Limiting Imperviousness

Research shows negative consequences for a stream when more than 10 to 15% of the land in its watershed is covered by impervious surfaces. Yet we find that how imperviousness is measured has a major effect on whether such thresholds are exceeded. First we show large differences between estimates of impervious ness obtained with land use and land cover data, and propose adopting a consistent standard for the future. Second, we illustrate very different results when watershed imperviousness is measured at a few discrete points on the drainage system or continuously along all stream segments. Based on these findings we recommend that planners allow a safety margin when regulating land based on imperviousness, steer development towards already urbanized locations and away from relatively undisturbed locations, and take advantage of methods that mitigate the deleterious effects of imperviousness on stream ecology.