Optimization of drip irrigation scheme for mature walnut in arid areas of Xinjiang, China

Scientific and reasonable irrigation schedules are the keys to improve the efficiency of irrigation utilization. In this study, the reliability of HYDRUS model combined with optimization model proposed used for irrigation scheme optimization was investigated. The arid area of southern Xinjiang, China was taken as the research area. In this area, the groundwater depth was target than 6 m. The soil dry bulk density was 1.39 g/cm3. The experimental data in 2018 and 2019 were used for model calibration and verification, respectively. In the experiments of both years, the irrigation quota was same as 45 mm and the irrigation norm was 360 mm. The irrigation stages were same. The walnut tree was eleven years old. It was drip irrigated every year. It started new growth period in April and was harvested in August. The soil water content was measured. In addition, the leaf area index was calculated and root-related indexes were determined. Meteorological parameter values were obtained. In this study, the rainfall amounts in 2018 and 2019 during the whole walnut growth stage were 133.3 and 24.8 mm, respectively. The years of 2019 and 2018 were respectively dry and wet years based on multi-year rainfall data. A total of 128 irrigation schedules were designed and they included eight irrigation quota (30-65 mm) and 16 irrigation intervals (5-20 d). The deep leakage and water stress under the 128 irrigation scheme were simulated by using HYDRUS-2D model. An optimization model was proposed. In this model, the target value for optimization was the difference between the amount of deep leakage and absolute of crop water stress. The values of amount of deep leakage and crop water stress both could be obtained by model simulation. By simulation, the irrigation scheme with small target value for optimization were considered to be optimal. The model calibration and verification results showed that the model accuracy was high with root mean square error of 0.016-0.017 cm3/cm3 and R2 of 83.03%-83.73%, which indicated that the model was well in simulating soil water content in the field of walnut of Xinjiang regardless of wet or dry years. Under the condition of the irrigation quota 45 mm and irrigation intervals of 8-20 days, the water consumption and the daily average water consumption intensity of mature walnut during its whole growing stage in Southern Xinjiang were 634.15 mm and 5.51 mm/d. The daily average water consumption intensity was the highest during the oil transformation stage (6.87 mm), followed by the hard core stage (6.14 mm), fruit expanding stage (5.29 mm), and flowering-fruiting stage (3.72 mm). During the whole growth stage, the total water stress was -25.57 mm, and the total deep leakage was 109.75 mm. It accounted for 21.33% of the total water consumption and exceeded the 20% of the total water consumption. Soil water flux in different irrigation scheme of mature walnut under drip irrigation was simulated by using the HYDRUS-2D model. Then, the optimal irrigation scheme was screened by using the optimization model. By the optimization model, the lowest target value for optimization indicated small water loss. Thus, two irrigation scheme were recommended: 1) the irrigation quota was 35 mm, irrigation intervals was 9 days, the irrigation times were 11 and the irrigation norm was 385 mm; 2) the irrigation quota was 50 mm, irrigation intervals was 14 days, the irrigation times were 7 and the irrigation norm was 350 mm. This study provided an effective way to formulate irrigation scheme of mature walnut under drip irrigation in Southern Xinjiang. © 2020, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.