ils in which
:low is link-
about regional
. GAM-model
lei.
:ic view about
water and the
nage. The
The avail-
piration is
the root zone;
subsoil ;
rowing season;
sprinkling
sndent on soil
reliable in-
ristics and
d if sprinkling
soil-plant-
mentioned
termined under
at for the ex
damage as de
ns with agro-
applied.
racteristics
fter a dry
of the peat
Netherlands
ly potatoes
0%) are grown,
i the available
u as well,
show high tem-
ight conditions
1 more peaty
¡how lower tem
iese soils are
r crops grown
i than 40 cm
»elow soil sur-
.mal water sup-
study area 1
at 13.30 MET.
the indicated
Figure 3. Thermal image of a part of study area 1 (see
Figure 1) taken on 8 August 1983 at 13.30 MET. Black
is cold and white is warm. Of the indicated plot only
the middle strip is unimproved
Figure 4. Evapotranspiration map of study area 2 (see
Figure 1). The map is composed from reflection and
heat images taken on 21 July 1983 at 11.30 MET. Crop
évapotranspiration decreases from potential (dark
grey) to about 30% of potential (white). Black areas
are not classified (scale 1:125,000)
ply. In this case the available moisture capacity is
sufficient to get through extreme dry periods in the
Netherlands.
In the mentioned peat area soil improvement has been
performed on a large scale. On several places the hy
draulic conductivity of layers in the subsoil was
very low, resulting in water excess in wet periods
and drought damage in dry periods. Effects of soil
improvement works on crop production have been studied
on experimental fields. With thermal images taken at
8 August 1983 it was investigated if information about
soil improvement could be obtained with the aid of
remote sensing images. One typical result is shown in
Figure 3. On the indicated plot potatoes were grown.
Two strips at the border of the plot were improved
while the middle strip was not. Parts of the plot
with deep peat soils show very low temperatures. Es
pecially where less peat was present a systematic
difference in temperature of 0.6 K was found between
the improved and unimproved part of the plot. With
eqs. (1) and (2) an increase in évapotranspiration of
15% is calculated thanks to the soil improvement. By
comparing improved and unimproved plots for each soil
type a positive effect for peaty soils was observed,
while for sandy soils like Haplaquods no effects were
found (Kok 1985). In spite of the application of soil
improvement crops grown on sandy soils show severe
drought damage under the prevailing weather condi
tions in 1982.
In the southwestern part of the Netherlands (study
area 2 in Figure 1) the influence of loam layers in
the subsoil on crop water supply was studied with
images taken at 21 July 1983 (Spaans 1986). In. gener
al no influence of the presence of loam was percep
tible. One subregion, however, indicated in Figure 4
showed significant high crop temperatures and hence
low relative évapotranspiration values. Because of
the shallow groundwater table depth in this area no
drought damage was expected.
From field observations it was found that the soil
physical characteristics of the loam layer in this
subregion differed a lot from those of the loam
layer that was found at other places in the project
area. The hydraulic conductivity was much lower and
therefore capillary rise was less for comparable
groundwater depths. This resulted in larger reductions
in évapotranspiration. With the aid of the évapotrans
piration map this subregion could be mapped more ac
curately.
It seems possible that under certain conditions the
hydraulic conductivity of the subsoil can be checked
or perhaps even can be determined with the aid of
évapotranspiration maps. A more detailed study on
this subject has not been carried out yet.
3.2 Influence of groundwater table depth on crop wa
ter supply
Thermal images taken at the boundary of the peat area
Figure 5. Thermal image of a part of study area 1
(see Figure 1) taken on 4 August 1982 at 12.30 MET.
Black is cold and white is warm
