Optimization of brackish water proportions for drip irrigation in arid regions of China: Field experiments and a crop growth model accounting for integrated salinity and root stress

Freshwater scarcity and soil salinization limit maize production in arid and semiarid irrigation districts, making the controlled use of brackish water necessary but risky. This study evaluated whether sequential drip irrigation with brackish water followed by freshwater could reduce freshwater use while maintaining yield and limiting root zone stress. A two year field experiment was conducted in the Hetao Irrigation District, China, with five brackish water proportions: 0%, 25%, 50%, 75%, and 100%. Each treatment received 11 irrigations of 22.5 mm, with brackish water applied first according to the prescribed proportion, followed by freshwater. Soil salinity, root length density ( RLD ), maize growth, and grain yield were measured. A salinity-root integrated stress index ( SRSI ( t )) was developed by integrating root weighted effective salinity stress and relative total RLD loss into a Logistic crop growth model. Increasing the brackish water proportion intensified salt accumulation mainly in the 0 −40 cm root zone, where RLD decreased by 12.70–36.14%. Although deep layer RLD increased slightly at filling, total RLD in the 0–100 cm profile decreased by 13.80–39.28%. Mean SRSI (t) increased by 34.11 −100.22%. The model satisfactorily reproduced maize growth dynamics, with a mean normalized root mean square error ( NRMSE ) of 8.22%. Scenario simulations showed that mean yield decreased by 20.49% from 0% to 100% brackish water. Considering yield maintenance and salinity-root integrated stress, the brackish water proportion should not exceed 70%, while 60–70% represents a practical range that can reduce seasonal freshwater use by 148.5–173.3 mm compared with only freshwater irrigation.