Geoscience Reference
In-Depth Information
Keep the same water levels
Excess air pressure h air
h air (m) = - π (m)
Salt solution
Pure water
Osmotic head π
Membrane: permeable for water, impermeable for salts
Figure 4.8 Experimental setup to measure the osmotic head of a water solution.
In clay soils the osmotic binding of water in diffuse electric double layers may
exceed the capillary binding (Koorevaar et al., 1983 ). Because water molecules have
a dipole moment, ions in the soil water are attracted by the electric ield around indi-
vidual water molecules and tend to cluster around them. The effect of this clustering
is to lower the energy state of water. If a membrane permeable to water but imperme-
able to solutes is used to separate pure water from a solution containing ions, water
from the pure side of the membrane will cross over into the solution side. This mass
transfer will continue indeinitely, unless stopped by an opposing force.
If the solution is contained in a sealed reservoir such as depicted in Figure 4.8 , the
pure water entering the volume will expand the salt solution until the increased air
pressure balances the ionic attraction of water through the semipermeable membrane.
To derive the osmotic head, we might increase the air pressure above the solution
until the water levels in both reservoirs are equal ( Figure 4.8 ). In case of common salt
solutions, the osmotic head π (cm) can be approximated by (Rhoades et al., 1992 ):
π =−
400
EC
≈−
625
TDS
(4.8)
with EC the electrical conductivity (dS m -1 ) and TDS total dissolved solids (mg cm -3 ).
The osmotic head plays an important role in irrigated agriculture, where soil water
salinity may hamper root water uptake ( Chapter 6 ).
For soil water movement in the vadose zone, we need to consider only the gravity
and pressure head, as water moves freely without a semipermeable membrane (Hillel,
1998 ). Therefore, similar to groundwater the hydraulic head H (m) is the sum of just
two elements, the gravity head and the pressure head:
Hzh
=+
(4.9)
Unlike the saturated zone, the pressure head will be negative in the unsaturated
zone ( Figure 4.9 ). At the groundwater level the pressure head is zero and the hydrau-
lic head is equal to the gravity head or elevation. To measure negative pressure heads,
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