The potential influence of increasing levels of atmospheric carbon dioxide (CO2) on water resources includes changes in evapotranspiration that result from control of stomatal resistance by C02 and changes in precipitation and temperature caused by 'greenhouse' warming. In this study we investigate the potential effects of CO2 change on the hydrological response of a forested catchment in Shenandoah National Park, Virginia. Steady temporal trends are superimposed on stochastically derived time series of precipitation and temperature. These input data, which account for the natural variability of the system and the hypothetical effects of climate change, are then used to drive TOPMODEL, a variable-source-area hydrological model. A variety of climate-change scenarios is simulated and temporal trends in annual average flow, peak flow, and basin yield are detected using Kendall's tau statistic. The direction and magnitude of the runoff trends are dependent on the relative magnitudes of the induced trends in precipitation, temperature, and stomatal resistance. Stochastic variability in temperature and precipitation obscure the runoff trends even when real trends in precipitation, temperature, and stomatal resistance are significant.