Study on Lag Law of Irrigation Return Flow Based on Unit Hydrograph and Hydrus

The Yellow River Diversion Irrigation District is a critical area for food security within the river basin; however, a significant contradiction exists between water supply and demand. The lag process of irrigation return flow is crucial for effective water resource management, yet this aspect has been overlooked in existing studies. This research focuses on the east-ern part of the Jingdian Irrigation District, where data related to agricultural hydrology was collected through monitoring efforts. The unit hydrograph method was introduced to construct a model, and numerical simulations were developed using Hydrus-2D to investigate the lag characteristics of irrigation return flow. The findings indicate that the lag time of return flow in response to precipitation and irrigation in the Hongbiliang Basin ranges from 0 to 2.3 months, while in the Nanshahe Basin, it spans from 0 to 5 months. The unit hydrograph model demonstrated high predictive accuracy, with a coefficient of determination (R2) exceeding 0.72 and a mean relative error (MRE) below 11.6% in both basins. The peak lag times recorded were 60 days and 110 days, respectively. The formation of return flow occurs in three stages: soil water infiltration, groundwater recharge, and channel drainage. Additionally, the unit hydrograph exhibited a strong fitting effect on silt loam and other soil types, confirming the validity of the “proportion and superposition” principle. This study contributes to the optimization of the water cycle model and the establishment of a comprehensive system within the irrigation district, thereby aiding in alleviating the pressure on water resources.