1065 
cycle; the Tr a in the LATE plots was 
low until the late irrigation; and in the 
DRY plots it remained lower than all 
others throughout the harvest cycle. 
Considering Eq. 8, it is apparent 
that this approach for estimation of Tr a 
accounts for both differences in 
vegetation density and stress response 
of the plants. As such, it is informative 
to look at both biomass and CWSI in 
relation to estimates of Tr,. For 
example, Fig.3 shows a pair of data 
from WET and EARLY treatments, 
both of which have similar biomass 
values. The transpiration of the WET 
plot was lower than that of the EARLY 
plot for the first half of the growth cycle 
and higher during the second half 
(Fig.3a); yet the biomass measurements 
taken in the two plots over the growth 
cycle were nearly identical (Fig.3b). 
The differences in Tr between the two 
a 
plots were due to corresponding 
differences in CWSI (Fig.3c). 
4.1.2 Three Continuous Harvest 
Periods Three lysimeters were 
subjected to EARLY treatment during 
the penod from DOY40-90 (a), from 
DOY91-130 (b), and from DOY131- 
154 (c), respectively, during three 
harvest periods from DOY40 to 151. 
The other two lysimeters were treated 
as WET plots in each growth cycle. 
Results here are based on the spectral 
and meteorological measurements 
centered on 10:30 each day. The value 
of a in Eq.8 was assumed to be 1.5, 
based on the analysis in section 4.1.1. 
The r a value was computed with the 
simple formula (Thom and Oliver 
1977), accounting for variations in 
plant height and windspeed. Based on 
this value of a and the CWSI 
computed using Eq.9, Tr values were 
calculated for the WET, EARLY, 
LATE, and DRY plots, and each 
lysimeter plot (a, b, and c) regardless 
of treatment. 
The values of Tr a for the three 
lysimeter plots correspond as expected 
with the ET a values measured by 
lysimeters in each plot (Fig.4). That 
is, Tr a was substantially lower than ET a 
when vegetation density was low (after 
harvest), and Tr a was nearly equal to 
ET a for full-cover canopy. 
Day of year 1985 
Fig. 1 Comparison of estimated Tr a and measured ET a for 
the EARLY and WET alfalfa treatment plots. Symbols ▼ 
with plots' initials indicate the dates of irrigation in each plot. 
Fig.2 Comparison of estimated Tr a values for the two 
replicates of WET, EARLY, LATE and DRY alfalfa treatment 
plots. Symbols ▼ with plots' initials indicate the dates of 
irrigation in each plot.