Authors: Tammy E. Parece and James B. Campbell
Urbanization alters the natural water cycle. One of urbanization’s most significant effects is increasing impervious surface cover which reduces infiltration, increases runoff volume and rate, and decreases evapotranspiration. Effective management of urban stormwater runoff and water quality issues can only be accomplished once drainage area and flow networks are accurately identified. Typically, geospatial evaluation of hydrologic impacts begins with identification of water flow and watershed boundaries, applying assessment techniques based on those designed for natural landscapes. However, urban hydrology differs from that of natural environments and thus urban watersheds require innovative evaluation techniques. This study identifies information gaps that originate from applications of standard geospatial techniques in urban hydrology. Delineation of a highly urbanized watershed in Fairfax County, Virginia was first accomplished using standard techniques. Next, using lidar data, the watershed was again delineated and both results compared. By overlaying both delineations on aerial photos and adding impervious surface and storm network layers, we identified regions redirecting the natural flow of water from the stream channel, then removed these regions from the watershed area. The results revealed a decrease in the watershed area by almost 17%, the natural watershed boundary was significantly altered, creating a much longer perimeter, and sink areas revealed. These results support the hypothesis that delineations of urban watersheds differ from those in natural settings. Anthropocentric alterations to land cover and landscape create a complex hydrology. The greatest complications are impervious surfaces and storm networks which redirect water flow.
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