Towards hydrological connectivity in the karst hillslope critical zone: Insight from using water isotope signals

•Runoff connectivity on a karst hillslope are identified by isotopic signals.•The proportion of young water and event water decreases from upslope to downslope.•Runoff from the upslope is connected to the downslope by the soil–bedrock interface.•Soil-epikarst structure controls the hydrological connectivity of karst hillslopes.