Page 204 - Biosystems Engineering

182 Cha pte r F i v e

Some of the most widely used software provided by these companies
include ArcGIS, ArcView, ERDAS Imagine, GRASS, Mapinfo, and
TIGRIS.
These softwares have ability of capture, manage, manipulate,
analyze, and output spatially referenced data such as points, lines,
and polygons (using vector data structure) or a unit space (using tes-
sellation or raster data structure). A vector data structure represents
geographic features and objects as points, lines, and polygons; the
information (attributes) associated with these features are stored in
relational database management systems. Examples of features rele-
vant to watershed modeling that use vector data structures include
streams (line features); watershed outlets, point sources, water qual-
ity and streamflow measurement locations, etc. (point features); and
soil types and land use/land cover (polygon feature). Most of the
vector data structures also store topological relationships (either
explicitly or implicitly), describing a feature’s spatial relation with
neighboring features, specifically its connectivity and adjacency. The
topological relationship provides a GIS a powerful facility for effi-
cient spatial analyses.
A raster (or tessellation) data structure divides space into two-
dimensional units of space. Square grids (square pixels) and triangu-
lated irregular networks (TIN) are most common raster data structures
used for storing raster data relevant for watershed modeling. In a
square grid, each square cell contains a representative value of the
attribute being mapped. In a raster data structure, a point is repre-
sented as a cell, a line by a string of connected cells, and areas by
groups of adjacent cells. Examples of raster datasets used for water-
shed modeling include digital elevation models (DEMs) and satellite-
derived land use/land cover maps.
Using the vector and raster data sets, a number of watershed
characteristics can be derived for distributed parameter watershed
modeling. Examples include watershed and subwatershed bound-
aries, stream networks, slope, aspect, and soil and land use for
each individual watershed elements (i.e., subwatersheds or hydro-
logic response units). These derived characteristics are important
state variables (i.e., the state of the watershed for a period for
which the watershed modeling is being performed) to a distrib-
uted parameter model. Once a watershed model is run, a GIS can
help manage and spatially display the vast amount of data gener-
ated by a model.

5.4 GIS and Watershed Models
Regardless of the level of sophistication, application of a dynamic
watershed model to preserve water quality on a site-specific basis