Temporal Trends in the Spatial Distribution of Impervious Cover Relative to Stream Location

Abstract

Use of impervious cover is transitioning from an indicator of surface water condition to one that also guides and informs watershed planning and management, including Clean Water Act (33 U.S.C. §1251 et seq.) reporting. Whether it is for understanding surface water condition or planning and management, impervious cover is most commonly expressed as summary measurement (e.g., percentage watershed in impervious cover). We use the National Land Cover Database to estimate impervious cover in the vicinity of surface waters for three time periods (2001, 2006, 2011). We also compare impervious cover in the vicinity of surface waters to watershed summary estimates of impervious cover for classifying the spatial pattern of impervious cover. Between 2001 and 2011, surface water shorelines (streams and water bodies) in the vicinity of impervious cover increased nearly 10,000 km. Across all time periods, approximately 27% of the watersheds in the continental United States had proximally distributed impervious cover, i.e., the percentage of impervious cover in the vicinity of surface waters was higher than its watershed summary expression. We discuss how impervious cover spatial pattern can be used to inform watershed planning and management, including reporting under the Clean Water Act.

Variability in stream ecosystem response to urbanization

“…As efficient transporters of water, the dominant function of stormwater drains is to export water and materials that fall on catchment impervious surfaces, independent of their position in a catchment (cf. Wickham et al, 2016). This is in stark contrast to the hydraulically inefficient flow paths in natural catchments, where organic matter deposited within the catchment is likely to be retained and become incorporated into the soil.…”

“…In fact, Walsh and Kunapo (2009) found that distance to the nearest stormwater drain was a stronger determinant than distance to the nearest stream. Estimation of distances of impervious surfaces to stormwater drains, which is likely to be a good predictor of whether they are connected to a drain, would certainly produce markedly different conclusions to those of Wickham et al (2016).…”

“…For instance, Wickham et al (2016) sought to represent the spatial distribution of impervious cover relative to stream location, and noted an increased separation of impervious surfaces from streams in more urban areas, which they attributed to reduced drainage densities in urban areas. Their study was based on the mistaken premise that studies such as Walsh and Kunapo (2009) found impervious cover near streams to be more influential on stream condition than more distant impervious cover.…”