W-RS infiltration process
Unlike the HS infiltration process, which is divided into rapid
infiltration, gradual decrease, and steady infiltration stages, the
shape of Fig. 5(b) suggests that the W-RS infiltration
process should be divided into
five sections (Fig. 7). In Phase I, the IR decreases
(stage I of Fig. 7 ). Because the soil air pressure is almost
zero at the initial point, there is a local maximum IR at\(i_{\text{start}}\). In Phase II , the infiltration process is
temporarily stable before the inflection point (stage II of Fig.
7 ), because the air-resistance of the deep soil layer, the repulsive
force of the water repellency, and the water gravitational and matric
potential are in a state of equilibrium.
There is a minimum IR of\(\mathbf{i}_{\mathbf{\text{bottom}}}\), and the IR caused by the
compact layer gradually decreases. In PhaseIII , the IR increases
(stage III of Fig. 7 ), possibly because of a gradual decrease
in WR. When the soil water content exceeds the critical value for the
disappearance of WR, there is a
maximum IR of \(\mathbf{i}_{\mathbf{\text{peak}}}\). In Phase
IV , the IR decreases (stage IV of Fig. 7 ), possibly
because the infiltration is
governed by gravity and the pressure head gradient, which diminishes as
the wetting front deepens. Grismer
et al. (1994) suggested that the water pressure gradient across the
transmission zone decreases because the soil air pressure balances the
capillary suction at the wetting front. Decreases in both the hydraulic
conductivity and the pressure gradient combine to reduce the
infiltration rate. In Phase V , the infiltration process
stabilizes after the inflection point (stage V of Fig. 7 ), and
the IR reaches a value of\(\mathbf{i}_{\mathbf{\text{stable}}}\).
This may be caused by the gravitational potential of free water in the
upper saturated soil layer being greater than the soil air-resistance,
similar to the infiltration flow becoming stable if the ponding head is
greater than the soil water-entry value (Wang et al., 2000). Ignoring
the initial section of rapid water infiltration, the W-RS IR is a
right-skewed single-peak curve.