5.2.4 CGMS and remote sensing derived crop transpiration
for wheat
To be able to make a visual interpretation, the remote sensing
results and the CGMS results have been integrated in one graph
(Figure 5) The remote sensing results are displayed as a
histogram. The CGMS model simulations have been marked with
arrows, pointing at a transpiration rate which was calculated by
CGMS for a particular simulation unit. Because two simulation
units can give the same transpiration rate, multiple arrows should
be placed at certain transpiration rates. This has been indicated by
a number above each arrow.
One must keep in mind that CGMS generate data about crop
transpiration, while the remote sensing approach results in a crop
evapotranspiration rate. Meteorological data show that there has
not been any precipitation during the two weeks before each
Landsat TM overpass and also the crop development stage is such
that soil evaporation might be neglected.
For wheat growing under rainfed conditions at the 25th of April
(Figure 5A) the graph shows a large deviation between modelled
and measured values. According to CGMS transpiration for
rainfed wheat is potential for most simulation units. This is
indicated by the cluster of arrows at the right hand side of the
graph. A few simulation units with poor soil physical properties
and/or shallow rooting depth show very low transpiration rates
which are indicated by the small cluster of arrows at the left hand
side of the graph.
This graph clearly shows that CGMS results deviate considerably
from the remote sensing measured values. The simulation results
show much more extreme values. The CGMS simulated crop
transpiration for irrigated wheat (Figure 5B) is also relatively
high. So under irrigated conditions, the potential transpiration rate
Daily ET for rainfed wheat
Seville 25/04/92
7%
6%
5%
3%
Percent pixels
2%
0%
0 0.5 1 1.5 2 2.5 3 3 4 4.5
Evapotranspiration mm/day
Daily ET for rainfed wheat
Seville, 11/05/92
9%
8%
7%
6%
5%
4%
Percent pixels
3%
0%
0 5 1 1
Evapotranspiration (mm/day)
as simulated by CGMS do not agree very well with the remote
sensing results. Also note that the difference between irrigated
and rainfed wheat is, according to the remote sensing results, in
fact very small, only 0.5 mm.
At the 11th of May the situation has changed dramatically,
according to CGMS. The crops are now experiencing serious
reductions in transpiration due to water restrictions and for every
simulation unit a reduced transpiration is calculated of 1 to 2 mm,
depending on the soil physical properties of the simulation unit,
the transpiration values for the several simulation units are now
distributed between 0 and 3 mm (Figure 5C), indicating a large
spatial variability.
The remote sensing information does indeed show a drop in
evapotranspiration of 0.5 mm, or 1 mm when the overestimation
is taken into account. Although the evapotranspiration values do
match better, the remote sensing information does not show a
larger spatial variability. It is thus unlikely that the situation is as
worse as CGMS simulates.
Figure 5D shows the distribution of the evapotranspiration for
wheat growing under irrigated conditions at the 11th of May. The
arrow represents the potential transpiration for wheat at the
concerning day. Again CGMS has strongly overestimated
transpiration of irrigated wheat and typically the difference
between irrigated and rainfed wheat is only 0.5 mm.
In general, it can be concluded that CGMS simulation results for
wheat deviate considerably from the remote sensing information.
Crop transpiration has been over- and underestimated at the 25th
of April and, while the absolute values do match better at the 11th
of May, the distributions are not in agreement. The spatial
distribution in transpiration of wheat has been overestimated by
CGMS. This implies that the water balance of CGMS is depleted
to early.
Daily ET for irrigated wheat
Seville 25/04/92
Percent pixels
1
Evapotranepiration mm/day
Daily ET for irrigated wheat
Seville, 11/05/92
Percent pixels
1
Evapotranepiration (mm/day)
Figure 5 Daily evapotranspiration rates as derived from remote sensing data for wheat. The arrows indicate CGMS simulations
-
148 Intemational Archives of Photogrammetry and Remote Sensing. Vol. XXXII, Part 7, Budapest, 1998
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