Linking High Frequency Variations in Stream Water DOC to Ages of Water Sources in Peat-Dominated Montane Watersheds

Abstract:

We combined time series of inferred DOC (from optical sensors) and stable isotopes in streams and watershed source areas to assess the link between water age and C fluxes. We monitored temporal dynamics of FDOM for 2 yrs at nested scales (0.9, 3.0 and 30km²) in a montane Scottish watershed. FDOM was strongly correlated (r² ~ 0.8) with DOC allowing inference of 15 min timeseries. Marked seasonality was observed, with highest DOC concentrations (~25 mg l^-1) in summer events and lower concentrations (~5mg l^-1) in winter. During events, anticlockwise hysteresis was observed; consistent with expansion of the riparian saturation zone, increasing hydrological connectivity across peat soils and mobilizing DOC. Lag times for peak discharge and DOC were 1-12 hrs depending on event characteristics and antecedent conditions. Isotope time series from precipitation, streams and catchment source waters (overland flow and hillslope drainage) were also generated. These allowed us to model the non-stationary characteristics of their ages. Stream water age ranges from 3 months at high flows when overland flow dominates runoff to 4 yrs under baseflow. Overland flow age was a dominant influence on DOC transport. Highest concentrations occurred in small summer events with relatively young (<1 month old) overland flow waters generated after dry antecedent conditions, reflecting displacement of DOC-rich soil water. With wetting up in late summer/fall, overland flow is younger (< 1 week) diluting soil water DOC. Similar patterns occur across scales, though at 0.9km² the dominance of a large peatland limits hysteresis, and maintains more constant DOC levels. At larger scales, DOC was more responsive but decreased more rapidly, reflecting a greater, but more transient influence of young overland flow. This shows the importance of riparian wetlands in regulating the quality and age of streams and the need to incorporate these sensitive areas in land management strategies.