Temporal Dynamics and Factors of Irrigation Water Effective Utilization Coefficient in Humid Agricultural Regions: Evidence from Hunan, China

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.