Lab 3

Goals and Background

          The goal of Lab 3 was to learn how to delineate watersheds and understand the concepts behind the analysis. Watersheds are geographically and environmentally important because the water within coverage at low points within the area and exit at a single point, forming rivers and lakes. This network of water sources make pollution particularly troublesome as it can effect all water sources downstream from it's point of origin. Therefore, delineating watershed is of special interest of land and water managers in order to help monitor the amount and quality of water networks within different watersheds.

Methods

          To begin Lab 3, download the Adirondack Park Boundary shapefile from the New York Stat Clearinghouse and unzip the data to your geodatabase. Open ArcMap and notice that the projection for Adirondack Park Boundary feature class is in NAD 1983 UTM Zone 18N in meters. All other features are going to be reprojected to this projection. But first, open the Buffer tool from the ToolBox within Analysis Tools > Proximity. Create a 20 km buffer around the park boundary, setting Dissolve to All. This will create smoother watersheds later in the analysis. 

          Use the Reproject tool from Data Management Tools > Projections and Transformation and reproject the hydrology feature class to the same projection as the park boundary. Utilize Import Projection to easily accomplish this. Use the Clip tool to clip the reprojected hydrology feature class to the original park boundary layer. 

          From Add Data From ArcGIS Online, add a raster called 30-arc-second DEM of North America. Since the DEM has a different project than the layers in the data frame, a window will appear. Click Transformations and set the transformation as convert from GCS_WGS_1984 to NAD 1983. Clip the DEM to the park boundary buffer and check Input Features for Clipping Geometry. Remove the original DEM as it's not needed anymore. Use the Project Raster tool to reproject the clipped DEM to the same projection as the park boundary layer. Use the same method as with the hydrology layer, but include the WGS_1984 to NAD_1983 transformation, choosing bilinear resampling method, and set the X and Y output cell size to 60 m. Once finished, display data that is only in the UTM projection.

          To delineate watersheds, flow directions need to be calculated for each cell. In the Spatial Analyst Tools > Hydrology, select the Flow Direction tool. Use the reprojected DEM as the input surface raster. Next, sinks need to be removed so that the water flow won't be disrupted falsely. Use the Fill tool (also in the Hydrology category) to fill sinks in the reprojected DEM. Determine flow direction for the filled DEM. Water accumulation areas need to be determined. Open the Flow Accumulation tool (Hydrology category) and use your second flow direction output as input. Lastly, a source raster is needed to create a threshold to determine the minimum number of cells that flow into any cell before it is designated as a stream cell. Open the Con tool from Spatial Analyst Tools > Conditional. Choose your water accumulation output as your input conditional raster. Set Type to Value > 50,000 and use 1 as your input true raster value. Label it as net_50k and run the tool. Open the Stream Link tool (Hydrology) to assign unique identifiers to each stream reach. Use net_50k as your input stream raster and use your second flow direction output as your input flow direction raster. Label it as source and run the tool. Open the Stream to Feature tool (Hydrology) to create vector streams using the source raster as your input stream raster and your second flow direction output as your input flow direction raster. 

          Finally, to delineate watersheds, open the Watershed tool (Hydrology). Use you second flow direction output as your input flow direction raster and your source raster as your input raster. Run the tool. Clip the output to the park boundary, checking Input Features for Clipping Geometry. Add the clipped hydrology layer to compare than generated watersheds to the stream locations (Figure 1). 

Results

          Comparing the results from the watershed delineation from a DEM with a cell size of 60 m (Figure 1) to a DEM with a cell size of 120 m (Figure 2), the differences are quit evident. Designating a larger cell size for the same DEM will simplify the raster, cascading its effects to the delineation. This creates less and oversimplified watershed areas. 

          The Vector Streams created from the Methods section are much more simplified versions of the hydrology layer (Figure 3). They follow the general trends of the rivers, being based of the flow direction, water accumulation, and an arbitrarily defined threshold value. 

Figure 1: Watershed delineation with a 50,000 cell threshold derived from a 60 cell size DEM.

Figure 2: Watershed delineation with a 50,000 cell threshold derived from a 120 m cell size DEM.

Figure 3: Vector streams from the watershed analysis compared to the hydrology feature class.

Sources

Barge, J. (n.d.). Adirondack Park Boundary [Downloaded Data]. Retrieved from http://gis.ny.gov/gisdata/inventories/details.cfm?DSID=303.

 National Aeronautics and Space Administration (NASA), the United Nations Environment Programme/Global Resource Information Database (UNEP/GRID), the U.S. Agency for International Development (USAID), the Instituto Nacional de Estadistica Geografica e Informatica (INEGI) of Mexico, the Geographical Survey Institute (GSI) of Japan, Manaaki Whenua Landcare Research of New Zealand, and the Scientific Committee on Antarctic Research (SCAR). 30-arc-second digital elevation model (DEM) of North America. Retrieved from ArcGIS Online.

Cornell University. hydrology. Retrieved from https://cugir.library.cornell.edu.