Layered control of stream-groundwater interactions: Insights into regional hydrological connectivity in China's Loess Plateau

Streamwater-groundwater interaction (SGI) plays a critical role in the exchange of water, energy, and contaminants in the terrestrial water cycle system. While shallow groundwater has traditionally been considered the primary contributor to streamflow, recent evidence suggests that deep fossil groundwater, which predates the Holocene, also discharges into rivers. However, the mechanisms driving SGI across different geological layers remain largely unknown. Here we examined SGI across five large watersheds (7,636-60,916 km2) in China's Loess Plateau (CLP), using 651 streamwater and groundwater samples collected during dry and wet seasons and analyzed for isotopic and hydrochemical indicators. Our results revealed clear distinctions in the interactions between river water and shallow loess pore groundwater (LPG) or deep fissure groundwater (DFG). LPG exhibited one-way connectivity to streamwater, with an average groundwater discharge ratio of 24 %, whereas a two-way recharge and discharge system exist between DFG and rivers, with an average discharge ratio of 19 %. Both groundwater systems showed higher discharge ratios during the wet season than the dry season. Spatially, discharge ratios from LPG and DFG were lower in central CLP but increased towards the north and south. Loess thickness and geological formations primarily govern these patterns. While LPG primarily influences localized water exchange dynamics, DFG drives regional hydrological connectivity across multiple watersheds. Our findings provide new insights into the stratigraphic mechanisms controlling SGI, offering targeted strategies for sustainable water resource management in the CLP and similar regions.