4.5 Relating SC-Q hysteresis, streamflow components and
watershed properties
On an annual scale, we detected a significant negative correlation
between quickflow proportion and the mean annual hysteresis index at the
50th percentile of discharge values (Table 7;
Pearson’s r = -0.77, p-value < 0.05). Watersheds with higher
proportions of quickflow tended to have larger negative hysteresis
indices implying these watersheds behaved the flashiest in response to
diurnal snowmelt pulses. The other runoff generation mechanisms
(baseflow and throughflow) were not correlated to any of the annual
hysteresis indices, supporting evidence that the youngest fraction of
runoff controls stream water solute concentration dynamics (Benettin et
al. , 2017).
We found strong relationships between direct runoff partitioning (i.e.
throughflow vs quickflow) and slope aspect of our study watersheds.
Watersheds with more land area facing north were associated with greater
annual proportions of quickflow (Table 7; Pearson’s r = 0.72, p-value
< 0.05). North-facing aspects tend to receive less direct
radiation, and snowpacks persist longer compared to south-facing slopes.
This allows snowpacks to melt later in the year at higher temperatures
and thus faster rates (Musselman et al. , 2017), which brings
about conditions favorable for quickflow. Longer persistence of
snowpacks on north-facing slopes leads to the development of
preferential meltwater flowpaths at the snow-soil interface, resulting
in as much as 170% more snow water equivalent accumulating along the
length of north-facing hillslopes compared to south-facing hillslopes
(Webb et al. , 2018).
The percentage of surficial geology covered with glacial deposits was
significantly negatively correlated to the proportion of annual
streamflow derived from quickflow (Table 7; Pearson’s r = -0.66, p-value
< 0.05). Watersheds with low proportions of quickflow
(NASH100, NASH200, NFLL100, NFLL200) had a larger percentage of glacial
deposits covering their surficial and bedrock geologies (Tables 2, 4 &
S1). In contrast, watersheds with larger proportions of quickflow tended
to have bedrock geology dominated by metasedimentary and metavolcanic
rocks associated with the Libby Creek Formation (Table S1) and surficial
geology dominated by glaciated bedrock (Table 2). Glacial deposits were
assumed to have higher infiltration capability and thus more drainage
capacity thereby reducing the occurrence of quickflow relative to the
bedrock associated with the Libby Creek Formation. This bedrock can be
considered to have lower infiltration capacity, increasing the
occurrence of quickflow.
The percentage of surficial geology covered by glacial deposits was
significantly correlated to annual and daily hysteresis indices (Table
7). Watersheds with a higher proportion of glacial deposits tended to
produce large, positive hysteresis indices. Waters flowing through
glacial deposits, compared to glaciated bedrock, likely have longer
residence times and thus more potential geochemical evolution before
arriving as streamflow. These factors would lead to more variability in
flowpath development and larger hysteresis indices observed at annual
and daily scales.
5. DISCUSSION
5.1 Dominant controls on hydrologic response
Using continuous records of SC and Q we showed that direct runoff
(quickflow and throughflow) contributes the majority of the total annual
streamflow - especially during the snowmelt period (Table 3). These
results align with other investigations near our study area that used SC
(Miller et al. , 2014; Rumsey et al. , 2015) or stable water
isotopes (Huth, Leydecker, Sickman, & Bales, 2004; Liu et al. ,
2004; Williams, Seibold, & Chowanski, 2009) for hydrograph separation.
Hydrograph components were significantly correlated to aspect and
surficial geology, while elevation was highly positively correlated to
snow depth, mean precipitation, total runoff, and runoff ratio (Table
7). The strong link between elevation and precipitation has been well
documented in other mountainous environments that span large elevation
ranges (Elder, Dozier, & Michaelsen, 1991). At smaller scales, however,
slope aspect impacts the ability of wind to redistribute snow (Dadic,
Mott, Lehning, & Burlando, 2010) and vegetation structure significantly
alters snow accumulation and ablation patterns (Varhola, Coops, Weiler,
& Moore, 2010).
Streamflow and SC data from our watersheds indicated a combination of
geology, topography, and precipitation inputs strongly affect the
hydrologic response in snow-dominated, headwater catchments (Segura et
al. , 2019). As temperatures warm, snowmelt will initiate earlier
in the season at slower rates (Musselman et al. , 2017) which
could bring about a slower hydrologic response and less near-surface
saturation, favoring groundwater recharge and less direct runoff. We can
already see evidence of this in our system where watersheds that have
more south-facing area are associated with greater proportions of
baseflow. Differences in slope aspect will be amplified in the future
when snowmelt occurs at earlier dates and the sun angle is lower
(Lundquist & Flint, 2006). In future warming scenarios, shaded
hillslopes will provide habitat to native species that developed under
cooler, wetter scenarios, whereas sunnier areas will be more susceptible
to drought and invasion from non-native species.
For the watersheds studied here, longer snow persistence on north-facing
or heavily shaded hillslopes enabled high snowmelt rates which bring
soils to field capacity and facilitates rapid shallow subsurface flow
and return flow (Dunne & Black, 1971). Water typically exfiltrated near
the break in slope where hillslopes are adjacent to riparian areas,
supporting the variable source area concept (Hewlett, 1961). Return flow
exfiltrating from soil and regolith layers is an important contributor
to groundwater recharge and has been shown to be resilient to drought
and can buffer recharge under climate change (Carroll et al. ,
2018). Conversely, south-facing slopes receive more direct radiation
during snowmelt compared to north-facing slopes such that snowmelt is
typically initiated earlier in the year. This likely contributes to the
low percentages of quickflow produced in the watersheds with the
greatest percentage of south-facing area (Table 4). This interpretation
is consistent with the results from Thayer et al. (2018). Using
time-lapse electrical resistivity tomography, they showed the preference
for deep drainage occurring on a south-facing hillslope located within
NONM100 and concluded that overland flow and lateral shallow-subsurface
interflow (i.e. quickflow) were negligible on south-facing hillslopes.
Interestingly, two of our smaller study watersheds and the two lowest
total streamflow producing sites (GOLD100 and NONM100; Table 4) had
remarkably different runoff generation mechanisms from one another
(Figures 2 & 5; Tables 3 & S5). Baseflow contributions during snowmelt
were the smallest for the GOLD100 watershed while baseflow contributions
were the largest during low flow conditions. This shift in baseflow
dominance, from low proportions during high discharge to high
proportions during low discharge, suggests that a smaller groundwater
storage capacity exists for this watershed. The surficial geology of
GOLD100, which is dominated by grus mixed with alluvium, residuum,
slopewash and colluvium, is remarkably different than the other study
watersheds and likely helps explain the hydrologic response (Table 2).
Unlike the other study watersheds, the Libby Glacier only overlaid a
small portion of this watershed (Atwood, 1937), resulting in few
glaciated deposits which we expect have greater infiltration capacity
and the ability to store more water which leads to more geochemical
evolution compared to the surficial geology units present in GOLD100
which resulted in a flashy hydrograph behavior during snowmelt.
In contrast, the surficial geology in NONM100 is almost entirely
glaciated deposits. The stream draining this watershed received a more
constant source of baseflow throughout the year, implying groundwater is
an important and stable source of stream water in this small catchment
(Carroll, Deems, Niswonger, Schumer, & Williams, 2019). Reductions in
snowpack, the primary water storage component in runoff-dominated
watersheds like GOLD100, will be more disruptive to the natural flow
regime in these types of catchments, resulting in a larger contraction
of the stream network compared to streams that are more groundwater
dominated (Tague, Grant, Farrell, Choate, & Jefferson, 2008).
Reductions in late season flow and subsequent contraction of the stream
network, for example under climatic shifts in seasonality, could
potentially reduce aquatic habitat heterogeneity and stream
macroinvertebrate biodiversity (Brown, Hannah, & Milner, 2007).
5.2 Specific conductance – discharge relationships
reflect the storage and release of snowmelt
water
While the SC-Q relationship remained consistently negative at annual and
daily scales, the direction of hysteresis during snowmelt-induced
diurnal cycles (clockwise) was opposite of the direction observed at the
annual scale (anti-clockwise). The clockwise hysteretic behavior
observed on a daily scale suggested throughflow, with its greater SC and
relatively longer residence time, was first flushed out of hillslopes,
followed by freshly melted snow, with lowest SC and shortest residence
times, that filled in pore space occupied by the discharged throughflow.
This behavior supports threshold behavior for runoff generation, where
connectivity must be achieved before daily meltwater with the lowest SC
contributes to streamflow (Tromp-Van Meerveld & McDonnell, 2006).
In watersheds with the largest mean daily hysteresis index (NFLL100 and
NFLL200) the shape of the daily SC – Q hysteresis during
snowmelt-induced diurnal cycles was relatively circular during the
rising limb and became flatter during peak flow and falling limb
conditions (Figure 7). This behavior was documented by Kobayashi (1986)
and was attributed to an increase in the subsurface component of
streamflow after a snow-free area emerged adjacent to the stream
channel. Our results from this change in hysteretic shape support
evidence that greater flow pathway variability was present during early
snowmelt where water was moving through both relatively shallow and deep
flow pathways. During the falling limb, less snow was present in the
watershed and throughflow from relatively deeper flow pathways draining
soil and regolith layers combined with baseflow were primarily
responsible for streamflow generation. This dominance of deeper flow
pathways leads to SC values measured in streamflow that vary more
linearly related to discharge over daily timescales.
5.3 Potential limitations and future
work
Based on field observations in our study watersheds, it is obvious that
the vast majority snowmelt infiltrates the ground surface for a period
of time. We make no distinction between surface (i.e. overland flow) and
subsurface runoff in the nomenclature of our selected hydrograph
components based on extensive field observations. During peak snowmelt
conditions, water stored in hillslopes was rapidly transported via
lateral flowpaths arising from macropores and / or interfaces between
depositional units with large density contrasts (Fullhart et al., 2019;
McNamara, Chandler, Seyfried, & Achet, 2005; Roberge & Plamondon,
1987; Uchida, Tromp-Van Meerveld, & McDonnell, 2005; ) indicative of
threshold runoff response (Penna, Tromp-Van Meerveld, Gobbi, Borga, &
Dalla Fontana, 2011; Spence, 2007; Tromp-Van Meerveld & McDonnell,
2006). This quick moving lateral runoff was observed in NONM100 with low
SC, exfiltrating from the base of hillslopes where fully saturated
riparian areas could not accommodate additional subsurface flow (Figure
S4).
As mentioned before, interpretation of our SC-based baseflow hydrograph
separation hinges on the ability to accurately assign concentrations to
end-members. Due to the fact that SC concentrations are effectively
constant throughout the low flow period, we believe the selection of the
baseflow end-member is fairly accurate. However, relatively few snow and
snowmelt samples were used to assign the direct runoff component.
Extensive field mapping and SC measurements of the stream network
expansion and contraction in NONM100 during snowmelt revealed that
exfiltrated shallow subsurface flow (i.e. return flow) often had an SC
concentration similar to or lower than the value assigned to the direct
runoff component (21.6 µS/cm; Figure S4). We were surprised by the lack
of geochemical evolution of this fast-moving return flow and hypothesize
that water flowing along these flowpaths had the shortest residence
times. The value we assigned to the direct runoff component is in
between two values chosen assigned by Miller et al. (2014) (10
and 33 µS/cm). Assigning a lower value to our direct runoff end-member
would have had little implications to our results since the difference
between direct runoff and baseflow SC is so large in our study
watersheds.
Persistent streamflow occurs in all watersheds throughout the winter,
except perhaps GOLD100 and NONM100 (not shown). In most of the
watersheds stream stage and SC data are constant, indicating stable
sources of baseflow contribution during the winter. Unfortunately, like
many other similar studies, we collected data only during the relatively
warm period of the year when ice is less likely to affect data quality
and freezing is less likely to damage equipment. Our results are, for
example, underrepresenting the amounts and proportions of baseflow by
only using data from the relatively warm period of the year where data
collection is more feasible.
While SC is a robust measurement of concentration and automated loggers
can record instantaneous values at relatively low costs, information is
lost when solute concentrations are aggregated together. Many studies
have shown that concentration-discharge relationships vary for different
individual solutes (e.g. Lewis & Grant, 1979; Godsey, Kirchner, &
Clow, 2009). Exploring individual solute concentration-discharge
relationships may help refine our knowledge on the number of different
flow pathways water may take before arriving as streamflow. Further,
stable water isotope composition sampled from our study watersheds did
not permit the use of hydrograph separation due to event and pre-event
waters having essentially the same concentration (analysis not shown).
This is consistent with previous work demonstrating that stable water
isotopes have proven less useful in snowmelt-dominated, seasonally arid
environments where recharge is dominated by snowmelt (Earman et al.,
2006), and thus indistinguishable from event water (Jin, Siegel, Lautz,
& Lu, 2012). Regardless, other process-relevant information may be
gained by exploiting the isotopic signatures and variability in the
translation of snowmelt to streamflow during different discharge
conditions to elucidate controls on runoff generation.
6. CONCLUSIONS
Specific Conductance (SC) recorded in our study watersheds showed large
amounts of dilution in peak streamflow periods indicating that seasonal
snowmelt dominated annual streamflow contributions. Streamflow and SC
also showed distinct hysteretic patterns at annual and daily scales.
Annually, most hysteresis indices that could be identified were
negative, suggesting faster flow pathways dominated streamflow on the
rising limb of the annual hydrograph compared to slower flow pathways
occurring during the falling limb. During snowmelt-induced diurnal
cycles, SC-Q hysteresis was consistently negative, indicating the daily
melt water with the lowest SC preferentially contributed to streamflow
on the falling limb of diurnal streamflow cycles after throughflow water
was displaced. Hysteresis indices derived from SC-Q relationships were
significantly correlated to proportions of quickflow and surficial
geology, supporting the idea that the youngest fraction of runoff
controlled stream water solute concentration dynamics in these systems.