Agriculture Reference
In-Depth Information
(a)
(b)
Water content
Transmission
zone
Water content
profile in a
structured
clay soil
showing flow
preferential
Wetting zone
Wetting front
Initial
Figure 6.5
(a) The wetting profile in an initially dry soil of uniform texture and little structure. (b)
The wetting profile in an initially dry, structured clay soil, with preferential flow.
. We speak of an average pore water velocity be-
cause the conducting pores are not all the same radius, and velocity is very de-
pendent on pore radius. The distance
z
traveled by the wetting front in time
t
is
given by
hence the higher the value of
J
w
t
z
(6.8)
This equation can be used to estimate the depth to which water has pene-
trated for a cumulative amount of irrigation (
J
w
t
in mm). This is also the depth
to which surface-applied soluble fertilizer could move under irrigation.
In practice, soils are usually not structurally homogeneous because of the pres-
ence of old root channels and worm holes (macropores), and gaps between large
aggregates. The presence of macropores and cracks predisposes to the
preferential
flow of water through the soil profile. Some of the infiltrating water flows rapidly
down the macropores and fissures (referred to as “bypass flow”), whereas the bulk
of the water moves more slowly into the micropores within aggregates (referred
to as “matrix flow”). As a result, the wetting zone is spread over a considerable
depth and the wetting front penetrates more deeply than predicted from equation
6.8 (fig. 6.5b). If preferential flow occurs, the depth of wetting can be estimated
using equation 6.7, provided the fractional volume (
m
) of the macropores is used,
instead of
for the whole soil. Some examples of average pore water velocities
and depths of wetting for different soil types are given in table 6.2.
The steady state condition applies well during the winter dormant period of
grapevines, especially in cool climates where evaporation rates are low. But this is
rarely the condition in the root zone of actively growing vines. Intermittent rain-
fall, irrigation, surface evaporation, and variable water uptake (through transpira-
tion) cause
to fluctuate with time at any particular point in the soil. Water flow
can still be described by Darcy's Law, provided it is combined with an equation
for the conservation of water mass. The solution of this equation is beyond the
scope of this topic, but it is discussed in texts such as Jury et al. (1991).
