SUMMARY AND CONCLUSIONS
Numerical simulations have been performed to analyze irrigation induced infiltration and soil water distribution. Factors influencing these processes and behaviors are identified by a set of scenario investigations. It is concluded that irrigation schedule, surface seal, residual water saturation, and soil heterogeneity can impose significant impacts on water infiltration and the resulting soil water regime. Recognition of these processes and mechanisms favors irrigation design and water management by improving soil conditions and optimizing irrigation strategies. Major findings are summarized as follows for the reader’s convenience.
(1) Irrigation schedules have significant impacts on SWC patterns for both the irrigation durations and intermittences. The VWCs for IS3 and IS4 have lower irrigation efficiencies than the other four schedules, due to their lengthy intermittences between irrigations. Commencing with lower-rate irrigation is likely instrumental to enhancing WUE since it favors saturating the upper soil pores to the largest extent, avoiding quick loss of water by leaking down to the deep depths. Of the six ISs examined, our simulations show that IS6 is probably the best practice for efficient irrigation for shallow-root crops. The IS5 and even IS2 may be comparable to IS1 in terms of irrigation efficiency. The IS4 would be the last option for agricultural irrigation, especially for shallow-root crops in arid areas.
(2) The infiltrability of the soil surface was determined by a bunch of ‘try-and-error’ simulations. The infiltrability drops exponentially or polynomially with the decrease in permeability of the immediate surface. Introduction of 5-mm soil surface seal with a permeability of 5 mD leads to about 97.1% reduction in the infiltrability. The strong effect on the infiltration of the surface seal is mainly due to the relatively high permeability of the loamy sand. The presence of soil surface seal hinders infiltration and hence renders remarkably lower VWCs along the depth. Exceptions can be observed in case the surface sealing is poorly developed.
(3) Residual water saturation or water content has positive correlations with VWC, due to the entrapment of the residual water. HigherSwr leads to higher VWC especially for the times free of irrigation. Enhancement of one-fold magnitude ofSwr causes 6.12% and 20.4% higher VWC for the loamy sand, but 7.57% and 5.71% lower VWC for the underlying sandy loam, at the 160 hr and 720 hr respectively. It is encouraged to take practical measures to alter the soil structure and physical properties to ensure a higher Swr in the root zone of the crops.
(4) The presence of soil fractures for C4 leads to apparent preferential flows along the fractures in the soil. VWCs that is as high as 0.491 are observed in the fracture network under condition of irrigation. The VWC plumes have more irregular forms due to uneven wet fronts induced by preferential flows. The presence of preferential flow causes the wet fronts to penetrate to 115.9% and 53.88% deeper depths at the 160 hr and 720 hr, respectively. Thus the presence of preferential flows in the soil can cause a loss of irrigated water and hence lower irrigation efficiency. The infiltrated water travels as fast as 0.2 m per day through preferential pathways in our model.