Seasonal inter-relationships between storage and the young water fraction
There is a marked seasonal variability in density distributions of Fyw for the three conceptual stores (Fig.6 (a) and (b)). Due to the thin epikarst, with only a mean depth of 30 cm on the hillslopes (Zhang et al., 2013), even small rainfall events can lead to remarkable increases in Fyw in the hillslopes The probability density distributions of Fyw show that water fluxes in the hillslope unit and the catchment outlet (from fast flow reservoir) have much more pronounced changes compared to the slow flow reservoir for each month during the dry period (Fig. 6 (a)). The high proportion of water contributions from the slow reservoir to the catchment outlet caused the similar mean Fyw (< 0.05) for these two conceptual stores, although their density distributions are notably different over the dry season due to different responses to low rainfall infiltration. In each month during the wet period, the mean Fyw of water flux from the hillslope unit was higher than from the outlet and slow flow reservoir, and also had the largest variety (from ~0 to more than 0.9 in Fig.6(b)).
Mean of the Fyw and its variability demonstrate that the dominant water source of runoff at the catchment outlet has a controlling influence on the Fyw of the water flux from the catchment. During responses to rainfall in the dry period, slow flow in the matrix or small fractures contributes a high proportion of older water to the underground channel at the catchment outlet, resulting in the low Fyw of the catchment outlet; meanwhile, hillslope flow and direct rainfall recharge occasionally contribute to the outlet for some of the small rainfall events, resulting in the great changes of Fyw at the catchment outlet. After heavy rain, the hillslope unit and direct rainfall recharge become the main water sources to the underground channel runoff, which can displace a large proportion of older water in the slow flow reservoir. Hence, the Fyw of water flux for the outlet became very high (Fig.6).
Fig.7 shows the respective modelled relationships between Fyw of water fluxes and the storage of the hillslope unit, and fast and slow flow reservoirs. For the hillslope unit, when storage was low for most rainfall events in the dry season, the Fyw of water flux usually increases sharply above a specific water storage, giving marked hysteresis (Fig.7(a)). Below the specific water storage, small rainfall amounts can rapidly recharge the aquifer, and substantially increase the relatively small hillslope storage. Furthermore, as there is no additional young water to maintain recharge the hillslope unit once rainfall stops, there is a rapid decline in Fyw. Under higher storage conditions (in the wet season), the Fyw of water flux tends to be a significant linear increase with storage in a prolong time.
For the slow flow reservoir, there was a strong season hysteretic pattern of Fyw of modelled flux vs. water storage (Fig.7(b)). During the dry period, water stored in small fractures is released to the underground channel without inputs of young water in rainfall. Therefore, the Fyw nearly stayed constant between November and April (<0.05), with the steady decrease in storage (from ~280 to 200 mm). During the wet season (May to August), the Fyw increased linearly with the increase in water storage, because of consecutively new rainwater recharge. In the later rainfall period (September and October) when the slow flow reservoir storage reaches the highest, there was a little change in storage with the gradually ageing water (Fig.7(b)), mainly sustained by the hillslope unit (Zhang et al., 2019).
For the fast flow contributions to outflows at the catchment outlet, there was a large variation in Fyw of water flux (from ~0 to 0.8) because water storage in the fast flow reservoir (conduits) was low (Fig.7(c)). Particularly, the very narrow range of water storage (<1mm) for low Fywindicates that variation in the outlet Fyw is caused by some fast flow (young water) that rarely has time to enter storage, as shown by the sharp increases of the hillslope flow in the dry period. As the Fyw becomes high, the band of water storage increases, which corresponds to the linear relationship between the Fyw and storage during the wet season (Fig.7(a) and 7(b)). This means that much more young water (high Fyw) from the hillslope unit and direct infiltration of rainfall fills in the underground channel, and then quickly mixes with or displaces the older water at the catchment outlet. During the wettest periods when the Fyw becomes highest (e.g. >0.7), a marked increase in storage occurs, however the Fyw sometimes did not increase dramatically. This means that after large quantities of young water from the hillslope unit and direct rainfall infiltration (via sinkholes) simultaneously drain into the underground channel in wet time, the fast flow reservoir has been almost filled with young water even though the storage changes with the rise and decline of hydrographs.