This study investigates changes in streamflow characteristics for urbanizing watersheds in the Portland Metropolitan Area of Oregon for the period from 1951 to 2000. The objective of this Study was to assess how mean annual runoff ratio, mean seasonal runoff ratio, annual peak runoff ratio, changes in streamflow in response to storm amount, the fraction of time that the daily mean flow exceeds the annual mean flow, 3-day recession constants, and dry/wet flow ratio vary among watersheds with different degrees of urban development. There were no statistically significant changes in annual runoff ratio and annual peak runoff ratio for the mixed land-use watershed (Tualatin River watershed) and the urban watershed (Johnson Creek watershed) during the entire study period. The Tualatin River watershed, where most of the urban development occurred in a lower part of the watershed, showed a statistically significant increase in annual peak runoff ratio during the 1976 and 2000 period. The Upper Tualatin River watershed illustrated a significant decrease in annual peak runoff ratio for the entire study period. With significant differences in seasonal runoff ratio, only Johnson Creek exhibited a significant increase in both wet and dry season runoff ratios. Streamflow during storm events declined rapidly in the urban watershed, with a high 3-day recession constant. At an event storm scale, streamflow in Fanno Creek, which is the most urbanized watershed, responded quickly to precipitation input. The fraction of time that the daily mean flow exceeded the annual mean flow and dry/wet flow ratio are all lower in Johnson Creek. This Suggests a shorter duration of storm runoff and lower baseflow in the urbanized watershed when compared to the mixed land use watershed. The findings of this study demonstrate the importance of spatial and temporal scale, climate variability, and basin physiographic characteristics in detecting the hydrologic effects Of urbanization in the Pacific Northwest of the USA. Copyright (c) 2006 John Wiley & Sons, Ltd.
