Optimizing the irrigation schedule for winter wheat-summer maize using APEX model

Water shortage has been one of the most significant constraints on grain production in the North China Plain. In this study, the irrigation schedule was optimized to enhance the efficiency of water use during groundwater extraction. A crop growth model was also developed using the APEX (Agricultural Policy Environmental eXtender) model. The "winter wheat - summer maize" rotation system was selected in Handan City, Hebei Province, China. The water dissipation was optimized for the irrigation schedule in farmland. Nine treatments of irrigation were set (irrigation quotas for the winter wheat ranging from 120 to 280 mm and for the summer maize from 0 to 160 mm). A series of field tests were carried out to validate the model. Some parameters were measured from the six experimental sites from 2022 to 2023, including the soil moisture content, crop yield, and plant height. The meteorological data was also collected from 2004 to 2023. The optimal schedules of irrigation were determined for the winter wheat and summer maize under the multiple years on average. The yield and water use efficiency (WUE) were taken as the indicators. The results show that: 1) The simulated content of soil moisture, crop yield, and plant height of the "winter wheat - summer maize" model with the APEX model were relatively close to the measured ones. The root mean square errors (RMSE) of yield were only 0.13 t/hm² and 0.16 t/hm², respectively, indicating the better performance for the growth and water use of winter wheat and summer maize; 2) A comparison was also made on the yields and water use efficiencies of the irrigation treatments. The appropriate irrigation quota was 200 mm for the winter wheat under the multi-year average, with a yield of 7.14 t/hm² and a WUE of1.78 kg/m³. In summer maize, the appropriate irrigation quota was 120 mm, with a yield of 9.05 t/hm² and a WUE of 2.78 kg/m³; 3) The total demand for irrigation water was 1.121×10⁹ m³ after optimization, which was reduced by 1.49×10⁸ m³, compared with the current. Excessive extraction of groundwater was effectively alleviated after optimization; 4) In water dissipation, the inter-plant evaporation of the winter wheat and summer maize accounted for a large proportion of the total evapotranspiration, 31.7%-35.2% and 23.8%-29.5% respectively, under the current furrow irrigation mode. Drip irrigation, subsurface irrigation, and straw mulching can be expected to control soil evaporation for the high efficiency of irrigation water use. An optimal irrigation schedule was achieved suitable for the multi-year average of the winter wheat and summer maize. As such, the WUE was enhanced to control the irrigation water consumption for the less exploitation of groundwater. The findings can also provide scientific suggestions to reduce the exploitation of groundwater.