The capacity of forests to mitigate global climate change can be negatively influenced by air pollutants-particularly tropospheric ozone, which may impair both photosynthesis and plant transpiration, thus ultimately affecting ecosystem productivity and watershed hydrology. Forest Service and collaborating scientists evaluated individual and interactive effects of ozone and climate on late season stream flow from six forested watersheds in the eastern and southeastern United States, using simulation models. In all cases, including ozone significantly improved the ability of the model to predict water use relative to climate-only models. Effects of ozone and its interactions with climate were consistently negative and varied with actual ozone exposures, both spatially across the region and over time. Conservative estimates of the influence of ozone on the variability of observed flow ranged from 7 percent in the area of lowest ozone exposure in West Virginia to 23 percent in the areas of highest exposure in Tennessee. Despite the frequent assumption that ozone reduces tree water loss, the team's findings provide evidence that ozone at near ambient concentrations can reduce stomatal control of leaf transpiration, thereby increasing water use by forests. These changes could increase the frequency and severity of episodic drought and affect flow-dependent aquatic biota in forested watersheds.