Keywords:snowmelt, carbon, nitrogen, solute flux, in-situ sensors,
NEON
1.
INTRODUCTION
Much of the world’s
streamflow is derived from mountain snowfall (Viviroli et al., 2007). In
the western United States, up to 70% of streamflow originates from
seasonal snowmelt (Li et al, 2017). Annual melt pulses provide a
critical resource (Sturm et al., 2017), supplying water for drinking,
irrigation, and power generation. Understandably, considerable work has
gone into characterizing the hydrology of melt pulses, particularly the
response to warming temperatures and reduced snowpack (e.g. Bales et
al., 2006; Rauscher et al., 2008; Dudley et al., 2017; Marshall et al.,
2019). As the principle hydrogeochemical and ecohydrological forcing in
most montane environments, equal effort has gone into understanding
processes controlling the chemistry of snowmelt (Campbell et al., 1995;
Williams et al., 1996a; Brooks et al., 1996;), and the role of melt
pulses dynamics in the retention or export of carbon and nitrogen from
catchments (Sickman et al., 2001; Meixner et al., 2003; Sickman et al.,
2003; Sebestyen et al., 2008).
Stream water chemistry represents the concatenation of all processes
occurring in the upstream watershed (Mulholland and Hill, 1997). For
snowmelt dominated catchments these processes include atmospheric
deposition (Williams et al., 1991a; Sievering et al., 1992),
biogeochemical cycling within the winter snowpack and subnivean zone
(Brooks et al., 1996; Williams et al., 1996a), and delivery to and
export in the stream (Brooks and Williams, 1999; Williams et al.,
1991b). Often, concentrations of dissolved carbon and dissolved nitrogen
tend to increase rapidly with the onset of the seasonal melt, reaching a
maximum several weeks before the peak in discharge (Q) and then decline
rapidly (Hornberger et al., 1994; Boyer et al., 1997; Sickman et al.,
2001). This “first flush” response has been interpreted as rapid
mobilization of limited stores which have built up within the catchment
soils during periods of lower flow (Baron et al., 1991; Sickman et al.,
2001). Unsurprisingly, inter-annual variability in the depth of snowpack
and melt pulse timing can exert considerable influence on carbon and
nitrogen processing and transport (Brooks and Williams 1999; Sickman et
al., 2003). Understanding these relationships is critical in predicting
the response of watersheds to a changing climate.
Notably, most of these studies have relied on grab sampling, with
frequencies ranging from daily to weekly. Advancements in field
deployable water chemistry sensors have revolutionized the frequency at
which measurements can be collected (Rode et al., 2016), allowing us to
better characterize processes varying over shorter time scales (Pellerin
et al., 2012). Within the melt pulse, rain-on-snow (ROS) events and
daily freeze-thaw cycles can generate “pulses within the pulse”.
Summer rainfall can also generate higher Q events outside the seasonal
melt pulse window. This additional Q variation can also profoundly
impact dissolved carbon and nitrogen concentrations in snowmelt
dominated streams (Casson et al., 2014; Koenig et al., 2017), but is
only apparent when high frequency sampling is employed (Pellerin et al.,
2012). While of short duration relative to the melt pulse, these events
may still constitute an important component of the catchment flux budget
and may provide insight into the potential for mobilization of
additional solute pools not depleted by the seasonal pulse. They are
also likely to become increasingly common as warmer temperatures result
in a greater fraction of annual precipitation falling as rain rather
than snow.
Here, we analyze in-situ sensor-based datasets from a snowmelt dominated
catchment in the central Rocky Mountains and characterize hydrologically
driven variability in stream concentrations (C) of dissolved organic
carbon (DOC) and nitrate-nitrogen (NO3-N) across
multiple scales. First, we test the hypothesis that a deeper and more
persistent winter snowpack will produce a larger and more prolonged
spring melt pulse. If Q is the dominant control on mass flux, this will
in turn result in greater export of solutes. Second, we hypothesize that
high-frequency measurements will reveal finer-scale, event-driven
variation of C-Q relationships. We test whether the C-Q dynamics of
these events (e.g. dilution versus enrichment) differ from those
observed at seasonal scales. We also test whether inability to account
for this high-frequency variation in C (e.g. relying on grab sampling vs
high frequency sensor-based measurements) substantially alters the
estimated annual solute flux. Finally, we place the results in the
context of climate change projections and what they suggest for
export/retention of carbon and nitrogen from alpine catchments in the
coming decades.