Characterizing the variability of transit time distributions and young water fractions in karst catchments using flux tracking

Abstract

Hydrological 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.

Optimal Transport for Assessing Nitrate Source‐Pathway Connectivity

“…Likewise, in our system, manure and sewage inputs from failing subsurface septic systems are directly injected into the epikarst and entirely circumvent surface pathways, such as runoff (Katz et al, 2010). Once material enters the epikarst, it can be stockpiled until hydrologic connectivity is restored and pathways are rewetted at which point considerable export occurs (Tritz et al, 2011; Z. Zhang, Chen, Cheng, & Soulsby, 2020) and can result in peak stream concentrations of nitrate (Husic, Fox, Adams, Backus, et al, 2019). Regarding the quick and slow pathways, representing runoff and baseflow, respectively, fertilizer likely dominates the quick pathway loading because crops in the region are often grown on gently rolling hills, which are suitable for row cropping, but also are susceptible to sinkhole formation in local depressions (Boyer & Pasquarell, 1995).…”

“…Loadograph separation only requires nitrate concentration and discharge, which are commonly becoming ubiquitous, particularly with the widespread use of aquatic nitrate sensors that provide continuous estimates of nitrate concentration (Burns et al, 2019). Further, with regard to nitrate sourcing, stable isotopes are commonly employed (Lutz et al, 2019; H. Zhang, Kang, et al, 2020; Wang et al, 2020; Z. Zhang, Chen, Cheng, Li, et al, 2020; Z. Zhang, Chen, Cheng, & Soulsby, 2020) but sources have also been unmixed with elemental concentration data alone (Katz et al, 2011; Yue et al, 2017). For example, the relationship between nitrate and chloride concentration provides a simple way to estimate whether nitrate is derived from wastewater or agriculture origin (Chen et al, 2009).…”

“…Likewise, in our system, manure and sewage inputs from failing subsurface septic systems are directly injected into the epikarst and entirely circumvent surface pathways, such as runoff (Katz et al, 2010). Once material enters the epikarst, it can be stockpiled until hydrologic connectivity is restored and pathways are rewetted at which point considerable export occurs (Tritz et al, 2011;Z. Zhang, Chen, Cheng, & Soulsby, 2020) and can result in peak stream concentrations of nitrate .…”

“…Zhang, Kang, et al, 2020;Wang et al, 2020;Z. Zhang, Chen, Cheng, Li, et al, 2020;Z. Zhang, Chen, Cheng, & Soulsby, 2020) but sources have also been unmixed with elemental concentration data alone (Katz et al, 2011;Yue et al, 2017).…”

Snow drought reduces water transit times in headwater streams

“…The young water fraction represents the proportion of a watershed outflow that is on average less than 2–3 months old. This metric has been used in a variety of environments, for example, boreal landscapes (Jutebring Sterte et al, 2021), karst environments (Simon et al, 2019; Zhang et al, 2020), and tropical watersheds (Trinh et al, 2020). In mountainous regions several studies have been conducted in European watersheds (Ceperley et al, 2020; Garvelmann et al, 2017; Stockinger et al, 2019) and North America (Campbell et al, 2020; Zhang et al, 2018).…”