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).