173 
by NASA, and grain yield samples were 
obtained prior to harvest in irrigated 
and dryland (rainfed) production areas 
in Hidalgo County, Texas. The data set 
comprised three weather years, several 
cultivars, various soils, and differing 
seeding rates and planting 
configurations. The highly significant 
functional relations demonstate that 
plant canopies, observable remotely, 
provide information on crop condition 
and portend yields under field 
conditions. It is the correspondence 
between canopy development and crop 
performance that makes the spectral 
observations generally applicable. 
Yields from Cumulative Seasonal VI 
The left sides of Eqs. [1] and [2] 
expressed as daily incremented 
cumulations, designated by £, can be 
coupled through growth analysis (Warren 
Wilson, 1981) to estimate yield (Y) 
from: 
Y ( ZVI) = zAPAR( ZVI ) x ADM ( ZAPAR )xY ( ADM), [3] 
The right hand side terms in Eq. [3] do 
not need to be known or observed to use 
the left hand side in applying remote 
observation capability; the right hand 
side terms provide the agronomic and 
physiological explanation of why the 
left side relation exists and is 
meaningful. 
Equation [3] provides the rationale for 
expecting yields (Y) to be estimable 
from cumulative daily vegetation 
indices (ZVI), that is, from the area 
under the seasonal VI versus time 
curves. In Eq. [3], APAR from Eq. [0] 
cumulated over the season has the units 
MJ/m 2 . By its very nature DM is 
integral growth so is expressed as the 
dry matter change, ADM, (g/m 2 ). 
The cumulations of all terms in Eq. [3] 
begin logically at seedling emergence 
so that the cumulations for all 
variables begin either at zero or the 
value for bare soil. At seedling 
emergence DM, DMi is so small that it 
is usually ignored. If economic yield 
(Y) of nonforage crops is of prime 
interest the ending date DM, DM2, 
corresponds to either physiological 
maturity or to harvest, forcing all 
other terms to be evaluated at this end 
time. 
Gallo et al. (1985) and Wiegand and 
Richardson (1987) have demonstrated 
that FPAR can be estimated almost as 
well from VI as from L. Consequently, 
because APAR is the energy available 
for photosynthesis and VI is a measure 
of the photosynthetic size of the 
canopy, ZAPAR and ZVI must be closely 
related. That is, we can anticipate 
that ZVI will relate functionally to 
and provide a good estimate of yield 
since ZAPAR does. We have termed the 
slope of the ZAPAR(ZVI) (the first 
right side term) relation the 
efficiency of absorption (ea, 
MJ/m 2 /VI unit). The relation should 
be stable for a given production area 
across years because the factors that 
control potential VI (temperature, 
insolation, soil properties, management 
practices, cultivars) change little 
year to year whereas stresses that 
determine actual VI (precipitation, 
diseases, poor seedbeds, carryover 
toxic residues, early season weather) 
are growing season dependent. 
The second right side term, ADM(ZAPAR) 
is often approximately linear for much 
of the season for a given planting 
(Monteith, 1977) and its slope is the 
efficiency of conversion of 
photosynthetically active radiation to 
dry mass, (ec, g/MJ). The results 
and discussion of Sinclair and Horie 
(1989) indicate that ec can be 
expected to vary among crops and be 
moderately site dependent within crops 
due to variation in adapted cultivars 
among sites, leaf nitrogen contents, 
and climatic variables such as air 
temperature. Within production areas 
the efficiency of conversion is stable 
over a moderate range in crop stresses 
because DM and APAR tend to respond 
alike to stresses. For example, when 
the canopy display is reduced by events 
as diverse as grazing by cattle, 
necrosis from foliar disease, hail 
damage, or wilting from a root zone 
water deficit, the common effect is a 
reduction in APAR. The reduction in 
APAR reduces the supply of assimilates 
of photosynthesis and consequently 
decreases ADM (growth) proportionally. 
For the full growing season, emergence 
to harvest, the third term in Eq. [3], 
Y(DM2), is by definition, the harvest 
index (HI). The harvest index is 
approximately 0.5 for starchy endosperm 
grain crops (maize, temperate cereals, 
grain sorghum, e.g.) when adapted 
cultivars and recommended agronomic 
practices are followed unless extreme 
stress such as foliar disease or 
drought truncates grain filling. 
However, HI is climate dependent; e.g., 
Howell (1990) reported HI of winter 
wheat (Triticum aestivum L.) and grain 
sorghum at Bushland, TX (lat. 35.2. N, 
long. 102.2. W) from experiments 
conducted over several growing seasons 
were 0.35 and 0.47, respectively, and 
"largely unaffected by fertility, water 
use, row spacing, many other cultural 
practices like tillage and profile 
modification, and growing season 
environment". 
The slope of the Y(ZVI) relation is 
termed the yield efficiency (ey,