The residence time of base flow in small catchments reflects the integrated ability of the soil and bedrock to transmit water to the stream and provides a means to calculate rates of biogeochemical processes in the subsurface. The ¹⁸O content of precipitation in temperate regions follows an annual cycle in which the greatest values occur in summer and the lowest values in winter. This seasonal signal progressively diminishes as precipitation infiltrates the soil and mixes with ground water. To calculate residence time, a ground-water model of the catchment is necessary to determine the temporal distribution of base flow residence times, and thereby derive a mean residence time value based on the diminution of the amplitude of the ¹⁸O seasonal signal. This approach was used to estimate the mean residence time of base flow at small catchments in the Catskill Mountains and lower Hudson River valley of eastern New York as part of studies that have examined nitrogen-cycling processes and the effects of land use on runoff processes. The results of these studies show that residence time estimates provide a comparison of the rate of the water cycle relative to the rates of biogeochemical processes in small catchments.