5. DISCUSSION
Despite the obvious difficulty of gathering data in high alpine
catchments, the complementary information derived from natural and
artificial tracer data and from a rainfall-runoff model (i.e. a water
balance) allowed a thorough quantification of recharge and discharge
components of the catchment of the active rock glacier Innere Ölgrube at
present. Recharge components vary seasonally as well as diurnally
reflecting varying contributions from rainfall, snowmelt and ice melt.
Discharge components are delayed to a certain extent due to the storage
capabilities of the shallow aquifer within the rock glacier catchment
(Wagner et al., 2020a), depending on the flow paths and saturation
status of the groundwater body (Winkler et al., 2016, 2018).
The tracer velocities are in the range of reported values in active
layers (Buchli et al., 2013; Krainer & Mostler, 2002; Tenthorey, 1992).
The low recovery rate (< 50 %) likely indicates that some of
the tracer is lost to the deeper domains of the rock glacier
(potentially the unfrozen base layer). The tracer is presumably stored
there for a longer period of time, thereby preventing it from reaching
the measurement device at a detectable concentration within the
observation period. Yet, the fast recovery of at least 40% of the
tracer clearly indicates a fast flow component likely related to lateral
flow on top of the permafrost table towards the rock glacier spring.
This is also reflected in the hydrograph analysis. Concerning the slow
flow component related to the unfrozen sediment layer (subpermafrost
flow) at the base of the rock glacier and potentially other fine
sediments within the catchment (e.g. till deposits and moraines between
the cirque glaciers and the rock glacier) EC and isotopic data was
analysed.
Figure 8 depicts the seasonal variations in individual “recharge” and
discharge component contributions. “Recharge” is inferred from the
rainfall-runoff model (Figure 8a) and does not take the loss due to
evapotranspiration into account, as this process is happening in the
production store (Figure 3) where the different input components are
already mixed and evapotranspiration is dependent on the saturation /
water level within that store. Nevertheless, evapotranspiration is
limited in such an alpine environment. Discharge is inferred from event
water analysis (Figure 8b), where higher mineralized water (higher EC)
is related to longer stored water within the groundwater body and low
mineralized water is related to event water derived from snowmelt, ice
melt or rainfall. The actual differentiation of event water into
rainfall, snowmelt and ice melt is not possible for that time frame (see
Figure 6c,d for a detailed analysis over a short period of time using
isotopic data in addition to EC).
Combining the information of Figure 8a and 8b allows for an interesting
interpretation:
The groundwater component (longer stored water within the aquifer) is
the most important component during periods of little to no recharge
(e.g. winter time; Wagner et al., 2020a). Snowmelt and (subordinately)
rainfall are the dominant recharge components during early summer,
rapidly increasing the event water contribution to ~60
%. In particular during the summer with little to no rain and after
snow has melted, ice (glacier or permafrost ice) runoff and groundwater
discharge become the relevant components. Separating groundwater and ice
melt water due to the diurnal pattern of melting process (related to
variations in air temperature) is possible. The pronounced change in
discharge pattern in early autumn observable in Figure 6b (reduced
discharge and attenuated diurnal variations caused by declining air
temperatures in early September) indicates the transition from the
event-water dominated period (late April - August) to the groundwater
dominated period (September until onset of the snowmelt in April-early
May). Some caution is required when interpreting relative discharge
component contributions during the onset of snowmelt in April (Figure
8), since the only available EC time series during that month starts on
April 25, 2018. Thus, the groundwater contribution during (early) April
is likely underestimated in Figure 8.
[Insert Figure 8]
To identify and quantify the main source of the ice melt water
potentially from permafrost ice melt or from the upper cirque glaciers
is more complex. Natural and artificial tracer test results show that
event water travels through the rock glacier within several hours.
Ice-derived meltwater can be observed with a time lag in the order of
~ 16 hours relative to air temperature indicated by EC
and isotopic data (Figure 6c). This time lag in combination with the
fast response due to event water suggests some distance of the melt
water source to the spring/rock glacier and favour the cirque glaciers
as the main melt water source. This is further substantiated by
comparing the time lag to the artificial tracer test results: The linear
distance between the lowermost point of the cirque glacier and the
gauging station is approximately 1500 m, indicating an actual flow
distance ≥ 1500 m. Typically, runoff from glaciers reaches a maximum a
few hours after the peak in meltwater production (Cuffey & Paterson,
2010), thus the runoff peak at the glacier precedes the peak at the
gauge by ≤ 16 h. Both estimates suggest that the water travels from the
front of the glacier to the gauging station at a velocity ≥ 0,026 m/s.
This is in good agreement with the artificial tracer test results
yielding slightly higher linear velocities. Note also that melting rates
at the cirque glaciers reflect diurnal variations in radiation, heat and
vapour content of the adjacent air (Cuffey & Paterson, 2010). The
coarse grained active layer covering the permafrost ice within the
catchment protects it from radiation and induces a damped and retarded
variation in temperatures, decoupling it from external weather and
climate conditions (Haeberli, 1985; Jones et al., 2019; Vonder Mühll,
1993; Wagner et al., 2019). Accordingly, melting of permafrost ice is
expected to follow changes in atmospheric conditions in a more damped
and retarded fashion, suggesting cirque glacier melting to account for
the observed diurnal variation in discharge and natural tracers.
The amount of ice melt is in the order of 30% of the annual recharge
and will decrease and finally disappear in the future due to glacier
melt. Thus, ignoring ice contribution from cirque glacier melt is
hypothetical, but interesting at least. This can be assessed by using
the calibrated rainfall-runoff model and assuming that ice melt from the
cirque glaciers is absent due to potential future complete glacier melt
(green line in Figure 9). This was simulated in a simplified manner by
actually eliminating the (glacier) ice storage, keeping all the other
parameters constant. Increasing runoff during snowmelt periods does not
change expectedly; however, the runoff in late summer indicates a
certain reduction. Interestingly, base flow during winter months does
not change significantly. This is an important finding that further
supports that base flow is mainly derived from the unfrozen base layer
of the rock glacier (and other fine-grained sediments in the catchment
area (e.g. till deposits, moraines); cf. Wagner et al., 2020a). In
total, a runoff reduction of almost 30% is expected in this
hypothetical scenario due to the disappearance of the cirque glaciers.
Another hypothetical scenario can be constructed by applying the model
parameters of a relict rock glacier where the same model was already
applied (Table 2; cf. Wagner et al., 2016) for the current setting of
the active rock glacier. This trading space-for-time approach accounting
for changing watershed behaviour under permafrost-free conditions (Singh
et al., 2011) allows to speculate how progressing climate change might
influence the discharge pattern, considering the associated
uncertainties (purple line in Figure 9).
[Insert Figure 9]
Changes are not great and only a slightly more buffered behavior during
storm events or snowmelt periods is visible compared to the scenario
where only the ice melt from the glaciers is neglected. Interestingly,
slightly slower recessions at the onset of winter periods are
observable. This can be explained by a higher routing store x3 for the
relict (Schöneben) rock glacier catchment than the active (Innere
Ölgrube) rock glacier catchment. This might indicate an increase in
storage capacities as permafrost thaw potentially leads to more
available pore space in the shallow aquifer system (cf. Rogger et al.,
2017). Nevertheless, in the case of the relict Schöneben rock glacier as
well as for the active Innere Ölgrube rock glacier, both are known to
have a rather fine-grained, unfrozen base layer that is 10-15 meters
thick. These similarities might explain their rather similar behavior,
although one may speculate about a further increase in storage capacity
within the Innere Ölgrube catchment as permafrost thaws eventually. This
potential change is depicted by computing the master recession curves of
the modelled runoff patterns for the “current” data, the hypothetical
scenario of vanished cirque glaciers and the other hypothetical scenario
of a relict rock glacier (Figure 10). The comparison to the observed
data further indicates the good model fit in addition to the efficiency
criteria and visual fit of the hydrographs presented before (and shown
again in the inset). The similar master recession curves of the
“current” data and the hypothetical one without the glacier ice melt
is to be expected, as the actual internal (or model) structure did not
change, but only a recharge component was “removed”. The discharge
pattern and consequently master recession curve of the hypothetically
relict rock glacier catchment (using the parameter set of the relict
Schöneben rock glacier; Wagner et al., 2016) indicates a slightly slower
recession during winter periods as storage capabilities increased.
[Insert Figure 10]
What remains to be identified is the actual contribution of permafrost
ice melt at present and future changes in the runoff pattern due to this
additional water in the short term and potential increases in the
storage capacity in the long term when the permafrost ice will melt
completely. The trading space for time approach presented herein does
suggest a slight difference in runoff patterns and supports what was
suggested by Rogger et al. (2017): Runoff might be dampened slightly and
storage capacities might increase to some extent. However, further
research addressing this particular case and ideally monitoring rock
glacier catchments without the influence of cirque glaciers within their
catchment is needed. Colombo et al. (2018) suggest establishing
baselines for future monitoring related to downstream water quality (and
quantity) as solute export from deteriorating rock glaciers might
provide valuable information.
In alpine catchments, rock glaciers should be seen as
hydro(geo)logically (and geomorphologically) conservative systems
(Giardino et al., 1992) that become increasingly important as ice
glaciers will continue to vanish. For a recent assessment of potential
changes in the European mountain cryosphere until the end of the 21st
century, see Beniston et al. (2018). There, improvements in
understanding changes in re- as well as discharge in high-alpine areas
are warranted. The here gathered information and developed understanding
of a high-alpine spring catchment that drains an active rock glacier and
two cirque glaciers is of importance and should be extended by including
downstream observations (cf. Wagner et al., 2016). Another open question
remains the actual contribution of permafrost ice melt relative to the
glacier ice melt and becomes increasingly important with regard to
climate change and a likely further warming trend. A recent review about
potential ecosystem shifts in alpine streams highlights the importance
of a better understanding of permafrost / rock glacier thaw (Brighenti
et al., 2019). Therefore, long-term monitoring of such high-alpine
catchments (and streams further downstream) are essential.