Pulses within pulses: Concentration‐discharge relationships across temporal scales in a snowmelt‐dominated Rocky Mountain catchment

Abstract

Concentration-discharge (C-Q) relationships can provide insight into how catchments store and transport solutes, but analysis is often limited to long-term behaviour 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 4 years of dissolved organic carbon (DOC) and nitrate-nitrogen (NO 3 -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. Finally, we observed low-level diel variation during summer baseflow following the melt pulse, likely driven by effects of daily evapotranspiration cycles. Additional contributions from in-stream metabolic cycles, independent from but covarying with diel streamflow cycles, could not be ruled out. The results clearly demonstrate that solute responses to daily cycles and individual events may differ significantly from the longer-term seasonal behaviour they combine to generate.

Shifted sediment-transport regimes by climate change and amplified hydrological variability in cryosphere-fed rivers

“…4). However, suspended sediment transport in cryosphere-fed rivers is also governed by temperaturedependent erosion processes in glacierized and permafrost landscapes, snowmelt erosion, rainstorm-triggered mass movements from unstable hillslopes (e.g., thermokarst slopes and glacially debuttressed slopes), and enhanced channel erosion by discharge pulses (1,28,30) as we have previously demonstrated (Figs. 3 and 4).…”

“…Rainfall represents the pluvially activated sediment sources, including rainstorm-triggered mass movements from unstable slopes and reactivated proglacial sediment storage ( 28 , 29 ). RSs capture the enhanced stream power and channel erosion and resuspend previously deposited sediment during meltwater/pluvial pulses and flash floods ( 30 , 31 ). Because of the integration of these sediment-transport mechanisms, the SAT-M model accurately replicates the clockwise sediment-discharge hysteresis with identical seasonal patterns as observed in studied basins (middle column in Fig.…”