Leaching of Contaminants to Groundwater - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 792

retention curve thus characterizes the energy status of water in the soil, and is one of
the two soil hydraulic functions necessary to describe the status and movement of
water in the vadose zone. Since capillary rise depends on the radius of a particular
capillary, the retention curve may also be interpreted as a curve that characterizes
the distribution of pores of different radii in the soil.
The dependency of the fraction of water-filled porosity (i.e., the water content

θ ) on the soil matric potential ψ m is formalized in the soil water retention curve,

θ ψ m ). This relation is of fundamental importance for the hydraulic characterization

of a soil since it relates an energy density (potential) to a capacity quantity (water
content). One may view the water retention curve as the curve that results when
water is slowly removed from an initially water saturated soil until the soil becomes
air saturated. This is the desorption curve. The adsorption (or absorption) curve
describes the reverse process. Note that air can only enter the porous medium after
the matric potential has fallen below a certain value

(

ψ m 0 , the so-called air-entry
value. This value is determined by the largest pore of the porous medium open to air.
Once air has entered the porous medium, the water content decreases monotonically
with increasingly negative matric potentials ψ m .

Rather than using the matric potential in the parameterization of the water
characteristic, the pressure head h is often used instead:

=− m
ρ w g

h (18.2)

where h has SI units of meter (m) and dimensions of L; h represents the energy state
of pore water and is expressed as energy per unit weight.
Figure 18.4 shows water retention curves for three different textural classes, i.e.,
for a clay, a loam, and a sand, as given by Carsel and Parish (1988) using the soil
hydraulic parameters given in Table 18.1 (to be discussed later). Notice that the sand
loses its water relatively quickly (at small negative pressure heads) and abruptly
above the water table (the pressure head h 0 cm at the water table), while the

more fine-textured loam and especially the clay soil lose their water much more
gradually. This reflects the pore-size distribution of a particular soil textural class.
While the majority of pores in coarse-textured soils (such as sand and gravel) have
relatively large diameters and thus drain at relatively small negative pressures, the
majority of pores in fine-textured soils (such as clays, silty clays and clay loams) do
not drain until very large tensions (negative pressures) are applied.
Commonly used mathematical expressions for the retention curve,

=

( h ), are
the Van Genuchten (Van Genuchten 1980a ) and Brooks and Corey ( 1964 ) equa-
tions since they permit a relatively good description of

θ

( h ) for many soils using
only a limited number of parameters. The Van Genuchten soil moisture retention
characteristic is defined as:

θ

θ s − θ r

θ ( h ) = θ r + (18.3)

m

n

( 1 + | ah | )