automated methods in GIS. The digital representation of the topography is called a
digital elevation model (DEM). The automated derivation of topographic watershed
information from DEM is faster and less subjective and provides more reproducible
measurements than traditional manual techniques applied to topographic maps
[44]. The technological advances provided by GIS and the increasing availability
and quality of DEM have greatly contributed to water resources and environmental
investigations [42].
The methods for linking GIS to analytical models are usually divided into three
broad categories: tightly coupled, closely coupled, and loosely coupled. In a tightly
coupled method, a GIS and a model share a common interface and the model is
embedded within the GIS software. In closely coupled method, the model and the
GIS can share a common interface, but the model is created as a program in the GIS
programming language. The third scheme for linking GIS to a model is known as
loose coupling. In this method, the GIS and model remain separated, including a
separate interface. However, there is a link between them that allows data to be
transferred from one to the other. A GIS dataset can be imported to the model,
analyzed, and sent back to the GIS for interpretation and mapping. This is probably
the cheapest method of integration, but controlling the data flow from two different
interfaces can be difficult and inefficient.
Watershed is the basic unit of hydrology and can be defined as an area contributing flow to a specified outlet. GIS is widely used to support water quantity and
quality modeling. Figure 5.9 presents the automated modeling system that incorporates both hydrologic and hydraulic models. This modeling system links the results
from the hydrology (discharge) to the input for the hydraulic model. After the
hydraulic models are executed, model output such as flood elevations, floodway
encroachments, and velocities can be extracted, displayed, and analyzed in a GIS
environment.
One of the first steps in hydrologic modeling is watershed delineation to a point or
a reach of interest. Using DEM or triangulated irregular networks (TIN), GIS can
automatically delineate a watershed for any outlet point. Further, watersheds can be
easily divided into subbasins by simply designating outlet points in any watershed.
As part of the delineation, basin data such as slope, area, mean elevation, distance to
centroid, and other common hydrologic parameters such as time of concentration
and Soil Conservation Service (SCS) runoff curve numbers are automatically computed. The curve number is a hydrologic parameter used to describe the stormwater
runoff potential for drainage area. It is a function of land use, soil type, and soil
moisture. SCS composite curve numbers can automatically be computed from land
use and soil data.
GIS automated watershed delineation consists of creating a depressionless DEM,
flow direction, flow accumulation, watershed outlet points, and delineating watersheds. The primary method involved in determining flow paths from DEM data is
the eight-point pour method developed by Puecker et al. [45]. The basic idea in
Puecker’s method is to assign a flow direction to each grid cell based on the
neighboring cell with the lowest elevation. Based on steepest slope, each cell is
assigned a flow direction in relation to one of its eight neighboring cells. The flow
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S. O. Darkwah et al.
digital elevation model (DEM). The automated derivation of topographic watershed
information from DEM is faster and less subjective and provides more reproducible
measurements than traditional manual techniques applied to topographic maps
[44]. The technological advances provided by GIS and the increasing availability
and quality of DEM have greatly contributed to water resources and environmental
investigations [42].
The methods for linking GIS to analytical models are usually divided into three
broad categories: tightly coupled, closely coupled, and loosely coupled. In a tightly
coupled method, a GIS and a model share a common interface and the model is
embedded within the GIS software. In closely coupled method, the model and the
GIS can share a common interface, but the model is created as a program in the GIS
programming language. The third scheme for linking GIS to a model is known as
loose coupling. In this method, the GIS and model remain separated, including a
separate interface. However, there is a link between them that allows data to be
transferred from one to the other. A GIS dataset can be imported to the model,
analyzed, and sent back to the GIS for interpretation and mapping. This is probably
the cheapest method of integration, but controlling the data flow from two different
interfaces can be difficult and inefficient.
Watershed is the basic unit of hydrology and can be defined as an area contributing flow to a specified outlet. GIS is widely used to support water quantity and
quality modeling. Figure 5.9 presents the automated modeling system that incorporates both hydrologic and hydraulic models. This modeling system links the results
from the hydrology (discharge) to the input for the hydraulic model. After the
hydraulic models are executed, model output such as flood elevations, floodway
encroachments, and velocities can be extracted, displayed, and analyzed in a GIS
environment.
One of the first steps in hydrologic modeling is watershed delineation to a point or
a reach of interest. Using DEM or triangulated irregular networks (TIN), GIS can
automatically delineate a watershed for any outlet point. Further, watersheds can be
easily divided into subbasins by simply designating outlet points in any watershed.
As part of the delineation, basin data such as slope, area, mean elevation, distance to
centroid, and other common hydrologic parameters such as time of concentration
and Soil Conservation Service (SCS) runoff curve numbers are automatically computed. The curve number is a hydrologic parameter used to describe the stormwater
runoff potential for drainage area. It is a function of land use, soil type, and soil
moisture. SCS composite curve numbers can automatically be computed from land
use and soil data.
GIS automated watershed delineation consists of creating a depressionless DEM,
flow direction, flow accumulation, watershed outlet points, and delineating watersheds. The primary method involved in determining flow paths from DEM data is
the eight-point pour method developed by Puecker et al. [45]. The basic idea in
Puecker’s method is to assign a flow direction to each grid cell based on the
neighboring cell with the lowest elevation. Based on steepest slope, each cell is
assigned a flow direction in relation to one of its eight neighboring cells. The flow
214
S. O. Darkwah et al.