Because field experiments were restricted by various factors, it was difficult to comprehensively carry out saline water irrigation experiments with waters of different salinities. The calibrated SWAP model can be used to simulate saline water irrigation with different water salinity. The simulated irrigation water salinities tested were 0.71, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, and 8.0 mg/cm
3. As the Shiyang River Basin is located in an arid desert area where rainfall is scarce, there is little difference between normal flow years, and dry or wet years. During the simulation, the meteorological data were based on the average annual method (
p = 50%). Drawing on 1960–2014 rainfall data, rainfall frequency analysis showed that the typical year with
p = 50% was 2011, when rainfall during the maize growth period was 118.8 mm. Based on local irrigation experience and previous studies, maize is generally irrigated four or five times over its growth period. Irrigating for the fifth time is mainly to maintain maize grain plumpness, thereby obtaining a greater yield and economic income. The added irrigation occurs in the mature stage of the maize crop cycle [
25]. The present simulation did not consider a fifth irrigation, but rather irrigated four times during the maize growth period, according to the crop’s water requirements. The overall irrigation water quota was 450 mm, including 120 mm at the seedling stage (5 June), 120 mm at the jointing stage (30 June), 105 mm at the heading stage (20 July), and 105 mm at the filling stage (10 August). The SWAP was simulated based on initial soil water content, soil salinity, and crop growth data of the s0 treatment in 2013. The simulated soil depth range was 0–100 cm. Under these conditions, the calibrated SWAP model was used to simulate saline water irrigation with nine different levels of irrigation water salinity.
Figure 8a shows that, although the irrigation water amount was unchanged, irrigation water of different salinities altered soil water transport. The soil water content increased gradually with an increase in irrigation water salinity. At the higher irrigation water salinities, salt stress worsened, inhibiting root water absorption by the crop, and allowing more water to remain in the soil [
26]. The soil water content under irrigation water salinities of 0.71, 1.0, and 2.0 mg/cm
3 was basically the same, indicating that soil water content was almost the same under low irrigation water salinities. Soil salinity in the 0–100-cm soil layer under saline water irrigation increased with an increase in irrigation water salinity (
Figure 8b). At the end of the maize growth period, the soil salinity under irrigation with waters of 0.71, 1.0, and 2.0 mg/cm
3 salinities was basically the same; however, for waters with salinities of 3.0, 4.0, 5.0, 6.0, 7.0, and 8.0 mg/cm
3, it increased by 3.73, 5.03, 6.20, 7.23, 8.18, and 9.05 mg/cm
3, respectively, compared to the 0.71 mg/cm
3 irrigation water. Simulated soil water flux and salt flux changed in a similar manner in the 100-cm soil layer (
Figure 9); the downward soil water flux and salt flux gradually increased with an increase in irrigation water salinity, reflecting the fact that soil water content could promote soil salinity movement to a certain degree.
Soil water–salt balance under saline water irrigation with different water salinities is presented in
Table 5. A negative sign for soil water change indicates that soil water content was diminished through consumption. A negative sign for bottom soil water flux indicated that soil water was leaking downward. The bottom soil salt flux was negative, indicating that soil salinity moved downward. The increase of soil salinity was negative, indicating that soil salinity was leached. Based on the principle that a lesser increase in soil salinity would result in a greater yield and water use efficiency for maize, suitable irrigation water salinity was sought for the study area. It can be seen from
Table 5 that the soil salinity of the 0.71, 1.0, and 2.0 mg/cm
3 irrigations increased below 50 mg/cm
2, the maize yield was above 5168 kg/hm
2, and the maize water use efficiency was above 1.0 kg/m
3. Therefore, brackish water of 0.71–2.0 mg/cm
3 can be used for irrigation in the study area. The soil salinity of 6.0, 7.0, and 8.0 mg/cm
3 irrigations increased more than 180 mg/cm
2. Compared with the simulated maize yield of 0.71 mg/cm
3, the maize yield decreased by more than 28% and the water use efficiency was less than 0.95 kg/m
3. Therefore, irrigation water salinity of 6.0, 7.0, and 8.0 mg/cm
3 was not suitable for irrigation. The soil salinity of 3.0, 4.0, and 5.0 mg/cm
3 irrigations increased between 80 and 160 mg/cm
2. The yield reduction of maize was between 12.5% and 23.0% and the water use efficiency was between 0.98 and 0.95 kg/m
3, which were slightly worse than brackish water of 0.71–2.0 mg/cm
3. Therefore, saline water of 3.0–5.0 mg/cm
3 can be used for irrigation over a short period of time. Whether saline water of 3.0–5.0 mg/cm
3 can be used for irrigation over a long period of time still needs further study.