Geoscience Reference
In-Depth Information
water tables may be temporary features reflecting
variable hydraulic conductivities within the soil and
rock, or they can be permanent features reflecting
the overall geology.
Table 4.1
Soil hydrological properties for
selected soil types
Soil type
Saturated
Porosity
hydraulic
conductivity
(cm/hr)
Groundwater flow
Sandy loam
2.59
0.45
The movement of water within the saturated zone
is described by Darcy's law (equation 4.5). Henri
Darcy was a nineteenth-century French engineer
concerned with the water supply for Dijon in
France. The majority of water for Dijon is aquifer
fed and Darcy began a series of observations on the
characteristics of flow through sand. He observed
that the 'rate of flow of water through a porous
medium was proportional to the hydraulic gradient'
(Darcy, 1856). There are many different ways of
formulating Darcy's law, but the most common and
easily understood is shown in equation 4.5.
Silt loam
0.68
0.50
Clay loam
0.23
0.46
Clay
0.06
0.475
Source
: Derived from measurements of different soil
types in the USA. From Rawls
et al
. (1982)
resources. It is the term 'hydraulic conductivity'
(
K
sat
) that is so important. This is the ability of a
porous medium to transmit water. This can be
related to the size of pores within the soil or rock
and the interconnectivity between these pores.
Table 4.1 shows some common values of hydraulic
conductivities for soils, in addition to their porosity
values.
One of the major difficulties in applying Darcy's
law is that hydraulic conductivities vary spatially
at both micro and macro scales. Although
K
sat
can
be measured from a small sample in the laboratory
(Klute and Dirksen, 1986), in the management of
water resources it is more common from larger-
scale well-pumping tests (see Freeze and Cherry
(1979) for more details). The well-pumping test
gives a spatially averaged
K
sat
value at the scale of
interest to those concerned with water resources.
QKA
dh
dx
(4.5)
=− ⋅ ⋅
sat
The
discharge
(
Q
) from an aquifer equals the
saturated hydraulic conductivity (
K
sat
) multiplied
by the cross-sectional area (
A
) multiplied by the
hydraulic gradient (
dh/dx
). The negative sign is
convention based on where you measure the
hydraulic gradient from (i.e. a fall in gradient is
negative).
The
h
term in the hydraulic gradient includes
both the elevation and pressure head. In an uncon-
fined aquifer it can be assumed that the hydraulic
gradient is equal to the drop in height of water
table over a horizontal distance (i.e. the elevation
head). In a confined aquifer it is the drop in phreatic
surface (i.e the level that water in boreholes reaches
given the pressure the water is under) over a hori-
zontal distance. The
h
term then includes a pressure
head.
Darcy's law is an empirical law (i.e. based on
experimental observation) that appears to hold
under many different situations and spatial scales.
It underlies most of groundwater hydrology and is
very important for the management of groundwater
The relationship between groundwater and
surface water
It is traditional to think of groundwater sustaining
streamflows during the summer months, which
indeed it often does. However, the interaction
between groundwater and streamflow is complex
and depends very much on local circumstances.
Water naturally moves towards areas where faster
flow is available and consequently can be drawn
upwards towards a stream. This is the case in dry
environments but is dependent on there being an
