation indices
yield the maximum correlation values at the heading
stage. At this biophysical phase the indices based
on the ratio concept yield the maximum correlation,
as well as at the booting stage.
At the first and second phenological stages
considered in this study, the indices yielding the
