other production inputs) (Wiegand and 
Richardson, 1984, 1987, 1990a). Thus 
SCA recognises photosynthesis as the 
fundamental plant process and that 
commercial producers are constrained by 
natural climate and soil 
characteristics as well as to those 
production inputs that are cost 
effective. 
Internal consistency of the equations 
is enhanced by the facts that FPAR, VI, 
and yield all have a functionally 
similar dependence on L (Wiegand and 
Richardson, 1984, Sellers, 1985). In 
addition, appropriate VI are 
insensitive at high L, the same way 
yield responds as L increases: yield 
of most crops increases rather steadily 
until a seasonal maximum L of about 4 
is reached (Fig. 2c) then approaches a 
limiting value asymptotically by the 
time a L of approximately 6 is 
reached. Yields of nonforage crops may 
even decrease at very high L because of 
lodging, competition for light, 
increased susceptibility to foliar 
diseases, and difficulties in getting 
total canopy coverage with 
insecticides. 
There will be cases where SCA will 
fail, notably when insects or diseases 
destroy the plant parts that constitute 
economic yield and the canopies remain 
vegetative. Commercial producers are 
watchful to prevent such occurrences. 
Economic yield is usually limited by 
apparent and insidious multiple factors 
that jointly control canopy size. For 
example, upland soils that receive high 
precipitation are usually lateritie or 
podzolic, nutrient-poor, very acid, and 
have a structure that impedes root 
penetration and proliferation; the 
natural climax vegetation is trees, and 
large canopies of economically 
important crops are obtained only with 
fertilizer and other production 
inputs. Even in tropical rain forests, 
leaf and stem growth is limited by 
solar radiation, nutrients, above 
optimum temperature, insects, or 
diseases. In desert areas irrigation 
relieves the water limitation and solar 
radiation may be high but soil 
salinity, nematodes, saturation 
deficits of the air, hot winds, and 
other stresses limit canopy size and 
yield. At northern latitudes cold 
temperatures at both its beginning and 
end limit the growing season and the 
canopy size that can be attained. SCA 
accomodates these diverse situations by 
letting the VI measure the 
photosynthetic size of the canopies 
actually attained. 
DISCUSSION 
Only the left side of Eqs. [2] and [3] 
need to be implemented to monitor crop 
conditions and estimate yields. Cloud 
171 
occurrence limits usable satellite 
coverages per growing season for many 
production areas (except for NOAA-AVHRR 
observations which are processed to 
save cloud-free data of daily 
overpasses). For such areas, Eq. [2] 
is the one that can most readily be 
implemented for yield estimation. For 
this purpose estimates will be best if 
the satellite observations occur when 
the canopies are well developed and the 
sinks for the assimilates of 
photosynthesis at the time of 
observation are the plant organs that 
constitute yield. 
Crop conditions can be monitored 
spectrally as soon as the plant 
canopies can be distinguished from the 
soil background. Relative vigor among 
fields of interest is revealed by the 
canopies because differences in growth 
attributable to differences in quality 
of seedbeds, carry-over fertility, and 
other factors that are hard to identify 
specifically start affecting vegetative 
development early in the growing 
season. Relative growing conditions 
among years can be interpreted provided 
the user is familiar enough with crops 
in the area to know how the plant 
canopies of interest normally appear on 
the date the data were acquired. 
Similarly, a smaller than normal 
increase in the VI between successive 
dates of observation during vegetative 
development quantifies the canopy 
response to a period of low 
temperature, deficit moisture, or a 
combination of factors that weather 
records and the spectral imagery itself 
may help verify. 
The yield as a function of vegetation 
indices calibration of Eq. [2] can be 
developed from paired yield and VI for 
a sampling of fields that includes both 
good and poor growing conditions. When 
nonirrigated and irrigated fields both 
occur in the same production area, data 
from both help speed the calibration 
(Wiegand and Richardson, 1984). 
Eq. [3] can be applied using satellite 
scenes in areas where clouds are not a 
problem, or anywhere with handheld or 
aircraft-mounted sensors under the 
control of the user organization that 
can be deployed on clear or partially 
cloudy days. LANDSAT TM or SPOT HRV 
data are appealing for worldwide 
applications because the same sensors 
can be used everywhere and their 
calibrations are known and published, 
e.g. Price (1987). The Eq. [1] 
calibrations, crop by crop, should also 
apply anywhere in the world when they 
are planted in similar configurations 
and observed at about the same local 
solar time. Variations in background 
soils are largely elimimated by 
vegetation indices referenced to the 
soil line (PVI, Richardson and Wiegand, 
1977) or the plane of soils (GVI, Kauth