Flood-season surface water-groundwater interactions across distinct geomorphic units in a Yellow River Great Bend...

Surface water-groundwater interactions are complex in the Great Bend region of the Yellow River. Clarifying water exchange and source differences among geomorphic units is important for understanding regional hydrological processes and improving water resources management. In this study, the Wulanmulun River Basin, a typical tributary of the Yellow River, was selected as the study area. A total of 90 water samples were collected, including 25 river water samples, 43 groundwater samples, and 22 precipitation samples. Gibbs diagrams, multivariate statistical analysis, PMF, and MixSIAR models were used to investigate the hydrochemical and isotopic characteristics, source contributions, and transformation relationships of surface water and groundwater. The results showed that, under the combined effects of rock weathering dissolution and evaporation concentration, the hydrochemical types of surface water and groundwater in the study area were dominated by Cl-Na and HCO₃-Ca types, respectively. The source contributions of hydrochemical components differed between surface water and groundwater. Groundwater was mainly controlled by geological processes (41.1%), followed by agricultural input (31.3%); surface water was also dominated by geological processes (36.1%), but the influence of domestic input was enhanced (34.1%). In the hilly regions, groundwater discharge mainly recharged surface water, with an average contribution rate of 32.4%. In the plain and built-up areas, surface water infiltration mainly recharged groundwater, with average contribution rates of 34.2% and 25.0%, respectively. These results enhance understanding of surface water-groundwater transformation in the Yellow River's "Great Bend" and support scientific management and efficient utilization of basin water resources.