Modeling the impact of development policies and climate on suburban watershed hydrology near Portland, Oregon
Maria Wright, Mary V. Santelmann, Kellie B. Vaché, David Hulse
Abstract
We simulated watershed hydrologic response to land use scenarios representing different sets of policies for suburban development and watershed management in an urbanizing watershed near Portland, Oregon. We asked (1) Can watershed-scale planning and development policies reduce the hydrologic impacts of urbanization? (2) How effective are different policies (including “green” neighborhood design, riparian protection, and representative Low Impact Development (LID) stormwater controls) in maintaining the pre-development hydrologic regime? and (3) Does climate change influence these results? We co-developed 50-year land and water management scenarios with a stakeholder advisory committee and used a spatially-explicit natural and human systems model to evaluate changes in water balance and streamflow characteristics under each scenario. We also used a factorial analysis to explore the influence of individual scenario elements on watershed hydrology. Results indicate that the biggest driver of change for this watershed was the spatial extent of development, followed by the nature of the development pattern, climate, forest harvest, and use of LID controls. Stormwater runoff increased as developed area increased, as did stream discharge for both periods of high and low flow. Increases in evapotranspiration (ET) with climate warming were offset by reductions in ET with development. Designs that condensed development and preserved upland and riparian forest showed the greatest potential for maintaining pre-development water balance and streamflow regimes, thereby reducing impacts of urbanization. These results bridge spatial and temporal scales important to decision-makers, highlighting the importance of considering impacts over time of neighborhood-scale development decisions on watershed-scale hydrology.