Geoscience Reference
In-Depth Information
Fig. 9.6
The fl ow rate
q
=
V/At
[(cm
3
water/cm
2
soil)/day] where
V
is the volume of water caught
after fl ow through soil column of cross section
A
and length
L
in time
t
, usually one day. The
greater is the difference of water levels in this experiments the higher is
q
. The greater is the soil
length
L
, the smaller is
q
. In order to get rid of those outside values, Darcy introduced the saturated
hydraulic conductivity
K
S
=
q/
(
∆
h
/
L
)
the apparatus in Fig.
9.6
because the value of
K
S
is high. For the same value of
h
when a loamy soil is in the apparatus, the fl ow rate is substantially lower because the
value of
K
S
for a loamy soil is much smaller. And because the
K
S
of clay is extremely
small, for the same value of
ʔ
ʔ
h
, the fl ow within clay could even look as if it were
stopped or standing still.
A similar laboratory equipment for measuring saturated hydraulic conductivity
K
S
is in Fig.
9.7
with the total potential head
H
=
h
+
z
where
h
is the pressure head
and
z
the gravitational head measured from the reference level
z
= 0. Maintaining a
constant level of water ponded on the soil surface causes water to fl ow steadily
downward through the soil a constant rate
q
. With a measurement of this rate, the
value of
K
S
can be determined from observations of total head
H
made at the top and
bottom of the soil column or by making observations of pressure head
h
using
piezometers placed at two vertical locations inside the column as illustrated on the
left and right sides of Fig.
9.7
, respectively.
Estimates of conductivity from fi eld measurements provide more realistic infor-
mation since their computed values relate to a natural soil area many times greater
than that of small, disturbed samples measured in the laboratory. A frequent, reli-
able fi eld procedure is to measure the decrease of groundwater level in the vicinity
of a well being pumped and steadily removing water from saturated soil below the
water table. Approximate analytical solutions or numerical simulations are used to
obtain the saturated conductivity representing the groundwater fl ow in the region
between a pumping well and nearby observation wells.
Equations and computation of saturated fl ow for solutions of practical tasks are
relatively simple. They have been used for decades in computing and managing
water from a system of wells for water supply and waterworks. Or, in agronomy, the
drainage of waterlogged soils and the management of water tables to optimal depths
are another example of the application of saturated fl ow equations for the benefi t of
practical life.
