Agriculture Reference
In-Depth Information
Box 6.7
(continued)
• The head gradient is made up of matric (
p
) and gravitational (
z
)
components. When the wetted zone is deep, the matric head gradient
d
(
p
)/
dz
becomes small relative to the gravitational head gradient of 1, so the
overall flux is downward at
J
w
K
(
h
).
• Suppose that after rain, the soil surface dries due to evaporation and the
matric head gradient becomes negative and its absolute value
1. The
K
(
h
) and the head gradient then becomes positive, and water
will flow upward to the surface.
• If the soil becomes saturated during rain (except perhaps for a few pockets of
trapped air), the matric head gradient becomes zero and equation B6.7.3
becomes
J
w
product of
(B6.7.4)
K
s
is called the
saturated hydraulic conductivity
, which is attained in the final
stages of infiltration, as illustrated in figure 6.4. The value of
K
s
ranges from
K
s
1 to
100 mm/hr, depending on the soil's structure and texture.
K
s
can be measured in
the field using a
ring infiltrometer
, and the variation in
K
for small decreases in
matric head near saturation can be measured with a
disc permeameter
. More details
are given in appendix 8.
Water Movement and the Wetting Front
Consider a soil that is structurally homogeneous and has no sharp changes in tex-
ture with depth. Examples are some of the deep loamy sands under vineyards in
the Sunraysia and Riverland regions of the Murray Valley, Australia, and some of
the deep alluvial soils of river valleys in Chile. Suppose such a soil is being wet
up from a dry state by rain or irrigation. As the water penetrates more deeply, a
zone of uniform water content, the
transmission zone
, develops behind a narrow
wetting zone and well-defined
wetting front
. A graph of water content
6.3.3
against
depth
z
, plotted after several hours' infiltration, shows a sharp change in
at the
wetting front (fig. 6.5a). This occurs because water at the wetting front takes up
a preferred position of lowest
in the narrowest pores, for which the hydraulic
conductivity is very small. Water does not penetrate further at an appreciable rate
until the large pores begin to fill.
Figure 6.5a shows that the water content in the transmission zone is ap-
proximately constant. This is described as the
steady state
condition (as in the fi-
nal stage of infiltration in fig. 6.4). The condition when a soil is wetting up (or
drying down) is described as non-steady state or
transient
. At steady state, and for
an infiltration rate
J
w
, the average
pore water velocity
in the wet soil zone is given
by
J
w
(6.7)
Equation 6.7 demonstrates that water entering 1 m
2
of soil surface is only
able to travel through the wetted pore volume, defined by
. The smaller the value
of
, the faster a given volume of water must travel through the pore space and
