Evaluation of Impervious Surface Estimates in a Rapidly Urbanizing Watershed

Accurate measurement of impervious surface (IS) cover is an essential indicator of downstream water quality and a critical input variable for many water quality and quantity models. This study compares IS estimates from a recently developed satellite imagery/land cover approach with a more traditional aerial photography/land use approach. Both approaches are evaluated against a high-quality validation set consisting of planimetric data merged with manually-delineated areas of soil disturbance. The study area is the rapidly urbanizing 127 km Cub Run watershed in northern Virginia, located on the fringe of the Washington, D.C. metropolitan region. Results show that photo-interpreted IS estimates of land class are higher than satellite-derived IS estimates by 100 percent or more, even in land uses conservatively assigned high IS values. Satellite-derived IS estimates by land class correlate well with planimetric reference data (r 0.95) and with published ranges for similar sites in the region. Basin-wide mean IS values, difference grids, and regression and density plots validate the use of satellite-derived/land cover-based IS estimates over photo-interpreted/land use-based estimates. Results of this site-specific study support the use of automated, satellite-derived IS estimates for planning and management within rapidly urbanizing watersheds where a GIS system is in place, but where time-sensitive, high quality planimetric data is unavailable. Introduction Urban growth, frequently occurring in the form of urban or suburban “sprawl” has been a consistent process in the Baltimore-Washington, D.C. metropolitan region over the last 50 years. Several authors (Crawford-Tilley, et al., 1996; Masek, et al., 2000; Jantz, et al., in press) have documented methods and data collection criteria to define urban growth in this region, which currently makes up ten percent of the 168,000 km Evaluation of Impervious Surface Estimates in a Rapidly Urbanizing Watershed Mark Dougherty, Randel L. Dymond, Scott J. Goetz, Claire A. Jantz, and Normand Goulet Chesapeake Bay watershed and includes over forty percent of its total population. Masek, et al. (2000) presented urban change detection methods that minimize confusion between urban green space and agriculture, reporting a notable increase in “built-up area” in the Washington, D.C. metropolitan region during the late-1980s. Jantz, et al. (in press) predicted future sprawl in the Washington, D.C.-Baltimore area, at current growth and policy trends, of more than 7,200 additional km by 2030. This paper evaluates the use of a newly developed approach to quantify a specific landscape feature, impervious surface (IS) cover, to support watershed management at the development or zoning level. The study compares two fundamentally different methods of estimating IS cover; a traditional aerial-photography/land use approach and a satellite imagery/land cover approach. Throughout this paper, the term land use refers to the economic or social function of the land, while land cover refers to the physical properties of the land surface (Aspinall, 2002). Photography, as used in this paper, is the process from which the results are photographs. The use of IS percent (from remotely-sensed or other data) as a measure urbanization and as an effective means for managing land and aquatic resources has been reviewed by a number of authors, including Schueler and Claytor (1996), Arnold and Gibbons (1996), CWP (1998), Prisloe, et al. (2000), Cappiella and Brown (2001), and Civco, et al. (2002). Accurate measurement of IS provides an essential indicator of downstream water quality and a critical input variable for many water quality and quantity models such as TR-20, TR-55, Storm Water Management Model (SWMM), Source Loading and Management Model (SLAMM), and Hydrologic Simulation Program Fortran (HSPF) (USDA, 1982, 1986; Huber, et al., 1988; Pitt and Vorhees, 1989; Bicknell, et al., 1995). In recent years, mean watershed imperviousness has become an indicator for assessing water quality impacted by urban growth. According to Schueler (1994), adverse water quality effects above the 10 percent imperviousness threshold appear as increased pollutant loads from urban washoff, warmer stream temperatures from reduced canopy cover, and increased scour and channel instability with accompanying loss of pool and riffle sequences. Long-term changes in a stream brought about by increased IS areas (such as parking lots, rooftops, airports, and sidewalks) can lead to reduced stream habitat and loss of biodiversity. In addition, pollutants transported downstream end up in the receiving water body. In the present study, the receiving water body was Cub Run and its tributary streams, which flow into the Occoquan Reservoir, and eventually into the Chesapeake Bay. P H OTO G R A M M E T R I C E N G I N E E R I N G & R E M OT E S E N S I N G November 2004 1 2 7 5 M. Dougherty was with the Department of Civil & Envir. Engineering, Virginia Tech, Blacksburg, VA 24061; he is currently with the Biosystems Engineering Department, Auburn University, AL 36849–5417 ([email protected]). R.L. Dymond is with the Department of Civil & Environmental Engineering, Virginia Tech, Blacksburg, VA 24061 ([email protected]). S.J. Goetz is with The Woods Hole Research Center, Woods Hole, MA 02543-0296 ([email protected]). C.A. Jantz was with the Department of Geography, The University of Maryland, College Park, MD 20742-8225; she is currently with The Woods Hole Research Center, Woods Hole, MA 02543-0296 ([email protected]). N. Goulet is with the Northern Virginia Regional Commission, 7535 Little River Turnpike, Annandale, VA 22003-2937 ([email protected]). Photogrammetric Engineering & Remote Sensing Vol. 70, No. 11, November 2004, pp. 1275–1284. 0099-1112/04/7011–1275/$3.00/0 © 2004 American Society for Photogrammetry and Remote Sensing 03-015.qxd 10/11/04 14:26 Page 1275