Geoscience Reference
In-Depth Information
Size of
the REV
Length or volume
Figure 4.3
Deinition of the representative elementary volume (REV).
equation. The soil is considered as a continuum, consisting of a mixture of solid
grains, water, solutes and air.
4.2 Field Water Balance
In
Chapter 1
we discussed the water balance of soil and an air-vegetation layer. If we
omit the air/vegetation layer, we may derive the water storage change Δ
W
(m) of a
soil volume near the soil surface by considering all in- and outlowing water amounts
(
Figure 4.4
):
(
)
∆
WPIERD t
=+−−−
∆
(4.1)
where
P
denotes precipitation rate (m d
-1
),
I
is irrigation rate (m d
-1
),
E
is evapotrans-
piration rate (including evaporation of intercepted water) (m d
-1
),
R
is surface runoff
(m d
-1
),
D
is drainage or deep percolation rate (m d
-1
), and Δ
t
is the considered time
interval (d). All terms are positive except for
D
and Δ
W
, which may be either posi-
tive or negative. A negative value for the drainage term implies that water is lowing
upward into the vadose zone volume (capillary rise).
In ield conditions, it is usually possible to measure
P, I
, and
R
with adequate pre-
cision. Also the proile water content and its changes Δ
W
can be measured accurately.
Evapotranspiration luxes are more dificult to measure, especially for longer periods.
To date no instruments exist to measure percolation luxes in a soil proile on a routine
basis. Also, unless a ield has subsurface tile drains, drainage luxes cannot be mea-
sured in the ield directly. Consequently, the drainage lux is often determined as the
closing term in the water balance. However, we should realize that any error we make
in one of the water balance components will affect the accuracy of the closing term.
This is especially the case for drainage, which is generally relatively small compared
to the evapotranspiration lux. The following question illustrates that relative errors of
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