×

You are using an outdated browser that does not fully support the intranda viewer.
As a result, some pages may not be displayed correctly.

We recommend you use one of the following browsers:

Full text

Title
Mesures physiques et signatures en télédétection

1067
eddy-correlation measurements on site. The a estimate for WG was 1.47,which was close to the a value for
the alfalfa crop. For most of the growing season, the test site was characterized by large expanses of hot, dry
soil with sparse vegetation; thus, an equation accounting for the additional resistance induced by the exposed
soil as a function of windspeed and surface temperature (Kustas et al. 1989) was introduced for the estimation
ofr a
Two different estimates of
aerodynamic resistance r were used for
J 3
the calculation of these parameters. It
was apparent that the water stress was
overestimated using r without excess
resistance and was more reasonable when
the excess resistance was included.
Values of Tr , Tr and ET for all
a p a
measurement days at the site were
compared (Fig.5). As expected, the Tr a
values were lower than ET values in
a
nearly all cases. The Tr p values were
also lower than ET a , especially in the
early growing season with lower
vegetation amount. However, it is
difficult to make any conclusions about
the feasibility of the application to the
rangeland site due to the questionable
methods of deriving a . It may be
possible to derive a in a more justifiable
manner using simultaneous
measurements of fAPAR, albedo and
spectral reflectances made by Daughtry
during 1990 at WG'92 (personal
communication, 1993)
4.3 Conclusion
A new concept was proposed for evaluating the potential and actual daily transpiration from vegetation based
on remote sensing. On the basis of the initial results presented here and the technique's sound theoretical
foundation, it is likely that this approach will be successful for evaluating potential and actual daily transpiration.
Several points require further investigation. 1) Behavior of the coefficient a will have to be examined in
detail from experimental and theoretical points of view, because it may not always be consistent for various
crops and sites. This a value is, by definition, determined as a function of full-cover reflectance and the slope
of the fAPAR and SAVI relation as long as the proportionality between the absorbed solar radiation and the
"potential" transpiration is assumed on a daily basis. From a practical point of view, nevertheless, both the
full-cover reflectance and the slope are not expected to vary much for most crop canopies, and that is why a
values for alfalfa and rangeland were similar (1.5 and 1.47, respectively). 2) Several questions related to the
estimation of CWSI values for sparsely-vegetated sites or immature crops still need to be answered; a) Which
formulation should be used to estimate r a in computation of ET p and CWSI ?, b) How can we calculate a CWSI
value using t (composite surface temperature) rather than t £ (plant canopy temperature) ?, or how can we derive
t £ from t s ? It is possible that the new stress index devised by Moran et al. (1993) may provide a good solution
for such questions. Also, high-resolution infrared thermal imagery will be useful, providing pixel-by-pixel
temperatures over the area of soil and plant mixture (Inoue 1990). We will be able to replace the present CS WI
by future improved stress indices. 3) The estimates of Tr p and Tr a values should be compared with "true" values
directly measured using the stem flow gauge. Since measurements of Tr a were not available in both experiments,
instead, we used ET values in the well-watered full-cover canopies. Therefore, these results represent simply a
demonstration, not a verification, of the approach. New experiments in which transpiration will be measured
directly with stem-flow gauges will be conducted for further examination of the concept.
The potential and actual values of daily transpiration rate may be estimated over wide vegetation areas
with simple remote sensing measurements and basic meteorological parameters. The method may also provide
information on biomass and stress status, because it accounts for the differences in both vegetation density and
stress response of the plant canopy
Day of year 1992
Fig.5 Comparison of estimated values of potential and actual
daily transpiration (Tr and Tr a ) with measured values of
ET at Walnut Gulch site.
a