The dynamics of VWC
Fig. 6a and b analyze the dynamics of VWC on a 10-thick layer basis. For each layer, the VWC varies with time for each layer and has high values after irrigation for the upper 0.6 m of soil. The response of VWC to irrigation decays with depth and manifests an increasing hysteretic effect with depth. This increase in hysteresis is an indicator of the average velocity of the wet front. Note also that the time lag becomes shorter at later times for each layer. For instance, the picks of VWC for the depth 0.4~0.5 m appear 80 hrs later than irrigation at about the 240 hr, as is illustrated in Fig. 8a. However, the time lag turns to be 40 hrs at about the 480 hr. This variation in the time lag for a specific layer indicates a faster arrival of the wet front at the later time, due to gradually increased area of the saturated or non-deficit zones.
The infiltration induced by irrigation causes soil water dynamics different between the upper and lower half of the thickness of the soil. The upper soil has higher VWCs than the lower when the infiltrated water has not arrived there yet. The VWC of the lower soil starts with a level of almost deficit and is elevated by irrigation layer by layer, which is supposed to be a reasonable and common situation in dry episodes of arid zones.
Since the VWCs in Fig. 6a and b are calculated on a layer basis at various discrete points of time, it is quite necessary to check out some specific locations for more consecutive and hence more accurate illustration of VWC dynamics. Fig. 6c and d present the variation of VWC at 10 selected positions in the ten intervals of depth counted in Fig. 6a and b. Periodic imbibitions and water recessions of soil pore are observed more clearly in these curves. The soil pores are increasingly saturated with water under and after irrigation, and then dewatered in the intermittences of irrigation due to pore drainage. The relatively sharp and gentle slopes of the dynamic curves are indicative of fast and easy infiltration whereas relatively slower and harder drainage (Fig. 6c). The regimes of VWC dynamics for the lower specific positions are basically similar to those for depth intervals (Fig. 6d). Analysis of infiltration and soil water dynamics based on point observations is thus well justified and meaningful for agricultural or hydrological research.
Figure 6. Temporal variation of the average VWCs of each depth interval in the BOI (a-b) and VWCs at selected individual positions (c-d). The irrigation rate IR applied is plotted as bars for convenience of analysis.