The Effect of Urbanization on Flow Duration Curves: A Case Study from Selected Streams in the Puget Sound Basin, Western Washington

Abstract:

Land use change associated with urbanization can alter natural flow regimes, typically resulting in larger peak flows for a given precipitation event than in a pre-urbanized watershed condition. The overall influence of urbanization on how flows of different frequencies might change over time, while important in hydrologic design, remains poorly understood. In this study, we investigate the effects of urbanization on flow duration curves (FDCs) and flow variability through a case study of watersheds in the Puget Sound Region of Washington State. Using different time windows of the flow record, we analyzed stream discharge, precipitation, and watershed urbanization for a minimum of 25 years between 1960 and 2010 to quantify how key FDC percentiles changed with time in response to urbanization in small watersheds with variable land use. Results of this analysis indicate that in watersheds with the highest degree of urbanization (average population growth rates in excess of 13 people/km²/year), the 95^th-99^th percentile of the daily-mean flow series increased by 0-94%, with an average increase of 35%. Similarly, the magnitude of small discharges (10^th percentile) increased by up to 34%, with an average increase of 15%. Stream flashiness in urbanizing watersheds was found to have increased by an average of 46% over the analysis period. As there were no significant changes in annual precipitation over the study period, increases in FDC magnitude and flashiness in urbanizing watersheds is most likely a result of increasing population density and watershed imperviousness. Rural watersheds were found to have decreasing FDC magnitude, which is possibly due to anthropogenic extractions of groundwater, and increasing stream flashiness, which is likely a result of reductions in base flow and increasing precipitation intensity and variability.