Optimal Irrigation Schedule For Winter Wheat In The Guanzhong Irrigation District Based On DSSAT Model

In this study, the CropWat, which calculates crop water demand and effective rain, and the DSSAT (Decision Support System for Agrotechnology Transfer) model, which can simulate crop yield response to different irrigation schedules, has been combined. The CropWat model was used to calculate water demand of winter wheat and relative effective precipitation during its growing season. Based on the calculated result, irrigation schedules were designed. The DSSAT model was used to simulte the effects of different irrigation schedules on winter wheat yield, water use efficiency, soil evaporation, and crop transpiration in the Guanzhong Irrigation District. Then the amount of water demand of winter wheat at different growing stages and the critical irrigation stage were determined through the analyses of effect of different irrigation quota at different crop growth stages with respect to the variables above. Finally, with comparisons of winter wheat yield, water use efficiency, and crop evapotranspiration, the high water yield and water use efficiency intervals have been determined. Based on intervals, the ultimate irrigation system was determined. The main results and conclusions have been drawn from this study as follows:1. The CropWat-DSSAT model simulation results had good consistency with previous experiment data, which indicated that it was feasible to calculate crop water demand and simulate the effects of different irrigation schedules on crop yields with the CropWat-DSSAT model. After the comparison of simulated and observed values, the results showed that DSSAT could precisely simulate the soil water balance and winter wheat phenology. Hence it could be used to simulate potential productivity of winter wheat with water limitation.2. In the past30years, the average water demand of winter wheat during its growing season was451.6mm and the average effective precipitation was203.2mm. The ratio between water supply and demand was less than50%. Precipitation and crop water demand were relatively consistent during the growing seasons. Both of them were slightly higher in the seeding stage and lower in the wintering period. They then increased sharply in the reviving-jointing stage, and finally reduced during the harvest. As for supply-demand ratio, it was at a low level at the stages of wintering and jointing-flowering, which indicated winter wheat suffered severe shortage of water. Because water demand in the wintering period was relatively low, the jointing-flowering stage was the most arid stage in the whole growth season of winter wheat. The best irrigation time was around150days after planting or the reviving stage, during which a given depth of irrigation increased the yield most. An irrigation of30-60mm at one time was advised. The real irrigation depth should be adjusted according to soil moisture and precipitation.3. To obtain the optimal irrigation schemes for different precipitation conditions, the years of2000-2001,1980-1981and2009-2010were chosen as typical of dry year, normal year and wet year in the past30years through P-Ⅲ curve, respectively. The research results showed that winter wheat yield, soil evaporation, crop transpiration, and water use efficiency (WUE) in wet year were slightly larger than those in normal year. Both of them were significantly larger than those in dry year. However, marginal benefit showed normal year> dry year> wet year.4. After analyses of the effects of irrigation at different growing stages on crop yield, soil evaporation, crop transpiration, and WUE, it could be found that irrigation at reviving stage had the most significant effect on above variables, followed by irrigations at jointing stage, and wintering stages. The irrigation at grain-filling stage had the relative smaller effects, which illustrated that wheat reviving was the critical irrigation stage. The reviving irrigation should be endured under the conditions of water shortage. Then the grain-filling and wintering irrigations could be considered to cancel under conditions of good soil moisture. This is because winter wheat grows rapidly and needs larger amount of water during the reviving stage. So there would be direct impact on crop leaves and lead to crop yield decline due to impacted photosynthesis, unless sufficient irrigation was conducted timely. The jointing-flowering stage was the stage winter wheat needed the maximum water, but precipitation also increased correspondingly during this period. So compared to the reviving stage, the jointing-flowering stage was secondly important.5. Through analysis of256irrigation schedules, it was found that a125-mm irrigation was enough (reviving water75mm+jointing water50mm) in dry year, a75-mm irrigation was advised (reviving water75mm) in normal year, and a50-mm irrigation was reasonable (reviving water25mm+jointing water25mm) in wet year.