Influence of hydrological connectivity on Fyw in karst critical zone
Although much recent research has focused on estimating the mean annual Fyw (von Freyberg et al., 2018; Wilusz et al., 2017) and short-term responses of Fyw to changes during precipitation events; the influence of storage (i.e. catchment wetness) and/or discharge on catchment dynamics is less clear (Kirchner, 2016b; Lutz et al., 2018; von Freyberg et al., 2018; Wilusz et al., 2017). In essence, temporal changes in hydrological connectivity, driven by storage dynamics, directly lead to changes in the Fyw of runoff. In karst environments, the change in hydrological connectivity and its impact on the Fyw is particularly pronounced. For example, the Fyw of water fluxes increases as hydrological connectivity between the hillslope and depression is strengthened during the wet period. In addition to infiltration through fractures within the epikarst matrix, concentrated infiltration from surface to underground flow systems via sinkholes is a distinct aspect of Fyw in karst catchments. This can make the Fyw at catchment outlet close to 1 (Fig. 3). In addition, the bidirectional flow exchange between matrix/small fractures and conduits or large fractures, controlled by their respective water levels, is a unique feature of karstic areas (Zhang et al, 2017). When the old water from the matrix enters the conduits, or the young water from the conduits flows back to the matrix, the Fyw of both water stores will be changed. Although new water from the conduits has a small impact on the age of the large volume of storage in the slow flow reservoir (the Fyw of fluxes from the slow flow reservoir did not change as drastically as other conceptual stores), there is still an impact on the Fyw of water in the small fractures around the larger conduits. Although this component of the younger entrance water will reduce the age of the water in the small fractures around conduits, the latter have the potential to be contaminated, which may have implications for water quality studies.
This is an important issue in the study catchment as fertilization of crops mainly occurs immediately prior to the rainy season, and large quantities of pollutants, such as inorganic nitrogen, remain on the surface (Yue et al., 2019). When heavy rain occurs, the component that enters the underground channel with water flowing through sinkholes or large fractures can transport this contamination into small fractures where it can be stored in the aquifer (Yue et al., 2019). This creates a small, but critical, part of the aquifer that is highly vulnerable to pollution, although its age may be relatively low in the wet season compared to the slow flow reservoir. High young water contributions reflect strong hydrological connectivity between underground channels and the catchment hillslopes via large fractures or from the surface via sinkholes. This means the high Fyw is likely related to cleaner water sources in hillslope draining non-agricultural forest and shrubs (Xiao et al., 2013) or surface contaminants in the lowland depression derived from agriculture (e.g. nitrogen). Hence, it is important to assess the role of hydrological connectivity and quantify its influence on the Fyw in the karst critical zone and associated interactions with biogeochemical processes. In particular, land and water management scenarios within karst watersheds need to not only consider the solute fluxes within the surface or underground system, but also the direct transfer of solutes between them via sinkholes. Young water comprises ~30% of the total runoff in this catchment, indicating that watersheds can transmit large contributions of soluble contaminant inputs to streams over much shorter time periods. This is consistent with recent global scale analysis that has emphasises the importance of young water contributions to larger rivers (Jasechko et al., 2016). However, in karst systems, due to the bidirectional flow between the fast and slow flow reservoirs, the soluble contaminant inputs to the small fractures or matrix in aquifer creates a “memory effect” of pollutant inputs that can be a significant pollutant source to streams, even the Fyw is high, especially at the beginning of wet season.
The turnover of old water in storage
The “old water paradox” has been a focus of interest for hydrologists; this addresses the question of how catchments store water for weeks or months, but then release it in minutes or hours in response to rainfall inputs (Kirchner, 2003). This is closely related to understanding dominant streamflow generation processes and associated solute fluxes, and provides insight into the difference between the velocity of water particles and the celerity of the response of hydrological systems (McDonnell and Beven, 2014). Fig. 5 shows that as discharge increases at Chenqi, the release of both young and old water at the hillslope spring and catchment outlet increase. The short-term rainfall-runoff dynamics (over hours) are controlled by the celerity of catchment or hillslope responses, which mobilizes water that has usually been stored in the catchment for much longer periods (months to years), but is constrained by low pore velocities. However, the increment is disproportionately larger for younger storage than it is for older, indicating that the increase in discharge is increasingly driven by the release of young water. This rapid increase in younger water contributions with increasing storage and discharge has been termed the ‘inverse storage effect’ by Harman (2015), which reduces the age of the water in the storage (Fig.10 in Zhang et al., 2019). Despite the ‘inverse storage effect’, at Chenqi, displacement of old water is also accelerated at moderately high flows, and this is consistent with the results of water age pdfs in Fig. 8. Although the Fywwas ~80% for runoff on 12 June 2017, the high discharge (0.0013 and 0.15 m3/s for the hillslope unit and catchment outlet, respectively) still resulted in large amounts of old water to be drained. This is consistent with findings on nutrient dynamics in the catchment reported by Yue et al., (2019). Based on nitrate [NO3–N] and the discharge time series of the study catchment in the wet season of 2017, they found that maximum nitrate concentrations during storm events generally lags behind peak Q , which indicates that initially water is low in nitrate and then sources with greater nitrate concentrations become increasingly important. In contrast, the Fyw of runoff in the dry period (30 November 2016 and 11 March 2017) were much lower than that in the wet period, but only small volumes of old water were released due to the low discharge at the hillslope and catchment outlet.
In this study, the changes in isotope composition from rainwater to the water flow at the hillslope spring and catchment outlet are also consistent with the hypothesis that old water acceleration is also occurring during high flows. The δD values at the sampling points in this catchment for the two largest rainfall events in 2017 are listed in Table 2. It shows that δD values at the outlet and hillslope spring are much less negative than rainwater. This is most likely explained by strong mixing of young rainwater with old stored water during and after the rainfall, although celerity of the response of discharge to rainfall is fast. This suggests that there was substantial old water delivered to the underground conduits at high discharge; and would be consistent with the aforementioned water quality studies which showed that the NO-3 concentration in runoff at the catchment outlet increased significantly after heavy rainfall, and then fell back to a low level as flows decreased (Yue, et al., 2019).