Study on the relationship between surface water and groundwater transformation in the middle and lower reaches of Songhua river basin.

To precisely delineate the transformation dynamics across several temporal and spatial scales in the middle and lower portions of the Songhua River basin in Heilongjiang Province, China, characterized by numerous irrigation zones and frequent transitions from surface water to groundwater. The SWAT-MODLFOW model, developed on the QSWATMOD2 platform, was validated using river runoff data and groundwater level observations. Consequently, the delay of precipitation on groundwater levels and the correlation between surface water and groundwater dynamics in the middle and lower portions of the Songhua River basin were modeled. The findings indicate: (1) The coefficient of calibration and Nash coefficient for monthly runoff during the calibration and validation periods yielded R ≥ 0.86, NSE ≥ 0.87, R ≥ 0.76, and NSE ≥ 0.77, respectively. Furthermore, the discrepancy between the simulated and actual groundwater table values is within 0.6 m, with R² values of 0.97 and 0.98 for the periodic and verification phases, respectively. The simulation outcomes of this test model are satisfactory and fulfill the criteria for scientific research. (2) The groundwater level in the research area typically declines from west-northwest to northeast and from south to east. The groundwater level lag time influenced by precipitation in Jiamusi, Fujin, and Tongjiang cities is around 10.56 days, 10.58 days, and 3.15 days, respectively. (3) The river channels facilitating surface water recharge to groundwater constitute 41.75% of the entire length of the Jiamusi-Tongjiang stretch of the Songhua River, with the annual average recharge representing 50.84% of the total exchanged water. Seasonally, the peak recharge value for each river section occurred in August, while the lowest recharge value was recorded in April. The peak recharge happened in 2009, while the lowest recharge was in 2014. The contribution of groundwater to surface water exhibits significant fluctuations, with seasonal variance between - 52% and 55%, and inter-annual variation between - 35% and 52%.