Component flow processes at four streams in the Catskill Mountains, New York, analysed using episodic concentration/discharge relationships

Plots of solute concentration against discharge have been used to relate st ream hydrochemical variations to processes of flow generation, using data c ollected at four streams in the Catskill Mountains, New York, during the Ep isodic Response Project of the US Environmental Protection Agency. Results suggest that a two-component system of shallow and deep saturated subsurfac e flow, in which the two components respond simultaneously during hydrologi c events, may be applicable to the study basins. Using a large natural sea- salt sodium input as a tracer for precipitation, it is argued that an addit ional distinction can be made between preevent and event water travelling a long the shallow subsurface flow path. Pre-event water is thought to be dis placed by infiltrating event water, which becomes dominant on the falling l imb of the hydrograph. Where, as appears to be the case for sulfate, a solu te equilibrates rapidly within the soil, the pre-event-event water distinct ion is unimportant. However, for some solutes there are clear and consisten t compositional differences between water from the two sources, evident as a hysteresis loop in concentration-discharge plots. Nitrate and acidity, in particular, appear to be elevated in event water following percolation thr ough the organic horizon. Consequently, the most acidic, high nitrate condi tions during an episode generally occur after peak discharge. A simple conc eptual model of episode runoff generation is presented on the basis of thes e results. Copyright (C) 1999 John Wiley & Sons, Ltd.