Geoscience Reference
In-Depth Information
Fig. 9.3
Schematic role of soil texture and soil structure upon saturated conductivity and water
percolation of the soil layer or of the whole soil profi le. Sandy soils are either structureless or with
a weak structure. The pores (indicated in
white
) between sand particles belong mainly to macro-
pores and the hydraulic conductivity is very high. The well-aggregated loamy soil contains macro-
pores between aggregates and micropores inside of the aggregates. Hydraulic conductivity is high
to medium. Structureless loamy soils contain mainly or only micropores between soil particles.
Macropores are either absent or they exist in minority if the soil is cracked when dry. Individual
macropores may appear after decomposition of dead roots of plants. Hydraulic conductivity of the
structureless loamy soil (except of cracks) is low
steadily loosens and removes particles of the loamy dam material and gradually
decreases the stability of the dam until it fi nally ruptures.
When water fl ows through a system of soil layers where the conductivities differ,
the overall total rate of fl ow depends upon the orientation of fl ow in relation to the
arrangement of the layers; see Figs.
9.4
and
9.5
.
The simplest procedure of measuring the saturated hydraulic conductivity is
illustrated in Fig.
9.6
. We measure the volume
V
of water percolating through the
horizontal soil column of the length
L
during a period of time
t
. When we know the
cross-sectional area
A
of the soil, we obtain the fl ow rate
q
=
V/At
[(cm
3
water/cm
2
soil)/day] or simply (cm/day). The volume of soil taken in the fi eld is usually not
bigger than 1,000 cm
3
, i.e., one liter. The greater the difference of water levels at the
beginning and at the end of the soil column
h
, the greater is the fl ow rate
q
in the
same soil. It looks logical and simple and this is the magic of Darcy's law whose
equation,
q
= -
K
S
ʔ
ʔ
h
/
L
, says that the fl ow rate depends upon the hydraulic gradient
(or “slope”)
h/L
and upon the property of the soil to allow water fl ow - the satu-
rated conductivity
K
S
. When the soil is sandy, water fl ows at very high rate through
ʔ
