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.