Pulses within pulses: Concentration-discharge relationships across temporal scales in a snowmelt-dominated Rocky Mountain catchment
Robert Hensley1*, Joel Singley2 and Michael Gooseff3
1Battelle - National Ecological Observatory Network, Boulder Colorado
2Colorado School of Mines, Golden Colorado
3Institute of Arctic and Alpine Research, University of Colorado, Boulder Colorado
Corresponding author : hensley@battelleecology.org
Abstract
Concentration-discharge (C-Q) relationships can provide insight into how catchments store and transport solutes, but analysis is often limited to long-term behavior assessed from infrequent grab samples. Increasing availability of high-frequency sensor data has shown that C-Q relationships can vary substantially across temporal scales, and in response to different hydrologic drivers. Here, we present four years of dissolved organic carbon (DOC) and nitrate-nitrogen (NO3-N) sensor data from a snowmelt- dominated catchment in the Rocky Mountains of Colorado. We assessed both the direction (enrichment vs. dilution) and hysteresis in C-Q relationships across a range of time scales, from interannual to sub-daily. Both solutes exhibited a seasonal flushing response, with concentrations initially increasing as solute stores are mobilized by the melt pulse, but then declining as these stores are depleted. The high-frequency data revealed that the seasonal melt pulse was composed of numerous individual daily melt pulses. The solute response to daily melt pulses was relatively chemostatic, suggesting mobilization and depletion to be progressive rather than episodic processes. In contrast, rainfall-induced pulses produced short-lived but substantial enrichment responses, suggesting they may activate alternative solute sources or transport pathways. The results clearly demonstrate that solute responses to individual events may differ significantly from the longer-term behavior these events combine to generate, something which only becomes apparent when high-frequency data are used.  
Keywords: concentration-discharge, snowmelt, DOC, nitrate, high-frequency 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.