QUANTIFYING STORMFLOW COMPONENTS IN A PENNSYLVANIA WATERSHED WHEN O-18 INPUT AND STORM CONDITIONS VARY

Four major storms occurring on a 7.4 km2 Pennsylvania watershed during May-June 1990 were used to examine methods for isotopically separatin g surface and subsurface stormflow components when the O-18 variabilit y in rainfall is known. Rainfalls and the resulting stormflows were in crementally sampled and analyzed for O-18, Si, Na, Cl, Mg, and NO3. Co mpared with conventionally used methods that input rainfall as a whole -storm value, analysis of the O-18 data by peak rainfall periods rathe r than by total storm changed the separations the most. In contrast, s ubstituting incremental for total-storm averaging of rainfall O-18 inp ut caused much less change. When the peak and incremental averaging ap proaches for O-18 were combined into a single method, the O-18-based s eparations corresponded to the silica-based separations for all storms and Na-based separations for the largest storms. The peak-based metho ds estimate subsurface contributions to stormflow to be substantially larger than would be estimated by conventionally used total-storm meth ods.