Geoscience Reference
In-Depth Information
Infiltration capacity is sometimes referred to as
the saturated hydraulic conductivity. This is not
absolutely true as the measurement is dependent on
the amount of water that may be ponded on the
surface creating a high hydraulic head. Saturated
hydraulic conductivity should be independent of
this ponded head of water. There are conditions
when infiltration capacity equals saturated hydraulic
conductivity, but this is not always the case.
The curve shown in Figure 4.3 is sometimes
called the Philip curve, after Philip (1957) who
built upon the pioneering work of Horton (1933)
and provided sound theory for the infiltration of
water.
The main force driving infiltration is gravity, but
it may not be the only force. When soil is very dry
it exerts a pulling force (soil suction; see p. 6) that
will suck the infiltrating water towards the drier
area. With both of these forces the infiltrating water
moves down through the soil profile in a wetting
front. The wetting front is three-dimensional, as the
water moves outwards as well as vertically down.
The shape of the curve in Figure 4.3 is related to
the speed at which the wetting front is moving. It
slows down the further it gets away from the surface
as it takes longer for the water at the surface to feed
the front (and as the front increases in size).
the water molecules together rather than allowing
them to be drawn apart. Equally, surface tension
creates a force to counter the removal of water by
evaporation from within the soil.
Adsorption is the force exerted through an
electrostatic attraction between the faces of soil
particles and water molecules. Essentially, through
adsorption the water is able to stick to the surface
of soil particles and not be drained away through
gravity. In Chapter 1 it was pointed out that the
dipolar nature of water molecules leads to hydrogen
bonding (and hence surface tension). Equally the
dipolar shape of water molecules lends itself to
adsorptive forces.
Soil suction
Combined together, adsorption and surface tension
make up the capillary force. The strength of that
force is referred to as the
soil suction
or
soil mois-
ture tension
. This reflects the concept that the
capillary forces are sucking to hold onto the water
and the water is under tension to keep in place. The
strength of the soil suction is dependent on the
amount of water present and the pore size distri-
bution within the soil. Because of this relationship
it is possible to find out pore size distribution
characteristics of a soil by looking at how the soil
moisture content changes at a given soil suction.
This derives a
suction-moisture
or
soil moisture
characteristic curve
.
A
suction-moisture curve
(see Figure 4.4) may
be derived for a soil sample using a pressure plate
apparatus. This uses a pressure chamber to increase
the air pressure surrounding a soil sample and force
water out of pores and through a ceramic plate at its
base. When no more water can be forced out then
it is assumed that the capillary forces (i.e. the soil
suction) equal the air pressure and the sample can
be weighed to measure the moisture content. By
steadily increasing the air pressure between soil
moisture measurements a suction-moisture curve
can be derived. This can be interpreted to give
important information on soil pore sizes and is also
important for deriving an unsaturated hydraulic
Capillary forces
It is obvious to any observer that there must be
forces acting against the gravitational force driving
infiltration. If there were not counteractive forces
all water would drain straight through to the water
table leaving no water in the unsaturated zone. The
counteractions are referred to as
capillary forces
.
Capillary forces are actually a combination of two
effects: surface tension and adsorption.
The surface tension of water is caused by the
molecules in liquid water having a stronger attrac-
tion to one another than to water molecules in the
air (vapour). This is due to the hydrogen bonding of
water molecules described in Chapter 1. Surface
tension prevents the free drainage of water from
small pores within a soil by creating a force to keep
