SEASONAL ISOTOPE HYDROLOGY OF THREE APPALACHIAN FOREST CATCHMENTS

Seasonal oxygen-18 variations in precipitation, throughfall, soil water, spring flow and stream baseflow were analysed to compare the hydrology of two forested basins in West Virginia (WV) (34 and 39 ha) and one in Pennsylvania (PA) (1134 ha). Precipitation and throughfall were measured with funnel/bottle samplers, soil water with ceramic-cup suction lysimeters and spring flow/baseflows by grab and automatic sampling during the period March 1989 to March 1990. Isotopic damping depths, or depths required to reduce the amplitude of subsurface oxygen-18 fluctuations to 37% of the surface amplitude, were generally similar for soil water on the larger PA basin, and baseflows and headwater spring flows on the smaller WV basins. Computed annual isotopic damping depths for these water sources averaged 49 cm using soil depth as the flow path length. The equivalent annual mean hydraulic diAusivity for the soil flow paths was 21 cm 2 d ˇ1 . Mean transit times, based upon an assumed exponential distribution of transit times, ranged from 0.2 y for soil water at a depth of 30 cm on the larger catchment, to 1.1‐1.3 y for most spring flows and 1.4‐1.6 y for baseflows on the smaller catchments. Baseflow on the larger PA basin and flow of one spring on a small WV basin showed no detectable seasonal fluctuations in oxygen-18, indicating flow emanated from sources with mean transit times greater than about 5 y. Based upon this soil flow path approach, it was concluded that seasonal oxygen-18 variations can be used to infer mean annual isotopic damping depths and diAusivities for soil depths up to approximately 170 cm. #1997 John Wiley & Sons, Ltd.

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