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.
Hydrological controls on dissolved organic carbon during snowmelt in the Snake River near Montezuma, Colorado
A quantitative understanding of the factors controlling the variation of dissolved organic carbon (DOC) in headwater streams is of scientific concern for at least two reasons. First, quantifying the overall carbon budgets of lotic systems is needed for a fundamental understanding of these systems. Second, DOC interacts strongly with other dissolved substances (heavy metals in particular) and plays an important role in the transport of contaminants.In the Snake River near Montezuma, Colorado, measurements of DOC from 1980 to 1986 show rapid decreases in concentration from a peak very early in the snowmelt period. Peak DOC concentrations occur approximately one month prior to peak discharge in the stream. The decline in DOC with time is approximately exponential, suggesting that a simple flushing mechanism can explain the response. We examined hydrological mechanisms to explain the observed variability of DOC in the Snake River by simulating the hydrological response of the catchment using TOPMODEL and routing the predicted flows through a simple model that accounted for temporal changes in DOC. Conceptually the DOC model represents a terrestrial (soil) reservoir in which DOC builds up during low flow periods and is flushed out by infiltrating meltwaters. The model reproduces the main features of the observed variation in DOC in the Snake River and thus lays the foundation for quantitatively linking hydrological processes with carbon cycling through upland catchments. Model results imply that a significant fraction of the soils in the Snake River catchment contribute DOC to the stream during peak discharge. Our work represents one of the first attempts to quantitatively describe the hydrological controls on DOC dynamics in a headwater stream. These controls are studied through the model by imposing mass balance constraints on both the flux of water through the various DOC source areas and the amount of DOC that can accumulate in these areas.