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.