Abstract
AbstractHydrological and biogeochemical processes in karst environments are strongly controlled by heterogeneous fracture‐conduit networks. Quantifying the spatio‐temporal variability of water transit time and young water fractions in such heterogeneous hydrogeological systems is fundamental to linking discharge and water quality dynamics in the karst critical zone. We used a tracer‐aided conceptual hydrological model to track the fate of each hour of rain input individually. Using this approach, the variability of transit time distributions and young water fraction were estimated in the main landscape units in a karst catchment of Chenqi in Guizhou Province, Southwest China. The model predicted that the mean young water (i.e., <~2 months old) fraction of ground conduit flow is 0.31. Marked seasonal variabilities in water storage and hydrological connectivity between the conduit network and fractured matrix, as well as between hillslopes and topographic depression, drive the dynamics of young water fraction and travel time distributions in each landscape unit. Especially, the strong hydrological connectivity between the land surface and underground conduits caused by the direct infiltration through large fractures and sinkholes, leads the drastic increasement in young water fraction of runoff after heavy rain. Even though the contribution of young water to runoff is greater, the strong mixing and drainage of small fractures accelerate the old water release during high flows during the wet season. It is notable that the young water may sometimes be the most contaminated component contributing to the underground conduit network in karst catchments, because of the direct transfer of contaminants from the ground surface with rain water via large fractures and sinkholes.
Highlights
Karst regions cover 12% of the Earth's surface and are the main source of potable water for more than 25% of the world's population (Ford and Williams, 2013)
The results show that the Transit time distributions (TTD) of infiltrating rain water is affected by the discharge
The results indicate that most of water entering in dry periods is stored in the catchment for a long time with high mean transit time
Summary
Karst regions cover 12% of the Earth's surface and are the main source of potable water for more than 25% of the world's population (Ford and Williams, 2013). The model was based on a generic structure initially developed to simulate catchment-scale water and solute transport (Mg and Ca) for at different scales in the karst critical zone (Zhang et al, 2017).The structure was subsequently improved by sub-dividing the landscape into hillslope and depression landscape units, and stable isotopes were added to simulate the flow and conservative tracer dynamics in karstic critical zones (Zhang et al, 2019) This improved tracer-aided model can track the water ages in different landscape units, which links hydrological connectivity to water storage. Linear and exponential relationships between the reservoir storage V and flow discharge Q are used to calculate the discharge for slow/fast reservoirs in depression and hillslope, respectively A total of 892 rainfall hours during the study period of November 11, 2016 to October 31, 2017 were separately tracked
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