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.