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).