## 4 Discussion
while the reason for the different rules of the W3 treatment was that irrigation was carried out during the filling stage among this study (Figure 3). [PERSON] et al. [32] showed that deep tillage among the fallow period can increase soil moisture storage, promote the absorption of nitrogen and phosphorus by wheat, and promote wheat spike formation. [PERSON] et al. [33] found that increasing soil moisture storage can increase spike number and yield as well as improve the WUE. In this experiment, since there was a lack of soil moisture storage before sowing in 2020-2021, the correlation between the yield increase rate and irrigation amount is higher. Moreover, the thousand-grain weight, effective spikes, grains per spike, and yield under different treatments in 2021-2022 are higher than those in 2020-2021. At that time, irrigation was needed to supplement the water demand for the winter wheat development. Under the W1 and W2 treatments, irrigation failed to supplement the water demanded by winter wheat growth and development, resulting in low yields. The W3 treatment supplemented the water required by winter wheat development at the later stage, and the yield increased significantly. However, when the irrigation amount is too high, although the wheat yield has increased, its ET will also increase significantly, and reducing the irrigation amount will lead to a decrease in wheat yield, but the WUE can be improved [34; 35]. [PERSON] et al. [5] found that when the irrigation times increased from 2 to 3, the yield increase was not significant. This study showed that the yield of the W3 treatment was the highest in 2021-2022, but the WUE of the W2 treatment was remarkably higher than that of the W1 and W3 treatments, which was consistent with the above research rules. However, from 2020 to 2021, the yields of the W1 and W2 treatments were too low and the water consumption under the W3 treatment was low, resulting in the highest WUE. [PERSON] et al. [36] found that irrigation at the jointing stage and flowering stage could increase the yield and WUE by increasing the biomass, harvest index, and grain number per spike of winter wheat after anthesis. In this experiment, the average grain number per spike and WUE of the W2 treatment were higher than those of the other irrigation treatments in 2021-2022, and the yield was higher than the W1 treatment. In 2020-2021, the WUE of the W2 treatment was also higher than that of the W1 treatment, which was consistent with the previous pattern. Irrigation affects yield by affecting the thousand-grain weight [24]. In this study, there was a notable and positive correlation between the yield and thousand-grain weight from 2020 to 2021, and there was an extremely remarkable and positive correlation between the yield and thousand-grain weight from 2021 to 2022, indicating that irrigation increased the wheat yield by increasing the thousand-kernel weight, which was in agreement with the previous studies. Due to different factors such as drought resistance, different varieties of winter wheat have different responses to irrigation [34; 37]. The regression analysis between the yield and irrigation amount showed that the yields of Hengmai4399 and Hengmai28 were highly significantly positively correlated with the irrigation amount from 2021 to 2022, and the fitting degree of Hengmai4399 was higher than that of Hengmai4399, but there was no notable linear relationship between the yield and irrigation amount of Hengguan35. The correlation between the yield and irrigation amount of Hengguan35 from 2020 to 2021 was also lower than that of Hengmai4399 (Table 8). The regression analysis between the grain yield increase rate and irrigation amount showed similar results (Figure 4). Hengmai4399 had the most positive response to irrigation, followed by Hengmai28, and Hengguan35 had a minor response. From 2021 to 2022, under the W1 treatment, Hengguan35 had the highest yield, indicating that its drought resistance was the strongest, followed by that of Hengmai28, and that of Hengmai4399, which was the weakest.