The structure of the water cycle and recharge mechanisms in the groundwater-influenced Hongjiannao Basin

Study region Hongjiannao Basin, an arid–semi-arid terminal-lake basin on the southeastern margin of the Mu Us Desert, China. Study focus This study investigates seasonal water-cycle structure, recharge relationships, and hydrochemical evolution in the Hongjiannao Basin. Hydrochemistry, stable isotopes (δ¹⁸O and δ²H), mineral saturation indices, and Bayesian isotope mixing analysis using MixSIAR were integrated to quantify water-source contributions and identify groundwater–river–lake connectivity and the effects of evaporation and seasonal freezing. New hydrological insights for the region Groundwater contributed 76.2%, 95.3%, 88.6%, and 59.1% of river water in April, July, October, and January, respectively, demonstrating its key role in sustaining river water across seasons, although winter estimates were more uncertain. The basin exhibits asymmetric hydrological connectivity, characterized by strong groundwater–river interaction, river-mediated transfer toward the terminal lake, spatially restricted direct groundwater–lake exchange, and strong evaporative modification of lake water. Evaporative concentration, coupled with water–rock interactions, drives pronounced solute enrichment in the terminal lake. During seasonal freezing, isotopic fractionation and solute exclusion at the ice–water interface produce relatively low-salinity ice and solute-enriched under-ice water. Together, these processes show that groundwater recharge, evaporation, and freezing jointly regulate the seasonal hydrological and hydrochemical evolution of this arid terminal-lake system.