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Storage dynamics, hydrological connectivity and flux ages in a karst catchment: conceptual modelling using stable isotopes

Abstract. We developed a new tracer-aided hydrological model that disaggregates cockpit
karst terrain into the two dominant landscape units of hillslopes and
depressions (with fast and slow flow systems). The new model was calibrated
by using high temporal resolution hydrometric and isotope data in the outflow
of Chenqi catchment in Guizhou Province of south-western China. The model
could track hourly water and isotope fluxes through each landscape unit and
estimate the associated storage and water age dynamics. From the model
results we inferred that the fast flow reservoir in the depression had the
smallest water storage and the slow flow reservoir the largest, with the
hillslope intermediate. The estimated mean ages of water draining the
hillslope unit, and the fast and slow flow reservoirs during the study
period, were 137, 326 and 493 days, respectively. Distinct seasonal
variability in hydroclimatic conditions and associated water storage dynamics
(captured by the model) were the main drivers of non-stationary hydrological
connectivity between the hillslope and depression. During the dry season,
slow flow in the depression contributes the largest proportion (78.4 %)
of flow to the underground stream draining the catchment, resulting in weak
hydrological connectivity between the hillslope and depression. During the
wet period, with the resulting rapid increase in storage, the hillslope unit
contributes the largest proportion (57.5 %) of flow to the underground
stream due to the strong hydrological connectivity between the hillslope and
depression. Meanwhile, the tracer-aided model can be used to identify the
sources of uncertainty in the model results. Our analysis showed that the
model uncertainty of the hydrological variables in the different units relies
on their connectivity with the outlet when the calibration target uses only
the outlet information. The model uncertainty was much lower for the
“newer” water from the fast flow system in the depression and flow from the
hillslope unit during the wet season and higher for “older” water from the
slow flow system in the depression. This suggests that to constrain model
parameters further, increased high-resolution hydrometric and tracer data on
the internal dynamics of systems (e.g. groundwater responses during low flow
periods) could be used in calibration.
