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.