Geoscience Reference
In-Depth Information
9
INFILTRATION AND RELATED
UNSATURATED FLOWS
This chapter deals with the flow of water in the partly saturated zone of the near-surface
soil, and with the transfer through the atmosphere-soil interface. At the local scale, as
precipitated water reaches the ground surface,
infiltration
into the soil takes place. In
between precipitation events, the atmosphere exerts its drying effect, and the water in
the soil profile may move to the surface by
vapor diffusion
and by liquid
capillary rise
,
where it evaporates.
As illustrated in Figure 8.23, a small decrease in water content below saturation
can cause a significant decrease in conductivity, so that in most soils the difference
in hydraulic conductivity above and below the water table can be large. At an inter-
face between soils with different conductivities, the streamlines are known to exhibit a
pronounced refraction; therefore, in many situations unsaturated flow above the water
table can be assumed to be nearly vertical, whereas the saturated flow below the water
table can be assumed to be more horizontal or parallel to underlying impervious layers.
Accordingly, in this chapter, infiltration and related flow phenomena in the partly satu-
rated zone of the soil are analyzed in a one-dimensional vertical framework. Similarly,
in Chapter 10, it is shown how many saturated flow situations can be analyzed in the
one-dimensional framework of hydraulic groundwater theory. Because the infiltration
capacity constitutes an upper limit of maximal rate of entry into the soil, it is treated first
in Sections 9.2 and 9.3; rain infiltration is treated in Section 9.4. Various parameteriza-
tions of the infiltration of precipitation and related processes at the catchment scale are
covered in Section 9.5. Finally, Section 9.6 describes a few elementary mechanisms of
capillary flow during inter-storm periods.
9.1
GENERAL FEATURES OF THE INFILTRATION PHENOMENON
Infiltration can be defined as the entry of water into the soil surface and its subsequent
vertical motion through the soil profile. In most situations of practical interest, the soil
profile is initially less than saturated. Therefore, if it can be assumed that the displaced
air can escape freely, the flow of the infiltrating water in the soil is governed by the
Richards equation (8.55). For vertical downward movement of water this can be written
as
k
∂
∂θ
∂
t
=−
∂
H
∂
−
∂
k
(9.1)
∂
z
z
∂
z
