Migration of infiltrated water
Infiltration of irrigation water accompanies advance of the wet front
and causes changes in soil water content. Fig. 4 presents the simulated
VWC plumes of C0 for analyzing the soil wetting process under
irrigation, and Fig. 5 shows the counter parts during the intermittences
of irrigation, to shed light on the process of water redistribution. At
the early times after the irrigation was applied, the soil sucks water
into the pores very quickly since the matric potential is large for a
drying condition. The superficial soil under the irrigation points (i.e.
nozzles) becomes saturated due to the infiltration. The saturated soil
has a VWC of 0.40, which is exactly the value of porosity of the loamy
sand, since we set a zero residual gas saturation to ensure easier
infiltration. The soil immediately inside the wet front is wet yet
unsaturated and hence still contains gases in pores. The wetting front
advances downward in the drive of gravity and matric potential. The
water under the middle nozzle flows downward more quickly than its both
sides, leading to forming a typical tongue-alike wet plume, although the
shape of the plume is also somewhat affected by the grid geometry. This
phenomenon occurs in a two-phase flow domain since the infiltrated water
flow is hindered to some extent by the displaced air, causing the air
migrates from the beneath to the upper outside.
The wet front develops downward with the time elapsed, no matter when
there is irrigation or not. After 6 hrs of irrigation, i.e. at t=16 hr,
the wet front arrives at depth of 0.35 m. At the 720 hr, the wet front
advances to approximately 1.20-m depth, which is the lower boundary of
the sandy loam. The wetting depths at the other times are found in Fig.
4 and Fig. 5. Due to the infiltration, after the last-time irrigation at
the 592 hr, the soil under the three irrigation nozzles has a planar
wetting scope of 1.08 m (517-625 cm, Fig. 4) at the surface and 1.73 m
(492-665 cm, Fig. 4) in the sandy loam. The corresponding values at the
720 hr are almost the same (Fig. 5) since the water redistribution
basically only changes the wetting depth. It is also found, by comparing
the plumes in Fig. 4 and Fig. 5, that the VWC in the topsoil (loamy
sand) is obviously higher than the subsoil (sandy loam) under the
condition of irrigation. In contrast, opposite characteristics are
observed during the irrigation intermittences. The significantly higher
VWC in the subsoil indicates that the soil water migration after the
irrigation ceased led to the water redistribution and increased wetting
depth (Fig. 5). It is straightforward to conclude that the infiltration
is dominantly a vertical process that the wet front advances under the
combined control of gravity and matric potential.
Figure 4. Vertical plumes of VWC for the base case C0 (IS1) at various
times under irrigation.
Figure 5. Vertical plumes of VWC for the base case C0 (IS1) during the
irrigation intermittences.