Pulses within pulses: Carbon and Nitrogen mobilization patterns across
temporal scales, from seasonal snowmelt to rain on snow events in a
Rocky Mountain catchment
Abstract
Seasonal snowmelt pulses are the dominant hydrologic feature of most
alpine catchments. The majority of annual export of water, carbon and
nitrogen occurs within a short window of only a few weeks. This
observation has largely been based on relatively infrequent manual
sampling, and our understanding of responses to finer-scale variation,
“pulses within the pulse”, is critically incomplete. Here, we combine
high-frequency sensor measurements of dissolved organic carbon (DOC) and
nitrate (NO3-N) with historical grab sample data from a high altitude
stream in the Rocky Mountains of Colorado. We characterize the linkages
between precipitation, snowpack, streamflow, and solute export, over
time scales ranging from decades to minutes. At all time scales,
discharge (Q) variation was several orders of magnitude larger than
concentration (C) variation, making it the dominant control on solute
flux rates. Interannual variation in Q, and by extension solute export,
appeared correlated to the depth of the winter snowpack, and how late
into the spring the snowpack persisted. Seasonally, we observed
clockwise C-Q hysteresis, with solute stores becoming depleted as the
melt pulse proceeds. Using the sensor data however, we were able to
observe individual events. In contrast to the seasonal patterns, these
events enriched concentrations, suggesting the persistence of additional
DOC and NO3-N stores which can be mobilized within, and even after the
main seasonal snowmelt pulse. The historical data suggest that reduced
snowpack and earlier snowmelt in the coming decades may result in
reduced export of DOC and NO3-N. The sensor data however make this
conclusion uncertain, as rain on snow events, which are expected to
become more prevalent, appear equally capable of mobilizing solutes.