Enhanced surface-groundwater interaction modeling in the middle and lower reaches of the Songhua River Basin using a coupled SWAT-MODFLOW model

The management of groundwater resources and the rehabilitation of groundwater levels in the middle and lower portions of the Songhua River basin have consistently garnered significant attention in our country. The SWAT-MODLFOW model, developed on the QSWATMOD2 platform, was calibrated and validated utilizing river runoff and groundwater observation data to precisely illustrate the transformation relationship across various spatial and temporal scales in the middle and lower reaches of the Songhua River basin, characterized by numerous agricultural and irrigation zones with frequent surface water conversion. The water cycle process in the middle and lower sections of the Songhua River basin is simulated and studied based on this foundation.The results show: (1) The SWAT-MODLFOW coupling model has a good simulation effect, and the simulation effect of menstrual flow in the periodic and verification periods is R ² ≥0.86, NSE≥0.87, R ² ≥0.76, NSE≥0.77, respectively. The simulated groundwater level and the actual error value are within 0.6m, and the R ² in the periodic and verification periods are 0.97 and 0.98, respectively. The simulation results of the model are satisfactory and meet the requirements of scientific research. (2) The groundwater in the study area generally decreases in the direction of west-north to northeast, and in the direction of east-south to north, and the groundwater level is affected by precipitation. Jiamusi, Fujin and Tongjiang, three major cities in the study area, are selected for characteristic study, and the lag time of their groundwater level to precipitation is about 10.56d, 10.58d and 3.15d. (3) The river channels of surface water recharge groundwater occupy 41.75% of the total length of Jiamusi - Tongjiang section of Songhua River, and the annual average recharge accounts for 50.84% of the total exchange water; On the seasonal scale, the maximum recharge value of each river section appeared in August, and the minimum recharge value appeared in April. On an annual scale, the maximum recharge occurred in 2009 and the minimum in 2014. The supply of groundwater to surface water fluctuates obviously, with seasonal variation ranging from -52% to 55% and inter-annual variation ranging from -35% to 52%.