The initial reports of the Intergovernmental Panel on Climate Change (IPCC 1990) and those that have followed (IPCC 2001) have consistently noted that our climate is changing and that one of the most important impacts of climate change is on the world's water supplies. These impacts include increased temperatures, changed precipitation, and shifts in the historic hydrologic cycle. The impact of climate change on water supplies in the Pacific Northwest is of particular interest because of the interplay of two factors, rainfall and temperature. Changes in temperature and precipitation alter the delicate interaction between the amount of precipitation that falls as either rain or snow, the eventual accumulation of snow during the winter, and temporal variability with which this snow melts and flows through the watershed.

This study explores the impact that climate change may have on the hydrology of the Bull Run basin and Portland Water Bureau's (PWB) ability to provide reliable water to its customers. The study uses a series of loosely linked models to address potential impacts of climate change. These models simulate three aspects of process: the climate, the hydrologic cycle, and water supply system management.