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GIS-Based W atershed Modeling Systems 181

are also available as are summary statistics on surcharging, volumes,
continuity, and other quantity parameters. Additional quality output
includes loads, source identification, continuity, residuals (e.g., sludge),
and other parameters. Some of the main limitations of the SWMM
model include lack of subsurface quality routing (a constant concen-
tration is used), lack of interaction of quality processes (apart from
adsorption), difficulty in simulating wetlands quality processes, and
a weak scour deposition routine in the Transport Block. The current
edition, version 5.0 (Rossman 2008), is a complete rewrite of the pre-
vious release and uses the Windows operating system to provide an
integrated environment for editing study area input data; running
hydrologic, hydraulic, and water quality simulations; and viewing
the results in a variety of formats. These formats include color-coded
drainage area and conveyance system maps, time series graphs and
tables, profile plots, and statistical frequency analyses. Although a
Windows-based graphical interface is present for this model, it has
not been integrated with GIS. SWMM has been extensively used in
United States, Canada, Europe, and Australia.

5.3 Geographic Information Systems (GIS)
A GIS is also known as a number of alternative names, including
Land Information System, Spatial Information System, Geoscience
Information System, Geomatics, and Geographically Referenced
Information System. It is a powerful collection of tools for capturing
and converting, storing and managing, manipulating and analyzing,
editing and updating, and displaying and outputting spatial and
nonspatial data. Several important components, including hardware,
software, data, methods, and people, are integral components of a
GIS. A number of disciplines have contributed to the development
and advancement of a GIS, including geography, cartography, remote
sensing, photogrammetry, surveying, geodesy, statistics, operations
research, computer science, and mathematics. The power of GIS
comes from linking real-world locations with the attribute data asso-
ciated with those locations. These locations could be a point, a line, or
a polygon feature, or a unit in space. This powerful linking allows us
to (1) tie data analyses to real-world locations, (2) visually represent
data, and (3) makes, at will, sophisticated spatial analysis feasible,
which are not feasible using only maps or only data. Some of the
basic questions that can be asked of a GIS include the following.
Where is it (i.e., location)? What has changed since (i.e., trend)? Which
is the best way (i.e., routing)? What is the pattern (i.e., pattern)? and
What if (i.e., modeling)? Currently, a number of vendors provide
GIS software including Environmental Systems Research Institute
(ESRI); Strategic Locations Planning, Inc.; ERDAS, Inc.; U.S. Army
Corps of Engineers; Mapinfo Corporation; and Intergraph Corporation.