Soil Water Flow - Transport in the Atmosphere-Vegetation-Soil Continuum - page 139

• Capillary rise is not considered.

• The drainage amount is very sensitive to the applied time step.

Therefore in the scientiic literature various modiications of this model have been
proposed (Kim, 1995 ). Most of these modiications require extra calibration param-
eters. In this chapter we explore a level deeper to derive a general framework for soil
water low analysis that uses more constant, physical-based input parameters and is
able to address capillary rise, soil layering and runoff.

4.3 Hydraulic Head
As the preceding paragraph shows, we need a more fundamental theory to describe
soil water low near the soil surface in a general way. The concept of potential energy
forms a solid base for such a theory. Potential energy is the energy a body has by
virtue of its position in a force ield. For example, a mass possesses greater poten-
tial energy in a gravitational ield than an identical mass lying below it, because
work is required to lift the mass to a higher position. Various forces act on water in
a porous material like soil. The gravitational ield of Earth pulls the water vertically
downward. Force ields that are caused by the attraction of water to solid surfaces
pull water in numerous directions. The weight of water and sometimes the addi-
tional weight of soil particles that is not compensated by grain pressure also exert
downward forces on water lying underneath. Ions dissolved in water have an attrac-
tive force for water and resist attempts to move it. An especially important force is
associated with the attraction of water molecules for each other and the imbalance of
these forces that exists at the air-water interface, the so-called surface water tension
(Jury et al., 1991 ).
The potential energy of water in soil may be deined relative to a reference or stan-
dard state, since there is no absolute scale of energy. The standard state is customarily
deined to be the state of pure (no solutes), free (no external forces other than gravity)
water at a reference pressure, reference temperature and reference elevation and is
arbitrarily given the value zero (Bolt, 1979 ). The soil water potential energy is deined
as the difference in energy per unit quantity of water compared to the reference state.
There are different systems of units in which the total potential and its components
may be described, depending on whether the quantity of pure water is expressed as a
mass, a volume, or a weight. Table 4.1 summarizes these systems and their units. In
soil physics commonly the energy/weight expression is used, which results in the very
practical dimension length. Consequently, in this chapter we use the energy/weight
expression, which is called head instead of potential. In the next sections we consider
the hydraulic head of groundwater, soil water and water vapour.

Question 4.3: Consider a water column of 2.0 m height. Express the soil water poten-
tial in J kg -1 , N m -2 and m. Which unit do you prefer?