Abstract
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.
Highlights
Karst aquifers are characterized by complex heterogeneous and anisotropic hydrogeological conditions which are very different to most other geological formations (Bakalowicz, 2005; Ford and Williams, 2013)
The model results show that the discharge and isotope dynamics were mostly bracketed by the simulation ranges at the outlet, though some peak discharges were underestimated (Fig. 4)
We significantly enhanced a catchment-scale flow–tracer model for karst systems developed by Zhang et al (2017) by conceptualizing two main hydrological response units: hillslope and depression, each containing fast and slow flow reservoirs
Summary
Karst aquifers are characterized by complex heterogeneous and anisotropic hydrogeological conditions which are very different to most other geological formations (Bakalowicz, 2005; Ford and Williams, 2013). The hydrological function of the critical zone in cockpit karst landscapes is dominated by the strong influence of this unique geomorphology and the structure of carbonate rocks. Z. Zhang et al.: Storage dynamics, hydrological connectivity and flux ages karst areas as the complex subsurface hydrogeology results in frequent and abrupt changes in hydrological connectivity. Understanding hydrological connectivity dynamics can provide key insights into the dominant processes governing water and solute fluxes (Lexartza-Artza and Wainwright, 2009). In the south-western karst area of China, the cockpit karst terrain encompasses flow paths sequencing in runoff generation from “hillslope to depression to stream”. Reaney et al, 2014), and the subsurface flow connections between the fractures and conduits at any landscape units The hydrological connectivity between these units is related to the catchment topographic features that affect water transmission (including slope length, gradient and flow convergence, e.g. Reaney et al, 2014), and the subsurface flow connections between the fractures and conduits at any landscape units
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