Soil Water Flow - Transport in the Atmosphere-Vegetation-Soil Continuum - page 148

h air

Steel core

Pressure chamber

Rings with soil

Rubber ring

Membrane

z tube

Ceramic plate

Steel base plate

Figure 4.14 Pressure membrane apparatus for measuring the soil water characteris-
tic at high suctions.

In the very dry range ( h < -20 000 cm) equilibration with salt solutions is a
suitable method. By adding precalibrated amounts of salts, the energy level of a
reservoir of pure water may be lowered to any speciied level. If this reservoir is
brought into contact with a moist soil sample, water will low from the sample to
the reservoir. If the sample and the reservoir are placed adjacent to each other in
a closed chamber at constant temperature, water will be exchanged through the
vapour phase by evaporation from the soil sample and condensation in the reser-
voir until equilibrium is reached. Another much applied method to determine water
retention in the very dry range is by equilibration with standard laboratory condi-
tions, which correspond to 20 °C and 50% relative air humidity. Using Eq. ( 4.10 ),
this implies a pressure head h = -9.2 × 10 5 cm in the soil sample after hydrostatic
equilibrium has been reached.
Hysteresis of the θ ( h ) relation may complicate the measurement and interpretation
of the soil water characteristic. The occurrence of various water contents at the same
pressure head can be caused by variations of the pore diameter (inkbottle effect),
differences in radii of advancing and receding meniscus, entrapped air and swell-
ing/shrinking processes (Hillel, 1998 ). Gradual desorption of an initially saturated
soil sample gives the main drying curve, whereas slow absorption of an initially dry
sample results in the main wetting curve ( Figure 4.15 ). In the ield partly wetting and
drying occurs in numerous cycles, resulting in so-called drying and wetting scanning
curves lying between the main drying and the main wetting curve. In practice, often
only the main drying curve is used to describe the θ ( h ) relation. For instance, a gen-
erally applied soil hydraulic database in the Netherlands, known as the Staring series
(Wösten et al., 2001 ), contains only θ ( h ) data of the main drying curve. This is due
mainly to the time and costs involved in measurement of the complete θ ( h ) relation-
ship, including the main wetting, the main drying and the scanning curves, especially
in the dry range. Nevertheless, it is obvious that the simulation of iniltration and
runoff events with the main drying curve can be misleading in case of signiicant
hysteresis.