Identifying Urban Watershed Boundaries
and Area, Fairfax County, Virginia
Tammy E. Parece and James B. Campbell
Abstract
Urban hydrology differs from that of natural environments, and
thus urban watersheds require innovative evaluation tech-
niques. Typical geospatial evaluation of urban hydrology begins
with identification of water flow and watershed boundaries.
This study identifies steps to delineate a highly urbanized water-
shed in Fairfax County, Virginia. Using standard techniques for
natural watersheds and one-meter2 resolution lidar, watershed
and flow accumulation raster datasets were derived. Then, mod-
ifications encountered within urban landscapes i.e., impervious
surfaces, stormwater inlets, pipes, and retention ponds along
with high-resolution aerial photos and lidar-derived contour
lines were integrated into the analysis. Regions redirecting water
flow from stream channels and areas redirecting water flow into
the stream channels were identified. These areas were removed
or added, reducing the area by almost 17 percent, and the
watershed boundary was significantly altered. This analysis il-
lustrates the significance of the distinctive characteristics of the
urban landscape in accurate delineations of urban watersheds.
Introduction
Substantial literature, dating back decades, has been devoted to
urban hydrology; most specifically to evaluation, management,
and engineering of urban hydrologic systems to address chang-
es the built environment has wrought on the natural hydrologic
cycle (e.g., McPherson and Schneider, 1974; Debo and Day,
1980; USDA, 1986; Sample
et al
., 2001; Debo and Reese, 2003;
Lhomme
et al
., 2004; Leonhardt
et al
., 2014). With the advent
of
GIS
, research has become much more robust in modeling
water flow and evaluating water quality issues (Rodriguez
et
al
., 2008). Yet, a better understanding of hydrologic impacts of
urbanization is required as current best management practices
implemented to address urban stormwater runoff are proving
to be inadequate (Burton Jr. and Pitt, 2002). Effective manage-
ment of urban stormwater runoff and water quality issues can
only be accomplished once drainage areas and flow networks
in urban settings are identified, with careful attention paid
to the urban landscape’s distinctive features (McPherson and
Schneider, 1974; Burton Jr. and Pitt, 2002; Quinn, 2013).
Urban hydrologic characteristics are unique i.e., quite
unlike those of natural environments (Kaufman
et al
., 2001;
Sample
et al
., 2001; Debo and Reese, 2003; Rodriguez
et al
.,
2003). Anthropogenic changes from land grading, channel-
ization, impervious surfaces, and stormwater sewer sys-
tems direct water flows from one catchment area to another
(McPherson and Schneider, 1974). Yet, geospatial evaluation
of hydrologic impacts begins with identification of overland
water flow and watershed boundary areas, and evaluative
techniques applied are based on similar techniques used in
natural landscapes (Sample
et al
., 2001; Rodriguez
et al
.,
2003). These conventional approaches fail to account for
transfers of runoff across topographic divides, creation of
sinks, and disruption by built topography, which modify orig-
inal natural surfaces and invalidate conventional delineations
of drainage systems.
Water bodies experience changes from stormwater runoff
with as little as 10 percent impervious surface cover within
its watershed (Center for Watershed Protection 2003). Anthro-
pogenic landscape changes due to removal of vegetative cover
and increased impervious surfaces have reduced infiltration,
amplified stormwater runoff volume and rate, diminished
groundwater tables, and decreased evapotranspiration
(DeBusk
et al
., 2010; Welker
et al
., 2010). Stormwater runoff
from impervious surfaces in urban regions degrades water
quality through higher water temperatures, and elevated lev-
els of contaminants in surface waters (Slonecker
et al
., 2001;
Davis
et al
., 2010; Welker
et al
., 2010). Stormwater runoff not
only effects water quality within a specific urban region but
also vitiates downstream waterbodies (Bhaduri and Minner,
2001).
On-the-ground surveys in an urban area can produce
watershed boundaries that do not compare to those of a nat-
ural watershed because they account for grading, and slope
changes from impervious surfaces. However, field surveys
cannot account for water inflows or outflows without evaluat-
ing the stormwater network’s inlets, pipes (including location
and flow direction), and retention ponds. In large urban areas,
field surveys can be quite complex, expensive, and disruptive
to daily human activities.
Many researchers recognize that stormwater networks
and impervious surfaces have altered urban water flows, and
the need to include these and aerial photographs with raster
based-delineations (Kaufman
et al
., 2001; Debo and Reese,
2003), yet few researchers alter standard geospatial methods
when delineating an urban watershed. In
Urban Drainage
Catchments
(Maksimovíc and Radojkovíc, 1986)
,
when identi-
fying watershed/catchment area, most authors recognized that
the built environment changed the natural water flow, and
therefore, included these changes in their delineations. How-
ever, these delineations were all accomplished without using
geospatial software and were completed for relatively small
areas. We located four articles evaluating stormwater flow,
which included stormwater networks and field data collection
with
GIS
to delineate catchments (Table 1).
While published research is sparse, many government
agencies and personnel, and other professionals have long rec-
ognized the deficiencies in applying routine methods in iden-
tifying urban catchment areas. As such, many of these entities
are including impervious surfaces, stormwater networks, and
remotely sensed data in analyses in local areas (Mauldin, per-
sonal communication, 2014; Quinn, personal communication,
2014). In addition, as more urban infrastructure is recorded as
Virginia Polytechnic Institute and State University, 220 E.
Stanger Street, Department of Geography (0115), Blacksburg,
Virginia, 24061 (
).
Photogrammetric Engineering & Remote Sensing
Vol. 81, No. 5, May 2015, pp. 365–372.
0099-1112/15/365–372
© 2015 American Society for Photogrammetry
and Remote Sensing
doi: 10.14358/PERS.81.5.365
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
May 2015
365
