Using StorAge Selection (SAS) functions to understand flow paths and age distributions in contrasting karst groundwater systems

•Flux age in karst groundwater system was tracked using SAS framework.•Flux age provides a basis for conceptualizing flow path actions in karst catchment.•Critical zone structure affects discharge selection preferences of karst catchment.•Karst system preferentially release younger water to evapotranspiration. Karst catchments have complex flow paths dynamics due to the unique, marked spatial heterogeneity of karst groundwater systems. We used the StorAge Selection (SAS) function framework to explore variations in age distributions of outflow and evapotranspiration from a small karst catchment in southwest China, as well as two hillslope spring systems within it. Daily hydrogen and oxygen stable isotopes (δD and δ18O) in precipitation and at corresponding discharge points were measured for 3 years in order to calibrate the model parameters. The seasonal variations in flux age of the three karst groundwater systems are similar, which indicates the generally strong hydrological connection between hillslope and depression at seasonal scales. The age distribution of outflow shows a “high-low–high” variation pattern in the whole catchment while a “high-low-low” pattern in hillslope system at the beginning of the wet season. In addition to sinkholes in depression, the distributions of soil and epikarst relating to the system storage capacity, resulting in variations of flow paths under different wetness conditions, is likely the main control on the age distributions in runoff for different karst system. The discharge selection preferences generally shift from for younger water in wet season to for older water in dry season in the whole catchment, while that for younger water in both wet and dry seasons in hillslope system. The dynamics of discharge selection preferences can be linked to the critical zone structure of karst underground system. The results in this study show that both hillslope and whole catchment preferentially release younger water to evapotranspiration in the study area.