Forest Hydro Exercise

Forest Hydrology GIS Watershed Delineation Exercise

Objective: to create watershed boundaries from digital datasets and analyze stream networks and land cover characteristics within those boundaries

The Process

Like watershed delineation using paper maps, watershed delineation in GIS is a multi-step process.

1. Set up the ArcGIS working environment

2. Delineate Watershed

1. Create a Depressionless DEM

2. Create Flow Direction

3. Create Flow Accumulation

4. Create Watershed Pour Points

5. Delineate Watersheds

3. Analyze Watershed Stream Network

4. Analyze Watershed Elevation Data

5. Analyze Watershed Land Cover

6. Export your map

7. Backup your work

As you work through this process, you will be reminded to save your work periodically.

The Scholars’ Lab :: Spring 2011

Forest Hydrology GIS Watershed Delineation: 2

Set up the ArcGIS working environment

1. Go to the Scholars’ Lab Forest Hydrology class page:

http://www.lib.virginia.edu/scholarslab/resources/class/foresthydro/ and click the foresthydrodata.zip link. Open this file with WinZip, click the Extract button, and navigate to c:\localdata and click Extract

This will move the data to your computer and preserve the folders within the data. Your data sets will not work properly if the folder structure is not preserved.

2. Exit WinZip

3. Check the contents of c:\localdata\hydro to be sure you see the charlottesville100K, wytheville100k and referenceIndex folders have been added in the hyrdro folder.

4. Open ArcMap (Start > Programs > GIS> ArcGIS 9.3 > ArcMap) and accept the defaults to create a new map or double click the .mxd file in hydro.

5. Set these environment properties (Tools > Options > Geoprocessing > Environments)

1. Under General Settings, set the Current Workspace and Scratch Workspace.

2. Under Raster Analysis Settings, set the Cell Size as below. NOTE: Your layer DEM layer may have a different name.

Click OK to accept the Environment Settings changes. Click OK on the Options dialog box.



6. Enable the Spatial Analyst Extension. (Tools > Extensions > check mark next to Spatial Analyst)

7. Click Close to accept the change and close the Extensions dialog box.

8. Open Spatial Analyst Toolbar. (View > Toolbars > Spatial Analyst). We will use this toolbar later in the exercise.

9. Verify the ArcToolbox window is open. (Window>ArcToolbox or by using the ( ) button. We will use the toolbox later in the exercise.

10. Save the map document (File > Save As) with this name: C:\Localdata\hydro\[area]100k\watershed.mxd.



Delineate Watershed

• Create a depressionless DEM

It is important to start with a digital elevation model that has no depressions.

1. Open the ArcToolbox toolset Spatial Analyst Tools > Hydrology. This is where you will find the surface hydrology tools.

2. Open the Fill tool. The input surface is the dem grid. Default output is C:\Localdata\hydro\[area]100k\Fill_[area]1001. Click OK. When complete, click Close.

3. When the Fill tool completes, a new layer, Fill_[area]1001, will be added to the data frame. This is identical to your original dem raster, but any areas of internal drainage are filled in.



Note the difference in the lowest elevation value in the legend; sink cells in the original data set have been filled in.

4. Remove the original dem layer from the data frame (right click on the [area]100kned layer and select Remove). The rest of the exercise will use the filled grid layer. It is important to have a depressionless DEM for all subsequent hydrological analyses. Areas of internal drainage can cause problems later in the watershed delineation process.

5. File > Save to save your work.

• Create Flow Direction

1. Open the Flow Direction tool (ArcToolbox > Spatial Analysis Tools > Hydrology > Flow Direction).

1. The input surface is the filled dem grid. 2. The output raster should default to C:\Localdata\hydro\[area]100k\FlowDir_fill1

3. Click OK to run the Flow Direction tool. When it completes, click Close to exit the Flow Direction dialog box. A new flow direction layer is added to your map.



2. The numbers in the Flow Direction layer show direction of flow.

Direction of flow must be known for each cell, because it is direction of flow that determines the ultimate destination of water flowing across the surface.

3. Turn off display of the filled dem layer by unchecking the box to the left of the Fill_[area]1001 layer.

4. To make your streams layer easier to see - [area]100khydro - click on the line symbol below your streams layer in the Table of Contents and select a new color and thickness in the Symbol Selector dialog box, then click Close.


• Create Flow Accumulation

1. Open the Flow Accumulation tool under Hydrology.

1. Set the input flow direction raster to the output of the last task: C:\Localdata\hydro\[area]100k\FlowDir_fill1

2. Set the output raster to C:\Localdata\hydro\[area]100k\FlowAcc_flow1 (default name).

2. Click OK to run Flow Accumulation, then Close to exit when complete.

3. Turn off the flow direction layer and the hydro layer by unchecking each in the table of contents.



4. The new flow accumulation layer has been added to your map. It may appear very dark and

difficult to interpret. Here’s an explanation:

Each cell contains a value representing the number of cells upstream from that cell. Cells with higher values will tend to be located in drainage channels rather than on hillsides or ridges. We will symbolize the flow accumulation layer to make these differences clear.

5. Alter the symbology for this new layer. It will be easier to visualize high-flow pathways by altering how cells are displayed.

1. Right click on [layer name] > Properties > Symbology. In the Show: box choose Classified. If you are prompted to compute a histogram, choose yes.

2. In the Classification box, set the number of classes to 2.

3. In the Layer Properties dialog, click the Classify button to open the Classification dialog. Under Break Values, click on the first break value and type 5000. Click OK to close the Classification dialog.

4. Change the symbology (no color for the first class, red for the second class) by double clicking each of the color swatches under Symbol and selecting the appropriate color. Click OK to apply your changes.




6. Now the cells that are displayed in red have the flow of at least 5000 upstream cells flowing through them.

You will need to zoom in before you can see the details of the flow network.

You should also see that the DEM-generated drainage network looks somewhat like the streams hydro layer, although if you turn on your hydro layer and zoom in you will see where the two data sets do not line up.

Flow accumulations are important because they allow us to locate cells with high cumulative flow. Each watershed has an outlet point called a “Pour Point”. Pour points must be located in cells of high cumulative flow or the resultant watersheds will be very small.



• Using Topographic Map Layer

To help locate the watershed you delineated from the paper topographic map, we will add a scanned version of that paper topographic map to our GIS project.

1. Make sure the topo layer appears below the flow accumulation layer in the ArcGIS table of contents. You can rearrange the layers by dragging the topo map layer in the table of contents.

2. Using the topo map as a basemap along with your flow accumulation layer, zoom in to the area surrounding the pour point for your watershed.

3. Turn off the topo map layer in the table of contents.


• Create Watershed Pour Points

We will create a new shapefile to store the pour point(s) we are about to identify.

1. Click the ArcCatalog icon ( ) in the ArcMap toolbar. In ArcCatalog in the leftmost panel, expand the C: drive to browse to your C:\Localdata\hydro\[area]100k\ folder and highlight the folder name. Open the Create New Shapefile dialog: File > New > Shapefile, then enter ppoints as the shapefile name.

2. Click the Edit button, then the Import button. Select your dem layer (either wyth100kned or char100kned). This defines the spatial reference for the new ppoints shapefile to be the same as your existing dem layer. Click OK to exit Spatial Reference.

3. Click OK to create the new ppoints layer.

4. Close ArcCatalog File > Exit

2. Back in ArcMap, add the new point layer (ppoints) to your data frame by using the Add Data

button ( ).



3. Activate the editor toolbar (View > Toolbars > Editor)

4. From the Editor Toolbar, choose Editor > Start Editing. In the Start Editing dialog, highlight the Source line which includes the Type of Shapefiles and click OK.

If you see the following dialog, click Start Editing.

5. In the Editor Toolbar be sure that Task: is Create New Feature and Target: is ppoints

6. In order to better see where you are adding a pour point, open a magnifier window (Window > Magnifier). Drag the magnifier window by the top bar and drop the crosshairs over your pour point. Experiment with different zoom levels until you can see individual raster cells.



7. You may need to zoom in and increase the zoom factor of the magnifier before adding any points. You must be able to see the individual raster cells to locate pour points.

8. Use the Identify tool ( ) from the Tools Toolbar to examine the values of the Flow Accumulation layer before adding any points. You will be able to tell which direction is upstream (lower accumulation value) and which direction is downstream (higher accumulation value).

9. Add a pour point by selecting the Sketch tool ( ) from the Editor toolbar and be sure that the target (in Editor toolbar) is ppoints. Click on the map to locate the pour point. You must zoom in closely to locate a pour point; otherwise your pour point may not be located within a high-flow pathway! Everything upstream from the pour point will define a single watershed.

10. If you are defining only one watershed, go to the next step. If you are defining more than one watershed, add one pour point for each watershed, then open the attribute table for the ppoints layer. (right click on layer, Open Attribute Table) Alter the ID field to represent unique values for each different record (e.g., 1 and 2). Once you have added the points and altered their IDs, stop editing the layer, making sure to save the edits. (Editor > Stop Editing > Save Edits? > Yes) Watersheds are defined by pour point IDs; if you do not alter the ID values for the points, there will be only one value, and you will not generate multiple unique watershed areas.



11. Save your newly defined pour point(s). (Editor > Stop Editing > Save Edits? > Yes). Close

the Identify and Magnifier windows and the Editor toolbar.

12. Select Tools > Options > Geoprocessing > Environments > General Settings > Extent > “Same as layer FlowAcc_flow1” to make sure the output extent will be correct and avoid truncating the watershed. Click OK. Then OK again to close the Options dialog.

Using the Spatial Analyst toolbar pull-down, set Spatial Analyst > Options > Extent > Analysis extent: “Same as layer FlowAcc_flow1”

Set Spatial Analyst > Options > Cell Size > Analysis cell size: “Same as layer FlowAcc_flow1” . Click OK to close the Options dialog.



If the Analysis Extent and Cell Size do not match an existing layer, there will be problems of registration between the pour point raster and the other raster layers necessary to delineate watersheds. Therefore, it is always a good idea to set your cell size and analysis extent relative to an existing raster layer

13. In ArcToolbox you will use the Spatial Analyst Tools > Hydrology > Snap Pour Point tool to ensure your pour point(s) are located on the highest flow accumulation cell within a specified radius.

The Snap Pour Point tool also converts your Pour Point(s) into the raster format needed later in the watershed delineation step.

Open the Snap Pour Point tool. Before running the tool, at the bottom of the Snap Pour Point dialog box, click Environments > Raster Analysis Settings > Cell Size > Same as layer FlowAcc_flow1.

Click OK to close the Environment Settings dialog.



Verify your Snap Pour Point dialog is filled in as above. The Snap Pour Point tool will search within a Snap Distance around your pour point(s) for the cell of highest accumulated flow and move the pour point to that location. Based on our datasets, a snap distance of 100 searches within an approximate 3 cell radius to find a cell with higher flow accumulation. So if your pour point is not located in a high-flow cell, the tool will move it for you. If you have already accurately located your pour point on a high flow cell, a value of 100 will serve to move the pour point 3 cells downstream. In this case, a Snap Distance of 0 is appropriate. Click OK to run the tool. When the tool completes click Close to exit.

14. When the tool completes, the new output SnapPou_ppoi1 layer is automatically added to your map and contains the new snapped pour point(s). Use the Zoom tool to see where the Snap Pour Point tool located your new pour point(s). Your colors may vary. In the example below using a snap distance of 100, the dot (orange) is the original pour point. The square (green) is the snapped pour point.

The new SnapPou_ppoi1 layer cell(s) identifies the place where the snapped pour point(s) were added.



If your original pour point(s) were already located within a high-flow pathway, you will find the snapped pour point(s) are moved downstream by the size of your snap radius. If it is important to your analysis to keep the exact cell location where you located the original pour point(s), re-run the Snap Pour Point tool with a radius of 0. If your pour point raster cell(s) are not directly “over” the high-flow pathways, the watersheds you create will represent very small drainage areas.

• Delineate Watersheds

1. Open the Watershed tool under Toolbox > Spatial Analysis Tools > Hydrology. Note that one of the options for the input data is the point feature dataset (ppoints). Selecting a point feature dataset will work only if the pour points fall in a high-flow pathway. In the previous steps, we used the Snap Pour Point tool to convert the pour point(s) to raster format and to ensure they fall within a high-flow pathway, so we will use the raster layer SnapPou_ppoi1 as input.

1. Select the flow direction grid as the input flow direction raster.

2. Select the raster version of the pour points (SnapPou_ppoi1 ) as the input raster.

3. Accept the other defaults and click OK. When complete, click Close.



2. The new watershed raster will be added to the data frame. Zoom out to see it all.

If your watershed is too small or otherwise not what you expected, it could be because you located your pour points outside of a high-flow pathway, or you did not fill the sinks in your input digital elevation model. To correct these errors, return to those steps in the exercise, correct the error, and follow through all the remaining exercise steps.

3. Convert the watershed raster to a polygon shapefile for area calculation and for use later to clip other datasets. (ArcToolbox > Conversion Tools > From Raster > Raster to Polygon). Set the parameters as below, then click OK to run the tool. When complete, click Close.

The new polygon is added as a layer in the Table of Contents with a default color fill. If you wish to change or remove the color fill, click the color swatch under the layer to change the symbology. Click OK when complete.



Calculating Watershed Area

1. Open the Attribute table for the new shapefile. Right-click on layer > Open Attribute Table.

2. Add an Area field by selecting Options > Add Field. 3. Type Area for Name and select Double as Type. 4. Right-click on header of new column and select

Calculate Geometry. 5. Select Units and hit OK.

Analyze Watershed Stream Network

To gather data about the streams inside our watershed, we will clip the original stream shapefile to our new watershed boundary polygon.

1. In ArcMap, open the Clip tool: ArcToolbox > Analysis Tools > Extract > Clip 2. Set Input features to [area]100khydro 3. Set Clip features to your watershed boundary polygon RasterT_watersh1 4. Accept the defaults for other values

5. Click OK to run the Clip tool. Click Close when complete. 6. The clipped stream layer is added to the map [area]100khydro_Clip. Turn off all the

other layers to better see the new clipped stream layer.



7. To view details of the streams inside our watershed, right click the

[area]100khydro_Clip layer > Open Attribute Table

The “meters” and “feet” attributes are not updated automatically in this case when the streams are clipped. We are still seeing the pre-clipped stream lengths. We need to update the lengths based on the newly-clipped streams.

8. Right click on the column heading “Meters” and choose Calculate Geometry. If you see a Calculate Geometry dialog, click Yes. Set the options as shown below and click OK to update the “Meters” values.

This column now contains the length in meters of each stream inside the watershed boundary.

9. Update the “Feet” column using the same technique with Units: Feet US (ft).

10. You may want to export this attribute table of stream length values to a file for further analysis. At the bottom of the Attribute dialog click Options > Export…. and navigate to the c:localdata\hydro\[area]100k folder (or another folder) to save your data. Name



the file streamlengths.dbf. This output file will contain the same rows and columns you see in the attribute table. The output format is .dbf, a file type which can be opened by Microsoft Excel. Click OK to export the data.

11. When asked if you would like to add the new table to the current map, click No.

Analyze Watershed Elevation Data

To analyze elevation data for our watershed, we will clip the filled digital elevation model raster to our new watershed boundary polygon.

1. In ArcMap, open the Extract by Mask tool: ArcToolbox > Spatial Analyst Tools > Extraction > Extract by Mask. Use the input values for your study area to clip the filled dem raster. Click OK

The new clipped dem is added to the map as a new layer. Click Close to exit the Extract by Mask dialog. Maximum and minimum elevation values for the clipped DEM are displayed in the table of contents and are also available here: Right Click layer name > Properties > Source tab

Analyze Watershed Land Cover

To analyze land cover data for our watershed, we will use the 2001 National Land Cover dataset (NLCD) described here: http://www.mrlc.gov/nlcd.php . We will clip the land cover dataset to the boundary of our new watershed polygon.

1. Open the Extract by Mask tool: ArcToolbox > Spatial Analyst Tools > Extraction > Extract by Mask. Use the appropriate input values to clip the land cover raster foryour study area.

2. The new clipped land cover raster is added to the map. When complete, click Close to exit the Extract by Mask dialog.

3. In the Table of Contents, turn off the [area]landcover layer to better see the newly clipped land cover layer.



4. To view the breakdown of land cover types in the watershed: For you new clipped land

cover layer Right Click layer name > Properties > Open Attribute Table.

5. The VALUE column contains the NLCD 2001 class definition categories which are

explained here: http://www.mrlc.gov/nlcd_definitions.php 6. The COUNT column describes the number of raster cells in the watershed for each

category. 7. The geographic area covered by each raster cell is found in the metadata for the clipped

land cover layer. To view the metadata, right click the clipped land cover layer name > Data > View Metadata.

8. Set the Stylesheet to FGDC. 9. Click Spatial Data Organization Information to find the cell size in the x and y

directions. 10. Click Spatial Reference Information to find the unit of measure in the “Planar distance

units” field. 11. Using these inputs, we can calculate areas and percentages of land cover by land cover

category for the watershed. 12. You may want to export this attribute table of land cover values and cell counts to a file

for analysis. At the bottom of the screen click Options > Export…. and navigate to the c:localdata\hydro\[area]100k folder (or another folder) to save your data. Give the file a unique name you will remember. This output file will contain the same rows and columns you see in the attribute table. The output format is .dbf, a file type which can be opened by Microsoft Excel. Click OK to export the data.

13. When asked if you would like to add the new table to the current map, click No. 14. Close the Attribute table dialog. Close the Spatial Analyst toolbar. 15. Click File > Save to save your ArcMap document.

Export the Watershed Map

Before exporting your map, zoom to the general area you wish to show, then turn on/off the layers you want to appear/not appear and adjust the symbology as you see fit.

1. Use View > Layout View to switch to layout view where you will see your map on a virtual 8.5” x 11” page in portrait mode.

2. Use File > Page and Print Setup if you want to switch to landscape mode. 3. Use the ArcMap Insert menu to add a Title, Legend, North Arrow, Scale Bar, text, etc.

to your map. These items are only available to insert when in Layout View. You can use View > Data View to switch back to data view at any time without losing the items you’ve added in Layout View.



4. File > Save to save your work. 5. At any time, use File > Export Map to create a .jpg, .pdf, or other file formats to export

your map. If you are in Layout View, the exported map will include the additional items (Title, Legend, etc.).

6. Exit ArcMap. File > Exit

Backup Your Work

If you’ve completed the exercise, gathered the required information, and will not need to access the exercise results or output files again, then no worries about backing up your work.

Otherwise, the c:\localdata directory is regularly purged from Scholars’ Lab machines. So files left on the Scholars’ Lab machines today may not be there tomorrow.

To back up your work for future reference check the size of folder c:\localdata\hydro\[area]100k (should be <150mb) and copy the complete folder and all subfolders to your home directory, a thumb drive, or other media.

Most important is to copy and move these folders as a group without renaming or modifying any individual file or folder.

To open the ArcMap project later, copy the [area]100k folder and all files and subfolders onto your local machine, open ArcMap, and navigate to the [area]100k folder to open your ArcMap project, watershed.mxd Your project will open with all the symbology and layers showing as they looked when you last did a File > Save.



Definitions

Clip - In ArcGIS, a command that extracts the features from one layer that reside entirely within a boundary defined by features in another layer.

Coverage - A data model for storing geographic features using ArcGIS software. A coverage stores a set of thematically associated data considered to be a unit. It usually represents a single layer, such as soils, streams, roads, or land cover. In a coverage, features are stored as both primary features (points, arcs, polygons) and secondary features (tics, links, annotation). Feature attributes are described and stored independently in feature attribute tables. Coverages cannot be edited in ArcGIS.

Digital Elevation Model (DEM) - The database for elevation data by map sheet from the National Mapping Division of the USGS. DEMs are rasters

Layer - The visual representation of a geographic dataset in any digital map environment. Conceptually, a layer is a slice or stratum of the geographic reality in a particular area, and is more or less equivalent to a legend item on a paper map. On a road map, for example, roads, national parks, political boundaries and rivers are examples of different layers.

In ArcGIS, a reference to a data source, such as a coverage, geodatabase feature class, raster, and so on, that defines how the data should be displayed on a map. Layers can also define additional properties, such as which features from the data source are included. Layers can be stored in map documents (.mxd) or saved individually as layer files (.lyr).

Overlay - Superimposing two or more maps registered to a common coordinate system, either digitally or on a transparent material, for the purpose of showing the relationships between features that occupy the same geographic space.

Pour Point – Outlet of flow from a given watershed area.

Projected Coordinate System - A reference system used to locate x,y, and z positions of point, line, and area features in two or three dimensions. A projected coordinate system is defined by a geographic coordinate system, a map projection, any parameters needed by the map projection, and a linear unit of measure.

Raster - A spatial data model that defines space as an array of equally sized cells arranged in rows and columns. Each cell contains an attribute value and location coordinates. Unlike a vector structure, which stores coordinates explicitly, raster coordinates are contained in the ordering of the matrix. Groups of cells that share the same value represent geographic features.

Shapefile - A vector data storage format for storing the location, shape, and attributes of geographic features. A shapefile is stored in a set of related files and contains one feature class.

Slope - The slope for a cell in a raster is the steepest downhill slope of a plane defined by the cell and its eight surrounding neighbors. Slope can be measured in degrees from horizontal (0–90) or percent slope, which is the rise divided by the run, multiplied by 100. A slope of 45 degrees equals 100 percent slope. As slope angle approaches vertical (90 degrees), the percent slope approaches infinity.

Vector - A coordinate-based data model that represents geographic features as points, lines, and polygons. Each point feature is represented as a single coordinate pair, while line and polygon features are represented as ordered lists of vertices.