STREAMFLOW GENERATION ON A SMALL AGRICULTURAL CATCHMENT DURING AUTUMNRECHARGE .2. STORMFLOW PERIODS

To increase understanding of hydrologic flow paths, two- and three-com ponent tracer models were used to determine stormflow components on a 19.8 ha agricultural catchment in central Pennsylvania. Three events w ere studied during the 1989 autumn recharge period with the first and largest storm having a 5-10 year return period, 80 mm rainfall. Contin uous precipitation and streamflow records were augmented by observatio ns and sampling of spring and seepage discharge along with shallow wel ls and soil water. During the largest storm event, a two-tracer, three -component model separation using O-18 and Si indicated that 42% of to tal flow was derived from shallow subsurface stormflow, 11% from surfa ce event water, and 47% from deep subsurface groundwater flow. Similar analysis using Si and NO3 for a 34 mm rainfall event showed 17% of to tal flow to be from shallow subsurface flow, 10% from surface event wa ter and 73% from deep subsurface groundwater flow. Based upon analysis of tracer concentration variations in soil water, shallow well water and springflow during these events, shallow subsurface stormflow was f ound to be a transient mixture of infiltrated event water (28% average maximum contribution) and stored pre-event water (72% average minimum contribution). Thus, two major pathways for transferring event water to streamflow existed in these two events: (1) overland flow and chann el precipitation (10-11% of total flow); (2) shallow subsurface stormf low (5-12% of total flow). In the third event analyzed, 9 mm of rainfa ll was insufficient to generate shallow subsurface stormflow, and two- component models using O-18, Si and NO3 indicated 10%, 4% and 3% event water contributions, respectively. Major early autumn storms, occurri ng before subsurface water becomes well mixed in this region, offer an opportunity to differentiate shallow and deep subsurface pathways for streamflow generation using two-tracer, three-component models.