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.