Interactive comment on “ Organic matter dynamics and stable isotopes for tracing sources of suspended sediment ” by Y . Schindler Wildhaber
The paper by Schindler Wildhaber et al. uses stable isotopes and other measures of organic components to determine sources of suspended sediment in a small watershed. The questions are of interest, the background laid out and the literature well represented and integrated. The use of the un-mixing model of Phillips and Gregg is not new for characterizing stream suspended sediment, but is a useful way of differentiating sources. The paper goes on to attempt to link the observed changes with the variations in flow regimes which is of interest hydrologically. However, there are a number of problems with the use of statistics in this paper and while I think there may be many significant findings in the data set presented I think the rationale and justification for the use of many of the statistical tests, including the validation of the tests assump-
Export mechanisms for dissolved organic carbon and nitrate during summer storm events in a glaciated forested catchment in New York, USA
Nitrate and dissolved organic carbon (DOC) concentrations during a summer storm for a forested catchment in the Adirondack Mountains displayed a clear separation in trajectories and timing of maximum values. Nitrate concentrations peaked early on the rising limb of the hydrograph, whereas DOC concentrations gradually increased through the rising limb with maximum concentrations following the discharge peak. Solute data from precipitation, throughfall, soilwater, and ground/till water indicated till water and near‐surface soil waters as the controlling end members for stream NO3− and DOC concentrations respectively. Streamflow concentrations of major base cations (Ca2+ and Mg2+), which were assumed to represent water originating from deep flow paths, matched the NO3− trajectory. These data suggest that streamflow NO3− concentrations are derived from till groundwater and that DOC is derived from near‐surface soil waters. We attributed the early expression of NO3− to the displacement of till waters by infiltrating precipitation. In contrast, we hypothesized that the delayed DOC concentrations occurred with surface and near‐surface runoff from near‐stream wetlands/peatlands and isolated saturated areas that became connected only under conditions of maximum water content in the catchment. Copyright © 2004 John Wiley & Sons, Ltd.
The role of soil water in stormflow generation in a forested headwater catchment: synthesis of natural tracer and hydrometric evidence
Quantifying the role of soil water in stormflow generation is a difficult but important goal for addressing catchment scale environmental problems. The purpose of this study was to combine natural tracer methods and hydrometric observations to estimate the contribution of pre-event (vadose) soil water to stormflow in a small forested headwater catchment in Shenandoah National Park, Virginia. Two storm hydrographs were separated, based on mass balance of naturally occurring 18O and chloride, into three physically based source components. For a large storm event in June 1992, pre-event soil water contributed 36% of total storm runoff and dominated peak flow (65%), based on the hydrograph separation. For a smaller storm event in November 1992, soil water contributed 25% of the total runoff and 50% during peak flow. A Monte Carlo error analysis of the hydrograph separations supported the interpretation for the June storm that pre-event soil water dominated peak flow (α = 0.12), but showed considerable uncertainty—the estimated soil water contribution ranged from 48% to 88% at peak flow at a confidence level of 68%—even under the relatively good conditions (i.e. relatively distinct source component chemical and isotopic compositions). Analysis of the November storm indicated greater uncertainty and a more poorly identifiable soil water component. The June storm was typical of ‘sharp’ responses that occurred coincident with the development of a transient saturated zone above the soil-bedrock interface on hillslopes, whereas the November storm was representative of slower delayed responses in which the hydrograph peak occurred 8–10 h after the end of precipitation. During the 6 month study, five other hydrographs displayed the ‘sharp’ response and three other hydrographs showed the delayed response. During all storms, water draining from the soil into zero-tension lysimeters maintained the isotopic content of pre-event soil water; this result was taken as evidence that transient saturated zones would consist of nearly all pre-event soil water. Field observations after large storm events suggest a threshold type expansion of the source area onto the hillslopes which may supplement rates of downslope flow of displaced pre-event soil water. This study provides evidence that mobilization of pre-event soil water may dominate peak stormflow in steep forested headwater catchments, and illustrates that quantitative error analyses are advisable for hydrograph separation studies.