Effects of irrigation schedule
Evaluation and improvement of WUE have become important issues for agricultural water management. Looking for an optimum irrigation strategy for a specific environment to promote the WUE will be important for saving water resources in this region. Six irrigation schedules have been investigated to save water resources and improve irrigation efficiency. Fig. 7 and Fig. 8 compare the VWC plumes between these six strategies (IS1-IS6) at the 160 hr and 720 hr, respectively. The plumes for the six schedules have hardly notable differences in terms of the shape and size, since they are subject to identical total amount of infiltrated irrigation water. However, differing irrigation schedules result in considerably differing soil VWCs no matter in the irrigation durations or in the intermittences. As has mentioned before, the IS for C0 (i.e. IS1) is 6 consecutive hours of irrigation per day every 6 days at the rate of 6.944×10-4 kg/s. The VWCs for IS3 and IS4 have significantly lower values than the others, indicating that these two ISs have lower irrigation efficiencies than the other four schedules, and IS4 leads to the lowest efficiency. This is because of their lengthy intermittences between irrigations. The VWCs turn to particularly low during these long post-irrigation periods. IS2 has relatively low VWCs at 160 hrs since that time is free of irrigation. Only marginally higher VWCs are observed at 720 hrs in Fig. 9d. For IS5 and IS6, the VWCs only have subtle differences than C0 and between each other. Higher VWCs can be observed under irrigation for IS6 than for IS5, due to the increasing IR applied for IS6. This indicates that commencing with lower-rate irrigation is likely instrumental to enhancing WUE since it favors gradual saturation of upper soil pores, avoiding quick loss of water by leaking down to the deep depths.
In order to take a closer look at the potential differences in the VWC distributions between the ISs, vertical profiles of VWC between the six ISs at four typical times have been plotted in Fig. 9. At 16 hrs, when the first day irrigation has completed for all six ISs, the VWC shows minor difference between ISs except for IS3 because of its less irrigation amount by that time. The IS4 causes slightly higher VWC in the upper part of the loamy sand and, the IS2 and IS6 result in slightly lower VWC in the same horizons. However, at the 160 hr, the VWC profiles for IS2, IS3 and IS4 show striking differences from C0 and others. Same characteristic has been observed in Fig. 9b. The lower VWCs above the depth of 30 cm for IS2 and IS3 can be explained by the actually less irrigation amount of water than the others. The slightly higher VWC between the depth 30~60 cm for IS2 is supposed to be a result of faster migration due to relatively more frequent irrigation. The particularly low VWC above the depth of 40 cm for IS4 for the 16 hr latter on, is attributed to its low frequency of irrigation. Application of the IS2 and IS6 leads to approximately same VWCs in the latter hours of the investigated duration (Fig. 9c and d). From a perspective of water saving, relatively high VWC in the root zone and low leak of water into the lower depths favors water uptake by crop roots and thereby higher irrigation efficiency. 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.
Figure 7. Comparison of the vertical plumes of VWC between six irrigation schedules (IS1-IS6) at the 160 hr.
Figure 8. Comparison of the vertical plumes of VWC between six irrigation schedules (IS1-IS6) at the 720 hr.
Figure 9. Comparison of vertical profiles of VWC between six irrigation schedules (IS1-IS6) at various times.