Geoscience Reference
In-Depth Information
Fig. 4.15 Successive soil water pressure
distributions within a Yolo loam
profile at Davis, California,
during a 60-day drainage
period following the end of
surface infiltration. The
negative pressure of the soil
water,
H
=
−
p
w
/γ
w
,is
expressed as equivalent water
column. (After Davidson
et al.
,
1969.)
H (cm)
0
50
100
150
200
0
0.5
2
5
15
25 35
60
days
0
0.5
1
1.5
2
The method is probably most useful insituations where
q
z
d
isnegligible, so that evapo-
ration is the only depletion mechanism of the moisture content of the soil profile. Still, with
some additional information it may be possible to obtain reliable estimates of
q
z
d
. If data
are available on the vertical water pressure gradient (
∂
p
w
/∂
z
)(infinite difference form)
and on the hydraulic conductivity
k
=
k
(
z
), the downward drainage rate can inprinciple
be calculated with Darcy's law (Equation (8.19)). However, in several field studies (see
Davidson
et al
., 1969; Nielsen
et al
., 1973) it has been observed that during the vertical
redistribution of soil water at depths of1mormore, where there isnodirect influence of
surface evaporation, the hydraulic gradient is rarely very different from unity. An example
of this phenomenon is shown inFigure 4.15. This allows the approximation of Equation
(8.9) by
q
z
d
=
k
(4.59)
Thus in such a case,
q
z
d
may be estimated with only a measurement of the soil water content
at
z
=
h
so
, provided, of course,
k
=
k
(
θ
) is known. In many situations, however, especially
during the second stage of drying (see Chapter 9), the downward drainage rate at some
depth may simply be neglected (see Jackson
et al
., 1973); but this needs to be checked in
each particular case.
Measurements of soil water content and water pressure at several levels in the profile are
not easy and they require many precautions. The soil water depletion method is probably
only useful for special experimental situations under favorable conditions, and it is clearly
not generally applicable on a routine basis. It may be hard, if not impossible, to apply when
the following conditions are present: a water table close to the surface, frequent and large
precipitation, non-negligible or net lateral inflows, a large drainage rate, and considerable
variability in the soil properties. Thus the accuracy obtainable with this method depends
largely on the local conditions. Some other practical aspects of the soil sampling have been
discussed by Jensen (1967).
A water budget-based instrument: the lysimeter
A lysimeter is a container placed in the field and filled with soil, on which vegetation can be
maintained for the purpose of studying various soil-water-plant relationships under natural
