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.