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We developed a water budget runoff model for the Salt and Verde River basins of central Arizona and used the outputs of 6 global climate models (GCMs) to estimate runoff in the future under assorted emissions scenarios developed by the Intergovernmental Panel on Climate Change (IPCC). We used a statistical downscaling routine to refine the GCM outputs for the 2 basins, and we found that all model-scenario combinations simulate a mean temperature rise in the study area of between 2.4 and 5.6 degrees C, using year 2050 greenhouse gas concentrations. Mean changes in precipitation vary substantially among the models and scenarios, and, as a result, changes in runoff vary from 50 to 127 % of historical levels. Assuming equal probabilities associated with each scenario and model run, the overall results suggest that runoff from the Salt and Verde will have an approximately 85 % chance of being less strong, the certainty of which is related to consensus on warming in the study area. The large variability among predictions of precipitation trends introduces substantial uncertainty.