infiltration

Infiltration is the movement of water into the soil. This is possible,
because soil is not solid matter; instead it is a porous medium
comprising a matrix of solid granular particles and voids that may be
filled with air or water (Figure 21). Flow in a porous medium may be
unsaturated when some of the voids are occupied by air, or saturated
when all the voids are occupied by water. Considering the cross
section of a porous medium illustrated in Figure 21, the porosity is
defined as
total volume
n = volume of voids

The range of n for soils is approximately 0.25 to 0.75 depending
upon the soil texture. A part of the voids is occupied by water, and
the remainder by air. The volume occupied by water being measured
by the volumetric soil moisture content is defined as
total volume
θ = volume of water (7)
Hence 0 ≤ θ ≤ n; the soil moisture content is equal to the porosity
when the soil is saturated. Soil moisture content is also sometimes
characterized by the degree of saturation, defined as
Sd = θ/n (8)
The degree of saturation varies between 0 and 1.

average density of the mineral grains making up a soil
ρm= Mm/Vm (9)
where Mm is the mass and Vm the volume of the mineral grains. The
value of ρm is rarely measured, but is estimated based on the mineral
composition of the soil. A value of 2650 kg/m3, which is the density
of the mineral quartz, is often assumed. The bulk density, ρb, is the
dry density of the soil

where Vs is the total volume of the soil sample which is the sum of
the volume of the air, Va, liquid water, Vw, and mineral components,
Vm, of the soil respectively. In practice, bulk density is defined as the
mass of a volume of soil that has been dried for an extended period
(16 hr or longer) at 105 oC, divided by the original volume. The
porosity (6) is given by
Here Mswet and Msdry are the masses before and after drying,
respectively, and ρw is the density of water (1000 kg/m3). This
method for determining soil moisture is referred to as the gravimetric
method. In the field moisture content can be measured in a number
of other ways. Electrical resistance blocks use the inverse
relationship between water content and the electrical resistance of a
volume of porous material (e.g. gypsum, nylon or fiberglass) in
equilibrium with the soil. Neutron probe moisture meters are
combined sources and detectors of neutrons that are inserted into