A novel method to optimize the limited irrigation schedule for cotton: HYDRUS-triggered irrigation simulation and plant water deficit index threshold analysis

Optimizing irrigation at the key growth stages of crops is essential for solving agricultural water shortages and improving the water-use efficiency. Using cotton from northern Xinjiang, China, as the research subject, this study proposes a novel method for optimizing cotton drip irrigation systems under mulch conditions, integrating field experiments with numerical simulations. A reliable water deficit index (PWDI) estimation method for cotton under mulched drip irrigation was established and the response characteristics of key cotton photosynthetic indicators to PWDI were analyzed, revealing appropriate PWDI thresholds for the cotton squaring (0.48) and boll stages (0.52). Based on HYDRUS, a soil water transport model for drip irrigated cotton under film mulch was constructed to simulate the trigger irrigation scenario and the PWDI variation characteristics during the growth period under 25 different irrigation schemes were characterized. When the cotton irrigation quota in northern Xinjiang was limited to 420–480 mm, the lower limit of irrigation at the squaring and boll stages should be controlled between 65 % and 70 % and 50 % and 55 % field capacity, respectively, to minimize water-stress days during key growth stages. The proposed method is promising for the optimal matching of cotton water consumption processes and irrigation systems under conditions of limited water supply and has reference value for optimizing water consumption allocation under limited water supply in water shortage areas. There is still room for improvement in the accuracy of this method for crop irrigation system optimization, particularly via the increased accuracy of PWDI estimation. • The accuracy of estimating PWDI based on root-weighted soil moisture was verified. • The PWDI threshold of the key growth period of cotton was clarified. • New irrigation optimization: Coupling HYDRUS simulation & PWDI threshold analysis. • Optimal cotton irrigation schedule in water-scarce Xinjiang, China, was proposed.