Geoscience Reference
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Table 4.2 Laboratory measurement methods of θ ( h )
Method
Range (cm)
Reference
Sand box apparatus
-200 < h < 0
Klute ( 1986 )
Pressure cell
-1000 < h < 0
Kool et al. (1985)
Pressure membrane
-20 000 < h < -1000
Klute ( 1986 )
Vapour equilibration
h < -20.000
Koorevaar et al. ( 1983 )
Soil sample
Nylon cloth
Fine sand
D = - h
De-aeration
or flushing
Coarse sand
Move up
or down
Perforated tubes
Reservoir
Figure 4.13 Sandbox apparatus for measuring the soil water characteristic at low
suctions.
reservoir. A certain pressure head h can be established by placing the reservoir level
D = - h cm below the soil samples. After hydrostatic equilibrium is reached, the water
content can be measured by gravimetric weighing.
In the range -20 000 < h < 0 cm so-called pressure cells or pressure membranes
are used. The apparatus consists of an air-tight chamber enclosing a water-saturated,
porous ceramic plate connected on its underside to a tube that extends through the
chamber to the open air ( Figure 4.14 ). Saturated soil samples are enclosed in rings
and placed on the ceramic plate. The chamber is then pressurized, which squeezes
water out of the soil pores, through the ceramic, and out of the tube. Similar to the
sandbox apparatus, the air entry value of the ceramic plate should be low enough to
keep the plate saturated at the applied air pressures. At equilibrium, low through the
tube will cease. At the soil-ceramic plate interface we may write:
hhz
=− −
air
(4.11)
tube
When equilibrium is reached, the chamber may be depressurized and the water con-
tent of the samples measured. This method may be used up to air gauge pressures of
about 20 bars if special ine-pore ceramic plates are used. Because these devices have
a very high low resistance, it may require one week or more before pressure equilib-
rium has been reached (Jury et al., 1991 ).
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