Young water fraction of water fluxes in different landscape
units
The Fyw of water fluxes from the different landscape
units were tracked using the tracer-aided model and calibrated parameter
sets (Fig. 3). The simulated mean
Fyw were 0.39, 0.31 and 0.10 for the hillslope unit,
fast flow and slow flow reservoirs during the study period. The seasonal
differences in Fyw are significant for all the
conceptual stores. The mean Fyw of water flux increased
between the dry and wet seasons: ranging from 0.30, 0.07 and 0.03 for
the dry season (December to the next April) to 0.48, 0.54 and 0.17 for
the wet season (May to October), for the hillslope unit, fast flow and
slow flow reservoirs, respectively. The mean Fyw of
runoff at the catchment outlet (fast flow reservoir) was mainly affected
by the varying contributions of the major water sources in different
seasons. In the dry season, a high proportion of flow was contributed by
the slow flow reservoir to the runoff at the catchment outlet (the
largest proportion was 78.4% from Zhang et al., 2019) leading to the
low Fyw of runoff at catchment outlet. In the wet
season, the Fyw of runoff had increased significantly
because the hillslope unit contributes much more water to runoff at the
catchment outlet (57.5%), which was much younger than the water in the
slow flow reservoir.
There is a noteworthy phenomenon that the mean Fyw of
the water flux in the hillslope unit was higher than that at the
catchment outlet in the dry season
(0.30 vs. 0.07), but it was reversed
in the wet season (0.48 vs. 0.54); this is likely to be due to the
impact of sinkholes in the lower catchment. During heavy storm events,
overland flow and epikarst water mainly directly generated by high
intensity rainfall were collected by sinkholes and large fractures and
recharged to the underground stream (flow out the catchment) directly.
After this influx of younger water, the Fywat the catchment outlet increased
rapidly, and therefore was higher than that in the hillslope unit during
the wet season. The
Fywof both flow in hillslope unit and catchment changed significantly
during rainfall events, especially during the wet season. For example,
the Fyw of flow from hillslope unit and catchment outlet
rose from 0.5 to close to 1 (Fig. 3).
We also calculated Fyw of runoff using sine-wave
approach proposed by Kirchner (2016a) in this study. Although this
methodology has been successfully used at a number of sites (Jasechko et
al., 2016; Song et al., 2017; Lutz et al., 2018; von Freyberg et al.,
2018), it has not been widely tested for karst catchments due to the
lack of data and high spatial heterogeneity. The seasonal cycle
amplitudes As and Ap in
Eq. (1) and (2) were determined according to the sinusoidal fits (Fig.
4), and the sine regression models of precipitation and runoff at the
hillslope spring and catchment outlet were all statistically significant
(p < 0.00001). Using this approach, the Fywfor the hillslope spring and catchment outlet were 0.07 and 0.09,
respectively. Although the sine regression was statistically significant
for daily stable isotope values of rainfall, runoff at the hillslope
spring and catchment outlet, the values obtained by sine-wave method
were notably smaller than those determined by tracer-aided model
tracking (0.39 and 0.31 for hillslope and outlet). This suggests that
the Fyw calculated using the sine-wave fitting method
with daily isotope data is underestimated in this study catchment. The
uncertainty of the sine-wave fitting method for Fyw has
been discussed in other studies. For example, Stockinger et al. (2016)
found that the tracer sampling frequency markedly influenced estimation
of Fyw, and weekly isotope tracer data lack information
about faster water transport mechanisms in the catchment. However, the
daily isotope tracer data used in some Fyw calculations
(Stockinger et al., 2016; Song et al., 2017) is also less well-suited
for determining Fyw using the sine-wave approach in a
karst catchment, because it does not adequately capture the sub-daily
variability in rainfall isotope signatures at resolution appropriate to
the response times of sub-tropical karst systems (Coplen et al., 2008).
Therefore, there is high uncertainty using the sine-wave approach with
relatively low-frequency measurements of tracer behaviour collected
daily or weekly, to calculate the Fyw of runoff in
catchments with rapidly variable flow dynamics, although it can reflect
the seasonal tracer cycles. At present, there is a significant challenge
in applying this method to determine the Fyw in karst
areas, because of the high cost of long-duration continuous sampling at
sub-daily, even hourly resolution.
Based on the Fyw calculated by flux tracking using the
tracer-aided model, the discharge of ‘old water’ (i.e. with and age
>69 days) were estimated for hillslope and catchment outlet
(Fig. 5). The model results indicate that more rapid old water turnover
occurs in wet season, although the Fyw in wet season is
obviously higher than in the dry season (Fig.3b). The mean discharge of
old water was 7.8×10-4 and 3×10-5m3/s
for the hillslope and catchment outlet, respectively, during the dry
period, and were 3.9×10-3 and
5.8×10-5 m3/s for the wet period.
This indicates that in the rainy season, the young water (rain) enters
soil and/or fractures/conduits and begins to displace the old water.
These results are consistent with the reasoning by Harman (2019), that
the old water release may be accelerated by high discharge or catchments
wetness.