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Seasonal oxygen-18 variations in precipitation, throughfall, soil water, spring¯ow and stream base¯ow 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¯ow/base¯ows 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¯uctuations to 37% of the surface amplitude, were generally similar for soil water on the larger PA basin, and base¯ows and headwater spring¯ows on the smaller WV basins. Computed annual isotopic damping depths for these water sources averaged 49 cm using soil depth as the¯ow path length. The equivalent annual mean hydraulic diusivity for the soil¯ow paths was 21 cm 2 d À1 . 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¯ows and 1 . 4±1 . 6 y for base¯ows on the smaller catchments. Base¯ow on the larger PA basin and¯ow of one spring on a small WV basin showed no detectable seasonal uctuations in oxygen-18, indicating¯ow emanated from sources with mean transit times greater than about 5 y. Based upon this soil¯ow path approach, it was concluded that seasonal oxygen-18 variations can be used to infer mean annual isotopic damping depths and diusivities for soil depths up to approximately 170 cm.

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