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

Zhicai Zhang*, Xi Chen, Qinbo Cheng, Chris Soulsby

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

11 Citations (Scopus)


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.

Original languageEnglish
Article number126785
Number of pages14
JournalJournal of Hydrology
Early online date6 Aug 2021
Publication statusPublished - 1 Nov 2021

Bibliographical note

Funding Information:
This research was supported by the National Natural Science Foundation of China ( 41971028 , 42030506 ), the National Key Research and development Program of China ( 2016YFC0502602 ), and the UK Natural Environment Research Council MIDST-CZ Project ( NE/S009167/1 ). And the SAS model program used in this study is modified from the original code reported by Paolo Benettin and Enrico Bertuzzo (2018). In addition, we thank the three anonymous reviewers and the editor for their constructive comments which significantly improved the manuscript.


  • Flow paths
  • Flux age distribution
  • Karst groundwater system
  • SAS function
  • Stable isotope


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