Optimization of irrigation schedule based on RZWQM model for spring wheat in Shiyang River Basin

The climate of Shiyang River Basin is dry and rainless.The average annual pre cipitation is only 160 mm with the time differences.The optimizing irrigation system has been widely recognized by achieving the water saving.In past researches, the upper limit of water was usually the field capacity, which could lead to the waste of irrigation water.When determining scheming wetting layer depth, they considered effects on crops inadequately.RZWQM(root zone water quality model) had been widely used in studies of the effects of sewage sludge on soil crop systems due to the accurate simulation of farmland water and nutrient cycling.To explore the scheming wetted soil layer depth and irrigation upper limits in different growth period for spring wheat, the field experiments were conducted in Shiyang River Basin of the Gansu province in 2014.According to the status of crop water requirement and crop growth characteristics, we set different irrigation upper limits and scheming wetted soil layer depth at different stages.We irrigated fields with border irrigation.The irrigation upper limits(expressed as field capacity) were set to 100%, 95%, 90%, 85% and 80%.In seedling stage, the scheming wetted soil layer depths were set to 30 cm, 35 cm, 40 cm, 45 cm and 50 cm.In jointing stage, the scheming wetted soil layer depth were set to 40 cm, 45 cm, 50 cm, 55 cm and 60 cm.In heading stage and filling stage, the scheming wetted soil layer depths were set to 50 cm, 55 cm, 60 cm, 65 cm and 70 cm.The experiments were adopted orthogonal design and chosen L₂₅ (55) orthogonal tables.All irrigation threshold was 65% of field capacity.The field experiments were chosen A1B1C1D1E1, A1B3C3D3E3, A1B5C5D5E5 to repeat 3 times.The field measured data was used to calibrate and verify the model.The RMSE of each layer of soil water content was not greater than 0.03%.The mean deviation of soil water content in five layers was 12.37%.The deviation of yield was relatively stable, which was around 20%.The results showed that RZWQM could be used to simulate the change of soil water content and growth of crop.Then the model was used to evaluate the effect of various irrigation upper limits and scheming wetted soil layer depth in different growth period on crop yield, irrigation amount, water use efficiency and the frequency of irrigation.The irrigation amount of A1B4C4D4E4 was the most, which was 431.1 mm.The irrigation amount of A5B3C2D1E5 was the least, which was 323.7 mm.The difference between A1B4C4D4E4 and A5B3C2D1E5 was 125 kg/hm².The yield in different irrigation treatments was not obviously changed compared with the needed irrigation amount so we could save water on the premise of not decreasing yield.The effect of the irrigation upper limit on irrigation amount was more remarkable than that of crop yield.The reducing of irrigation upper limit could increase the irrigation frequency to improve the yield of spring wheat.The differences of GIWUE were from the differences of irrigation amount.The GIWUE of A5B3C2D1E5 was the most, which was 2.48 kg/m3.The GIWUE of A1B4C4D4E4 was the least, which was 1.84 kg/m3.The difference between A5B3C2D1E5 and A1B4C4D4E4 was 0.64 kg/m3.The appropriate scheming wetted soil layer depth could effectively improve the utilization efficiency of irrigation water and achieved the purpose of water saving.Through regulating irrigation upper limit and scheming wetted soil layer depth in different growth period, we could save water and improve the yield of spring wheat.Considering the influence of the control factors on the yield, irrigation amounts and GIWUE, we choose 80% of soil moisture as irrigation upper limit, and the optimal scheming wetted soil layer depths are 30 cm, 60 cm, 50 cm and 70 cm at seeding, jointing, heading and filling stages, respectively.