Improving the irrigation water effective utilization coefficient (IWEUC, η) is important for improving agricultural water management and reducing pressure on agricultural water resources. However, in humid agricultural regions, the main factors associated with observed IWEUC improvement remain uncertain. They could be either interannual climate variability or structural improvements in irrigation engineering and management. Using measurement data from 202 representative irrigation districts in Hunan Province, China, from 2014 to 2022, this study combined the head–tail measurement method, comparative trend analysis, and correlation analysis to distinguish climate-related variability from structural change. Province-wide η increased from 0.4884 to 0.5502, and differences among large, medium-sized, and small districts narrowed over time. Structural differences also remained evident. Pumped systems generally showed higher efficiency and faster improvement than gravity-fed systems, and medium-sized gravity-fed districts remained the main low-efficiency category. No stable significant relationships were found between η and hydrometeorological indicators, whereas water-saving investment showed a positive association with η, with the strongest positive association observed in medium-sized gravity-fed districts. These results suggest that IWEUC improvement in humid agricultural regions is more closely associated with structural modernization related to engineering investment and management improvement than with annual hydrometeorological variability.

The intricate interplay among soil physicochemical properties, root physiological traits, and flowering gene expression fundamentally influences rice reproductive success and grain yield. This study elucidates the effects of the Aerobic and water-controlled irrigation (AWCI) method on rhizosphere soil parameters, root biochemical responses, pollen viability, and temporal expression of the circadian-related flowering gene OsFKF1 during the heading and flowering stage, illuminating their integrated impact on rice productivity. Application of the AWCI regime modulated soil pH and nitrogen dynamics at critical growth intervals, enhanced rhizosphere oxygen availability, and shifted redox potential, thereby contributing to nutrient bioavailability. Simultaneously, AWCI treatment influenced root antioxidant enzyme activities-including peroxidase (POD) and catalase (CAT)-as well as abscisic acid (ABA) and MDA concentrations, which positively correlated with elevated pollen viability and upregulated OsFKF1 expression. Multivariate analyses identified key determinants of yield enhancement, notably augmented root surface area during the jointing-booting phase, balanced soil nitrogen content, and finely regulated oxidative stress markers at the mid-tillering stage. Hierarchical clustering robustly designated the T3 treatment as a promising candidate AWCI protocol for maximizing reproductive performance and grain yield. Collectively, these findings underscore the pivotal nexus among irrigation management, soil biochemical milieu, root physiology, and floral gene regulation in modulating rice yield, offering a theoretical foundation for precision water management strategies tailored to sustainable productivity enhancement. Future research should extend to encompass additional flowering-related genes and a broader spectrum of rice cultivars to generalize these mechanistic insights under multiple abiotic stresses, including heat and drought combination.