Conclusions
This study determined the TTD and Fyw in different landscape units within a karst catchment in Chenqi in Southwest China, using a tracer-aided conceptual hydrological model. The simulated mean Fyw were 0.39, 0.31 and 0.10 for the hillslope, fast flow and slow flow reservoirs, respectively. We also estimated the young water fraction using the sine-wave method based on the daily observations of stable isotopes. Such relatively low-frequency measurements of tracer dynamics fitted by the sine-wave approach significantly underestimate Fywin karst systems marked seasonal variability in sources of runoff generation, and hydrological connectivity between different conceptual stores, were the main drivers of Fyw dynamics in each landform unit. Affected by sinkholes and large fractures, young water recharges the underground channel quickly after heavy rainfall, leading to significant increases in the Fyw of runoff (close to 1). The results of transit time pdfs for three rainfall events representing different catchment wetness conditions indicated that the forward TTD of new water was markedly affected by the water storage of corresponding conceptual stores. Most rainwater is stored in the aquifer (matrix or small fractures) when rainfall events occur during the dry period, and this stored water is released quickly during floods in the wet period, which has an important influence on water quality in the catchment. After heavy rainfall events in the wet season, there is rapid and direct rainfall conversion to underground flow through large fractures and sinkholes. Even though the contribution of young water to runoff is greater at high flows, the old water contribution is generally accelerated as well. Meanwhile, there was strong mixing of the younger rainwater with old stored water during and after the rainfall, decreasing turnover times. However, the tracer-aided model used here is a conceptual, and can only to assess the spatial variations in transit time and water age between the dominant landscape units in complex karst terrain. Consequently, more complex tracer-aided process-based modelling is needed for finer spatial insights.
Data Availability Statement: The data that support the findings of this study are available from the corresponding author upon reasonable request.
Acknowledgments. This research was supported by the UK-China Critical Zone Observatory (CZO) Programme (41571130071), the National Natural Science Foundation of China (41971028, 41571020, 41601013), the National Key Research and development Program of China (2016YFC0502602) and the UK Natural Environment Research Council (NE/N007468/1).