The Effects of Urbanization on Local First-Order Streams: One Year Later
Kristen K. Cecala, Steven J. Price, William Ringle, and Michael E. Dorcas
Davidson College
GIS is a typical abreviation of geographical information systems. This information can be used for a variety of purposes by a wide range of people. Individuals can use this information to assist with urban planning or understanding the distributions of people throughout a landscape or it may be used to predict species presence or absence throughout a landscape. These tools are extremely powerful and have allowed individuals to examine not only fine, local scale variables, but also examine how these local variables play into the surrounding landscape. ArcGIS (ESRI; Redlands, CA) is one program available to examine landscapes that has many additional extensions such as Spatial Analyst and other downloads to perform other more specific functions. One particular add-in is a program known as ArcHydro. Most planners are more familiar with the download Basin1 used in ArcView, but ArcHydro can compute all of the same variables and plots in ArcGIS. Because we sought to examine watersheds and first-order streams, we used ArcHydro to develop and calculate watersheds and streams. The first step towards developing streams is to obtain elevation data for the region one is working within, which provides much more information than a simple aerial photograph. We obtained elevation data from the North Carolina Department of Transportation that was collected in March, 2005. This data was collected by Lidar and is composed of 20 ft cells. Lidar is a remote sensing option to collect information about the range to an object due to light scattering. This type of remote sensing data is particularly appropriate for collecting elevation information. |
An aerial photograph of one region of northern Mecklenburg County, North Carolina.
A hillshade sketch of the elevation of the same region of northern Mecklenburg County, North Carolina. |
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To develop a stream layer for a region, the first step is to determine the direction water or rain would move for all points of a region and how fast it would move. This is very similar to determining the slope of a region.. Depending on the scale one would like to use, clipping or extracting a smaller region of the elevation data will allow individuals to examine finer scale detail such as first-order streams.
The flow direction of this region. The light blue indicates flows that should have lower velocities than areas in purple. This information is calculated based on the change in elevation of slopes.
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The next step is to illustrate areas in which the elevation data indicate water accumulation may occur. Water accumulation may be above or below ground. This is the first step in determining where streams run and where they initiate. |
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Next, ArcHydro requires another preprocessing step to delineate where streams occur. This step outlines where stream reaches have been identified using the flow accumulation calculations and flow direction calculations to determine direction. This step is known as stream definition.
This is the stream definition of the region. Although difficult to see at this scale, stream reaches are more connected than in the flow accumulation figure, but still not a fully connected network.
The next step is the stream segmentation step. This step separates the streams into segments that will be used to delineate the stream catchments.
In this figure, the stream segments are colored different colors to reflect different drainage areas.
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This step defines catchment regions. These regions are colored in a range of black to white in order to see a gradient throughout the landscape. This is not the final step in delineating a first-order stream catchment. Green areas reflect regions in which this program could not define a catchment or did not include this region with other catchments. This can be typical of regions with urban development. |
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Next, we define each catchment as a separate polygon. These are the final catchments for each stream segment. These regions define the area of the landscape that drains into one stream segment. Although we have proceeded through many steps to acquire this information, essentially we compute these basins based on simply the elevation data available to many on public websites.
Each different colored region reflects a different drainage area and shows the catchment for each stream within this region.
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The final step is to define the drainage line of each catchment. This is analagous to defining the stream layer for this area. Depending on the scale at which one begins, different order streams can be delineated. For this analyses, we used a small scale to identify first-order streams, but this program may also be used to delineate large watersheds such as the Chesapeake Bay or Catawba River. |
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