Variability and Hysteresis in Streamwater Dissolved Organic Carbon during Hydrologic Events and its Implications on Hydrologic Flow Paths at Panola Mountain Research Watershed, Georgia, U.S.A.

Abstract:

Dissolved organic carbon (DOC) has been a useful tracer of hydrologic flow paths that generate streamflow during hydrologic events. This is due to the distinct strong source of DOC in shallow soil horizons and specific landscape positions within a watershed. The variability in stream DOC concentration was examined in 76 hydrologic events between 1985 and 2014 at Panola Mountain Research Watershed, a small 41-hectare forested watershed near Atlanta, Georgia, U.S.A. Stream DOC concentrations ranged from about 1 to 2 mg/l (as C) in base flow and increased with discharge to a maximum of about 11 mg/l. DOC concentrations in shallow soil water at 15 cm depth varied between about 5 and 25 mg/l and were highest in the summer. Maximum stream event DOC concentrations were lower during winter and spring when conditions are wetter. The timing of the maximum event DOC concentration compared to the stormflow peak was variable. Summer events displayed an almost exclusive counterclockwise (ccw) hysteresis (DOC maximum occurring after the streamflow peak). During other seasons, events exhibited both clockwise (cw) and ccw hysteresis, no hysteresis, and occasionally a figure-8 shaped concentration-discharge response. The hysteretic patterns were compared to various attributes such as precipitation amount and intensity, base flow, streamflow response, and soil moisture antecedent conditions and event response at various soil depths. The ccw response was most related to shallow soil moisture response at a depth of 15 cm that either peaked at or after the hydrograph peak indicating higher contributions of DOC from shallow soils during the storm recession. A cw response was related to a rapid increase in soil moisture that plateaued well before the peak event discharge and occurred when the soil profile was wet. Other DOC response patterns occurred during either intermediate wetness conditions or more varied hydrologic dynamics. Spectral ultraviolet absorption (SUVA), an index of the aromatic fraction of DOC, increased in five high intensity storms during particularly wet antecedent conditions and is likely the result of overland flow, which apparently has a distinct SUVA signature. DOC was a useful tracer indicating the importance of the contribution and timing of shallow flow paths in generating streamflow during hydrologic events.