Rice, K.C.1, Chanat, J.G.2, Hornberger, G.M. 2, and Webb J.R. 2
1U.S. Geological Survey, P.O. Box B, Charlottesville, VA 22903
2Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22903
To provide insight into runoff processes, we examined variability of concentration-discharge (c-Q) plots of acid-neutralizing capacity (ANC) using a multiyear, multistorm hydrological and hydrochemical data set for three forested catchments in Shenandoah National Park, Virginia. The shape and rotational direction of c-Q plots (clockwise or anticlockwise) have been explained in the context of conservative mixing of three end members—surface-event, soil, and ground water. The streams draining the catchments represent a gradient in baseflow ANC, which is controlled by the underlying geology. We observed a trend in the fraction of anticlockwise rotation patterns, from the highest in the most acidic catchment to the lowest in the least acidic catchment. On the basis of previous modeling of c-Q plots, the trend across the catchments can be explained by differences in the concentration of the three end members, whereby the greater the differences (i.e., in the least acidic catchment), the fewer anticlockwise rotation patterns were observed. Differences in the relative volume and timing of the end members contributing to stormflow, ultimately controlled by the underlying geology, also may have contributed to the trend. Discriminant function analysis indicates that pre-storm baseflow ANC is an important predictor of rotation direction in two of the catchments; however, the predictive strength of the model decreases from the most acidic to the least acidic catchment. Variation of rotation patterns within a given catchment and across similar catchments can provide insight into factors and processes that influence runoff generation and solute transport.
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