Modeling Sunflower Root Water Uptake Under Soil Water and Salinity Conditions Across Soil Depths

This study aims to quantify the response of sunflower root water uptake to stratified soil water and salinity stress. Based on field observations, the root water uptake function in the existing model was improved by developing a new equation for the root water uptake rate that accounts for spatial differences in root response. Field experiments were conducted in 2021 and 2022 using irrigation water with four salinity levels: CK (0.87 g/L), S1 (1.0 g/L), S2 (1.5 g/L), and S3 (2.0 g/L). Soil moisture and salinity in five soil layers (0–100 cm) were continuously monitored using sensors. The actual crop water requirement (ETa) was estimated using the soil water balance method, while the actual (Ta) and potential (Tp) plant transpiration rates were calculated based on the canopy-scale water consumption principle. Results indicated that with increasing irrigation water salinity, both soil moisture content and electrical conductivity exhibited an overall increasing trend. Significant differences were observed in the combined soil moisture and salinity conditions across soil depths. In particular, salt accumulation in the surface layer reduced root water uptake in the upper soil profile. Based on the differential root response to soil water and salinity stratification, the root water uptake function was further optimized, and the parameters representing water and salinity conditions in each soil layer were calibrated using the least squares method. Model validation with 2021 and 2022 data demonstrated good agreement between simulated and observed Ta values, with RMSE = 11.41 mm and MRE = 0.32%, R2 ranging from 0.66 to 0.98, NSE between 0.52 and 0.96, and regression slope b between 0.90 and 1.10. This enhancement in the root water uptake rate formulation significantly improves model simulation accuracy and provides a robust basis for optimizing irrigation management in saline–alkali environments.