Agriculture Reference
In-Depth Information
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
With full cover crop
Without cover
April
May
June
July
Aug.
Sept.
Oct.
Figure 12.1
Approximate crop coefficients for apple orchards in
Central Washington relative to alfalfa-based evapotranspiration.
Reproduced from Evans (1982),credit Washington State University
Co-operative Extension. Based on data given
by James
et al.
(1982).
irrigation to only part of the root zone, or where water is an expensive or
very limited resource, depends on a much more detailed knowledge of tree
water relations.
Basic concepts in tree water relations
Water movement is governed by driving forces, originating from differences
in solute concentration and pressure, and by the conductance of the flow
pathway (Boyer,
). The driving forces can all be expressed in units of
pressure. They are described in units of water potential (
ψ
w
) which is the
chemical potential (free energy mol
−
) divided by the partial molal volume
of liquid water (
cm
mol
−
). This water potential is expressed in units of
.
=
dynes cm
−
=
ergs cm
−
=
.
bars (
bar
atmospheres). It is also
quoted in megapascals (MPa),
ψ
w
arealwaysexpressedrelativetopurefreewateratatmosphericpressureandthe
same temperature as the system being measured. This reference potential of
pure water is taken as
MPa equalling
bar. Measurements of
. Other water potentials are governed by equation
.
.
)
ψ
w
can be measured as the water potential in particular organs or tissues,
e.g. in leaf (
ψ
w
=
ψ
s
+
ψ
p
(
.
ψ
l
), xylem (
ψ
x
) or fruit (
ψ
fr
).
ψ
s
is the solute or osmotic potential
