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
techniques. Typical geospatial evaluation of urban hydrology
begins with identification of water flow and watershed boundar-
ies. This study identifies steps to delineate a highly urbanized
watershed in Fairfax County, Virginia. Using standard tech-
niques for natural watersheds and one-meter2 resolution lidar,
watershed and flow accumulation raster datasets were derived.
Then, modifications 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 redirect-
ing 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 illustrates the significance of the distinctive char-
acteristics 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 hydrolog-
ic 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
original natural surfaces and invalidate conventional delinea-
tions 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 (De-
Busk
et al
., 2010; Welker
et al
., 2010). Stormwater runoff from
impervious surfaces in urban regions degrades water quality
through higher water temperatures, and elevated levels 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 wa-
tershed boundaries that do not compare to those of a natu-
ral 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 iden-
tifying watershed/catchment area, most authors recognized
that the built environment changed the natural water flow,
and therefore, included these changes in their delineations.
However, 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 col-
lection 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
