492 
in the northeastern part of the Netherlands (study 
area 1 in Figure 1) show the influence of the ground- 
water table depth on crop water supply (Figure 5). 
Crops grown on the high situated sandy area in the 
western part, with groundwater table depths of more 
than 5 m, show high temperatures and hence strong re 
ductions in évapotranspiration. 
The seepage zone clearly manifests itself as a zone 
with low crop temperatures. Here reduction in évapo 
transpiration is negligible. More towards the east 
temperatures vary between the two mentioned extremes. 
In this area patterns on the heat image coincide more 
or less with relief and soil type. 
In a sandy area in the eastern part of the Nether 
lands (study area 3 in Figure 1) the effect of a low 
ering of the groundwater level caused by groundwater 
extraction for domestic purposes on the water supply 
of agricultural crops has been emphasized. 
As phreatic groundwater is extracted by the pumping 
station 't-Klooster the groundwater level and there 
fore tne occurrence of drought damage is influenced 
by the groundwater extraction in the area (Figure 6). 
Around the centre of the extraction a more or less 
conical depression of the groundwater table occurs. 
The évapotranspiration of crops has been studied in 
relation to tne distance from the centre of the ground- 
water extraction. As crop évapotranspiration depends 
on soil type and groundwater level a systematic anal 
ysis has been performed for each soil type and aach 
groundwater table class separately. Figure 7 shows 
two typical results. 
The évapotranspiration rate for grassland on a 
Typic Haplaquod soil in combination with a relatively 
high groundwater table under natural conditions, de 
creases in the direction of the centre of the extrac 
tion (Figure 7A). Natural conditions are defined as 
the situation without groundwater extraction. At dis 
tances of more than 1300 m from the centre of the 
extraction the drawdown of the groundwater table is 
negligible and because of sufficient water supply by 
capillary rise from groundwater crops are transpiring 
potentially. 
Figure 7B shows a quite different result. For maize 
on a Typic Haplaquod soil with a relatively deep 
groundwater table the évapotranspiration rate is low 
and independent on the distance from the centre of 
the extraction. This means that under these circum 
stances even without lowering of the groundwater ta 
ble the water delivery to the root zone by means of 
capillary rise can be neglected. 
The present example shows that the effects of a 
lowering of the groundwater level can be studied with 
remote sensing if additional information about soil 
type and groundwater level in the original situation 
is available. 
3.3 Remote sensing and hydrological model calculations 
With remote sensing detailed information about the 
regional distribution of évapotranspiration on flight 
days is obtained. In Sections 3.1 and 3.2 we have il 
lustrated that drought damage can be caused by dif 
ferent factors. This means that for the explanation 
of drought patterns on évapotranspiration maps ob 
tained with remote sensing additional information is 
necessary. For this aim the SWATRE-model (Feddes et 
al. 1978, Belmans et al. 1983) has been applied. 
Moreover this model has been applied for the transla 
tion of drought damage as determined with remote 
sensing into seasonal effects. 
Evapotranspiration was simulated during the entire 
growing season of 1982 for grass and maize grown on 
a Typic Haplaquod soil with groundwater table class 
V. In the calculations measured groundwater depths 
were taken for situations without extraction. More 
over, model calculations were performed assuming a 
constant drawdown during the entire growing season. 
Figure 8 shows that for the growing season of 1982 
the cumulative effect of a lowering of the groundwa 
ter table can amount to 15% for grass and 20% for 
maize. 
Except cumulative effects for 30 July 1982 the ef 
fect of a lowering of the groundwater table on daily 
crop évapotranspiration was also studied (Figure 9). 
In a dry period the water supply of crops depends 
on suppletion from groundwater by means of capillary 
rise. The effect of a lowering of the groundwater 
table could be serious if a relatively shallow ground- 
Figure 6. Evapotranspiration map of study area 3 (see 
Figure 1) situated around the pumping station 
't Klooster (P). The map is composed from reflection 
and heat images taken on 30 July 1982 at 12.00 MET. 
The grid applied in the GELGAM-model has been indi 
cated. Crop évapotranspiration decreases from poten 
tial (dark grey) to about 30% of potential (white). 
Black areas are not classified 
Figure 7. Relative 24-hour évapotranspiration rate (LE /LE ) on 30 July 1982 derived from the évapotranspira 
tion map shown in Figure 6. A. grass on Typic Haplaquod soil with drainage class V. B. maize on the same soil 
with drainage class VI (depending on the distance to the centre of the groundwater extraction)