Lab 3: Watershed Analysis
Lab 3: Watershed Analysis
Goal
Watersheds are land surface areas, that drain water to one specific location, a group of streams that all lead to one place, would all have the same watershed.The goal of this lab is to delineate watersheds utilizing watershed analysis tools, from a DEM raster image. In this lab, Adirondack Park in New York is the site for attempting to delineate watersheds.
Methods
To begin the lab, an Adirondack Park boundary shapefile was obtained, from the New York State GIS Clearinghouse. Additionally a hydrology shapefile from Cornell Universities Geospatial Information Repository (CUGIR), and a #0_arc_second DEM of North America from ArcGIS online was utilized.
In order to determine watersheds, the obtained data needs to be processed. I began by creating a buffer around the park boundary in order to allow for a little more room around the park boundary, that way extra raster values can be included so more raster calculations can be performed especially around the edges of the park boundary. This buffer is used to clip streams to, this just limits the streams to one specific area.
Once all the data is manipulated, the DEM image is brought in and clipped to the study area.
Then the image is resampled to a 60m cell size which allows for a clearer image, where better watersheds can be delineated.
First a flow direction raster is created. this raster creates an output that will later be used to delineate streams prior to delineating the watersheds. This flow direction raster is then automatically filled using the fill tool. This tool corrects for Sinks,areas in the data that are very low. These Sinks are brought to the same level as there nearest neighbor creating a more level and accurate surface value. Then to delineate the actual streams, Flow accumulation tool is used which produces an output that looks faintly similar to streams. Then a source raster is created from this which will be used as an input for watershed delineation. This is don through using the conditional con tool, and creating a threshold that states, Value > 50000. This tool creates a threshold value that allows the watershed delineation tool to align watersheds too.
Finally the source, and flow direction raster are combined in the Watershed tool which creates a raster image showing where different watersheds are located.
Results:
The final output after performing all these methods to delineate watersheds is in figure 1. This first map shows 96 watersheds delineated from a 60m pixel value raster, and 50000 threshold value. With using a 60m pixel image, the surface is seen much better than a greater pixel value because it is a higher resolution. Additionally the threshold value used was at the right level for determining the watersheds. Figure 2 shows a 120m pixel value image. By having a higher pixel value, there is less detail in the image causing some of the smaller watersheds to be left out possibly.
Then the image is resampled to a 60m cell size which allows for a clearer image, where better watersheds can be delineated.
First a flow direction raster is created. this raster creates an output that will later be used to delineate streams prior to delineating the watersheds. This flow direction raster is then automatically filled using the fill tool. This tool corrects for Sinks,areas in the data that are very low. These Sinks are brought to the same level as there nearest neighbor creating a more level and accurate surface value. Then to delineate the actual streams, Flow accumulation tool is used which produces an output that looks faintly similar to streams. Then a source raster is created from this which will be used as an input for watershed delineation. This is don through using the conditional con tool, and creating a threshold that states, Value > 50000. This tool creates a threshold value that allows the watershed delineation tool to align watersheds too.
Finally the source, and flow direction raster are combined in the Watershed tool which creates a raster image showing where different watersheds are located.
Results:
The final output after performing all these methods to delineate watersheds is in figure 1. This first map shows 96 watersheds delineated from a 60m pixel value raster, and 50000 threshold value. With using a 60m pixel image, the surface is seen much better than a greater pixel value because it is a higher resolution. Additionally the threshold value used was at the right level for determining the watersheds. Figure 2 shows a 120m pixel value image. By having a higher pixel value, there is less detail in the image causing some of the smaller watersheds to be left out possibly.
Figure 1: This image shows watershed delineation using a 60m pixel value raster image The output shows that there are 96 watersheds Within the Adirondack Park Boundary.
Figure 2: This map depicts 41 watersheds after changing the Pixel value of the raster from 60 to 120.
Sources.
30-sec North American DEM
30-sec North American DEM
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