Simulation cases and parameterization
The base case C0 of the model is a ‘benchmark’ simulation of infiltration induced by drip irrigation in the irrigation schedule coded IS1. The irrigation rate of the nozzle for C0 is determined according to Li (2014). For the purpose of analyzing the potential factors influencing the soil water content in the vadose zone, four additional simulation cases (C1-C4) encompassing 11 sub-cases (scenarios) were investigated to look into the effects on soil water distribution of the irrigation schedule, soil surface seal, residual water saturation, and preferential flow. Besides IS1 for C0, C1 considers additional 5 ISs: (i) IS1, 6-hr irrigation per day at an irrigation rate of 6.94×10-4 kg/s every 6 days; (ii) IS2, 5-hr irrigation per day at an irrigation rate of 6.94×10-4 kg/s every 5 days; (iii) IS3, 3-hr irrigation per day at an irrigation rate of 6.94×10-4 kg/s every 3 days; (iv) IS4, 3-hr irrigation per day at an irrigation rate of 1.388×10-3 kg/s every 6 days; (v) IS5, 6-hr irrigation per day at an exponentially reducing irrigation rate every 6 days; (vi) IS6, 6-hr irrigation per day at an exponentially increasing irrigation rate every 6 days. More concrete details are found in Fig. 2.
C2 was designed to investigate the effects of soil surface seal on water infiltration. A surface seal of 5 mm thick is included in this case, to simulate surface crust formed by raindrops splash or soil salinization. C3 run the exactly same model as C0 but using differing residual water saturations to investigate the effect on infiltration. Preferential flow was simulated in C4 by introducing a set of synthesized fractures in the soil profile.
The hydraulic properties for these simulation cases or scenarios are presented in Table 1 or described later when analyzing specific cases. The assignment of soil properties such as porosity, permeability and grain density are based on actual measurements from the field test site (Wang, 2013). The relations of relative permeabilities and capillary pressures with liquid-phase saturation are described with the van Genuchten-Mualem function (Mualem, 1976; van Genuchten, 1980) and the van Genuchten function (van Genuchten, 1980), respectively. The corresponding curves assembled with parameters from the experiments in Wang (2013) and Li (2014) are illustrated in Fig. 3.
Table 1. Major soil properties used for the base-case simulation.
Figure 2. Irrigation schedules considered in the model.
Figure 3. The relative permeability (a-c) and capillary pressure curves (d) defined for the three soils for the base-case simulation.