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DM
a
(g)
300
200
100
r
2
= 0.90
0
0
50
100
150
T
a
(L)
Figure 6.16
Cumulative dry matter
DM
a
of above-ground parts in Kubanka wheat
(
Triticum durum
) grown in a single pot, as function of cumulative transpiration
T
a
.
(Data from Briggs and Shantz, cited by de Wit, 1958.)
40
30
y
= 0.0893
x
-17.36
r
2
= 0.78
20
10
0
0
100
200
300
400
500
600
700
Cumulative evapotranspiration (mm)
Figure 6.17
Relationship between cumulative evapotranspiration and dry matter
yield of forage maize. The crop was grown in Paulinenaue, northeast Germany,
on a lysimeter with a groundwater table 125 cm below the soil surface. (Based on
Mundel,
1992
.)
humidity varies during the course of a day and from day to day. This means that the
vapour pressure deicit (VPD) of the air (
D
a
=
e
sat
(
T
a
) -
e
a
, here
T
a
is air temperature)
is highly variable and therefore also the amount of water lost by diffusion. Taking this
into account, Bierhuizen and Slayter (
1965
) incorporated a mean
D
a
(Pa) in the rela-
tionship between cumulative transpiration
T
a
(m) and dry matter
DM
a
(kg m
-2
):
DM
T
1000
ยต
WP
T
=
a
=
(6.35)
D
a
a
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