Impacts of Climate Change and Urban Development on Water Resources in the Tualatin River Basin, Oregon

We investigated the relative importance of future climate change and land use change in determining the quantity and quality of freshwater resources in northwestern Oregon's Tualatin River Basin using the U.S. Environmental Protection Agency's Better Assessment Science Integrating Point and Nonpoint Sources (BASINS) modeling system. Models were calibrated and validated using historic flow and water quality data between 1990 and 2006. The goodness of fit for the calibrated models was high, with coefficients of determination ranging from 0.72 to 0.93 in the calibration period. The calibrated models were run under a range of eight statistically downscaled climate change, two regional land use change, and four combined scenarios. Results included average increases in winter flows of 10 percent, decreases in summer flows of 37 percent, and increases in fifth-percentile flows of up to 80 percent as a result of climate change in the Tualatin River Basin. For land use change, the results included an increase in annual flows of 21 percent for the development-oriented scenario and a decrease of 16 percent for the conservation-oriented scenario. For combined scenarios of high climate change and high urban development, there is a projected increase in winter flows of up to 71 percent and decrease in summer flows of up to 48 percent. Climate change scenarios were more significant than urban development scenarios in determining basin hydrological response. The results are relevant to regional planners interested in the long-term response of water resources to climate change and land use change at the basin scale.