Geoscience Reference
In-Depth Information
1
Fig. 8.5
Soil water characteristic
curve for a fine sand (Oso
Flaco) measured during a
stepwise drainage process,
showing the degree of
saturation,
S
=
θ/
n
0
, against
negative pressure in the water
H
=
−
p
w
/γ
w
, expressed in
cm of equivalent water
column; in this particular
experiment the porosity was
n
0
=
0
.
405.
0.8
S
0.6
0.4
0.2
0
0 0000000
H =
−
p
w
/
γ
w
(cm)
Fig. 8.6
Soil water characteristic
curves for two loam soils
(Adelanto, circles, and
Pachappa, diamonds)
during drainage (1 hPa is
roughly equivalent with
1.02 cm of water column).
(After Jackson
et al
., 1965.)
0 . 5
0 . 4
θ
0 . 3
0 . 2
0 . 1
1 0
1
1 0
2
1 0
3
1 0
4
1 0
5
1 0
6
1 0
7
N e g a t i v e p r e s s u r e ,
−
p
w
( h P a )
the locus of points in the soil profile, where the water pressure is atmospheric, and thus
p
w
=
0, whenever atmospheric pressure can be taken as the reference. Below the water
table the water pressure
p
w
is positive, and above the water table it is negative. The
pressure of the water in the soil profile above a water table can be measured by means of
a tensiometer; this instrument appears to have been introduced by Richards (1928). An
example of a tensiometer is illustrated in Figure 8.8; in this case it is simply a tube filled
with water, closed at one end by a sensing element consisting of a porous material, and
connected at the other end to a manometer. The material of the sensing element must have
pores that are sufficiently fine to ensure continuous contact (without air leakage) between
the water in the soil and that in the tube; recall that this is the same requirement as that
of the porous plate PP in Figure 8.3. The porous tip of the tensiometer is installed at the
point where the water pressure is to be measured and that pressure can then be recorded
with the manometer at the surface. Although many different types of tensiometer have
been developed, with various types of pressure gauges, they all function in essentially
the same way. A smaller soil water content is indicated by a larger suction, that is a larger
negative pressure measurement by the tensiometer.
