Hydroclimatic Coupling and Its Linkage with Baseflow Dynamics in the Karst Catchments of Southern China

Baseflow sustains streamflow during dry periods and provides hydrological support for low-flow conditions, but its monthly partitioning can vary with coupled hydroclimatic conditions in karst catchments. This study examined monthly baseflow index ( BFI ) dynamics in 8 gauged karst catchments in southern China during 2012 to 2019. Baseflow was separated from original daily discharge records using the Eckhardt recursive digital filter, and monthly BFI was checked against the Lyne–Hollick filter, an Eckhardt parameter-sensitivity grid, and a hydroclimatic coupling index ( HCI ) logistic shape sensitivity analysis. Relationships between BFI and precipitation ( P ), potential evapotranspiration ( PET ), temperature ( T ), soil moisture ( SM ), the humidity index ( HI ), and HCI were then evaluated. HCI is treated as a process-informed empirical descriptor of coupled supply–demand–wetness conditions, rather than as a strict water-balance equation. The logistic shape parameter was fixed at k = 10 before model evaluation; sensitivity checks with k = 5 and k = 15 produced highly correlated HCI series. Results show clear intra-annual BFI variability and catchment-dependent associations with hydroclimatic predictors. Adding HCI improved prediction in some catchments, whereas the Base predictor set or Base + HI performed as well or better in others, and model skill remained weak in the Erlangba and Leigongtan catchments. These findings indicate that the added value of HCI is conditional on catchment context. The results represent statistical linkages with baseflow-related low-flow support, not evidence for causal mechanisms or ecological endpoints.