GIS Fundamentals: Supplementary Lessons with ArcGIS Pro Watershed Analysis

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Watershed Analysis What You’ll Learn:

- Practice various skills using ArcGIS Pro. - Hydrologic processing, including DEM pit removal, calculation of flow direction,

upslope contributing area, stream location, and watershed boundary identification.

Data are in the L:\FNRM5131\Semester_Assignment\Watershed directory on the server and also in a zip file on the FNRM5131 course website. What You’ll Produce: A series of maps, in the four parts of this exercise, depicting various aspects of terrain, landcover, and extracted hydrologic properties. Background: These exercises re-iterate tools taught in labs, and in part 3, introduce additional extensions. The lab descriptions “hold your hand” through most processes, and here we provide a synthesis and application with just the goals, and a few pointers, to help you better learn the material to solve problems. Part 1 – (uses skills taught in lessons 1 & 2) • Create a workspace for this project and copy

all the data into a directory. • Create a base map using ArcGIS Pro.

• Load the elevation (app_dem), roads

(ClayRDS for Clay County, MaconRDs for Macon County), and USGS water discharge gages (usgs_gages) into a data view.

• Create and print a properly titled map, with north arrow, scale bar, your name and simple description in place of a formal legend. Save the map composition (.aprx file) for later work.

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Part 2 – Projections, Hillshade, Editing (uses skills taught in lessons 3, 4, 5, & 6) • Add the NLCD2001 grid to the project created in Part 1.

Examine the properties for each data set, particularly the map projection of each data set (right click the data name in the table of contents, then properties-source). Some data layers are not in our preferred projection, UTM, NAD83 zone 17 meters. Project all offending data layers into our preferred UTM (refer to Lesson 3). Note you should use the appropriate datum transformation, NAD_1927_To_NAD_1983_NADCON. Display the UTM data in your data frame. Make sure the data frame coordinate system is also set to UTM, NAD83 zone 17 meters. From now on, any reference to data layers is to these new, projected layers.

• To increase the speed of redraws, turn off the display of the NLCD data. You may often do this for large, detailed rasters, when working on complex projects.

• Create a hillshade layer from the DEM, and save it as an output grid

(ArcToolboxSpatial Analyst Tools-SurfaceHillshade, specify an output data file – see the graphic on above, but also discussed briefly in Lab 6).

• Edit both the Clay and Macon County roads data layers (UTM NAD83 zone 17N versions), first by removing the roads that lie outside the area defined by the elevation data set (see labs 4 and 5 or see the video (Raster_Clippin_Video.mov). Don’t be too worried about cutting exactly at the edge, within 15 meters is good.

• Now improve a small portion of the Macon County roads data. The original data are from 2000 U.S. Census, 1:100,000 scale source files. They lack many roads, and put many in the wrong locations. You will interpret/edit/add road locations for a small portion of Macon County. Display the color infrared images CartSubIR1.img and CartSubIR2.img. These images were collected in 1998, about the same time as the Macon County roads data were collected. Using the images as a guide, digitize and clean up the errors on the existing roads, and add new roads for the entire area within the bounds of these images (see lab exercises 4 and 5 for methods). Save the roads data to a new data layer, named something like “NewMaconRoads.”

• Print a properly composed map that shows the

hillshade, land cover, updated roads, and gage locations. Place the land cover on top of the hillshade and make the landcover somewhat transparent (30 to 50%) to make the hillshading visible. Symbolize the roads and gages so they stand out.

https://www.youtube.com/watch_popup?v=K3DVWDUmYqQ

GIS Fundamentals: Supplementary Lessons with ArcGIS Pro Watershed Analysis

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Part 3 (skills from labs 9, 10, and 11) We’ll now identify the stream locations, slope, flow direction, and watersheds for each of the gage stations, through a series of manipulations on the DEM. Use the hydrology tools included in ArcGIS and used in lab 11. Hydrology tools may be found in ArcToolboxSpatial Analyst ToolsHydrology (see video Watershed.mov). Note that the documentation for the ArcGIS hydrology tools is best found at the using the Help on the ArcMap main toolbar and doing search of Help using the phrase “An overview of the Hydrology toolset”. Read the introductory, Fill, Sink, Flow Direction, Flow Accumulation, Snap Pour Point and Watershed descriptions. Then: • All data and the data frame coordinates should be set to

NAD83, UTM Zone17N meters. • Note that the elevation data must be stored as and ESRI GRID

file. If your data are in an .img format, you must change your .img to a GRID layer. If your data are already projected to NAD83 UTM, they may most easily be converted to GRID by - adding the .img file in a new ArcMap session - making the .img file active (left click on it) then right click and

DataExport Data. - Give the new output layer a name and choose the export file

format of GRID. - Load the GRID elevation data into a new data view.

• Remove pits from the elevation data using the Hydrology/Fill tool. Enter the reprojected DEM as the input surface raster; name the output DEM something descriptive like fill_dem and leave the Z limit blank (fill all pits).

• Calculate the flow direction for your filled DEM with the Hydrology-Flow Direction tool. Again, name the output file something descriptive, like flowdir, force all edge cells to flow outward, and leave blank the designation for the output drop raster. Read the description of the numbering system for flow direction. Note that this method, called the D8 algorithm, forces flow to one of 8 directions – there is no partial flow to cells. There are other methods that are generally more accurate, but this is the easiest to program and ESRI’s choice.

• Calculate the flow accumulation grid (Hydrology-Flow Accumulation), using the just created flow direction grid as input, naming the flow accumulation something like flowacc, leaving the weight surface blank, and specifying FLOAT output.

• Next, we need to create a vector stream network. Streams form when there is between a 7 and 15 hectare contributing area. This translates to on average about 110 of the 30-meter square cells. Reclassify the flow accumulation grid so that cells with flow accumulation values of over 110 cells are given a value of 1 (one), and all

GIS Fundamentals: Supplementary Lessons with ArcGIS Pro Watershed Analysis

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other cells are given a value of 0 (zero). Remember, flow accumulation is a grid, so use the reclassify tool in the Spatial Analyst toolbox (see lab 11 for reclassification). Then, convert this raster layer to a vector layer.

• Use the USGS gauges as guides to create the outlets for the pour points. Make sure establish a field to differentiate between the two gauges.

• Calculate the watershed using the watershed function, the pour points, and the calculated flow direction surface from above.

• Convert the two raster watersheds into separate vector layers

Calculate and record the areas for the two vector watersheds (open the table, then calculate geometry on an appropriate column). Map 1, Part 3: Create a layout of the data you created, including in order from bottom to top, the DEM, the hillshade (with approximately 50% transparency), the watershed boundaries, the streams, and the USGS gaging stations. The data view should look something like the figure to the right, although your streams may look slightly different (different threshold used here). Annotate the layout with the two-watershed labeled by their USGS gage names, with the usual north arrow, scale bar, legend, and descriptive title. Be sure to include the area of each watershed, in square kilometers, somewhere on the map. Submit a pdf via Moodle.

GIS Fundamentals: Supplementary Lessons with ArcGIS Pro Watershed Analysis

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Part 4 Folks do many analyses with watershed, hydrology, and other data. We’ll finish with a sample a very simple analysis relevant to watersheds and water quality. Open a new map composition and add the elevation, stream, roads, watershed boundaries, and NLCD data to the data view. We are going to perform two main exercises. First, we are going to summarize the NLCD land cover by watershed. We want to calculate the percentage of each watershed in each of four broad classes: 1) Developed/urban, and barren, 2) agricultural (includes pasture/hay, grassland, and cultivated crops), 3) forest and shrub, and 4) water and wetlands. There are several ways to summarize land cover, and any way you choose that works is fine. We will provide the general outline for a method that we think is conceptually straightforward but requires a few more steps than some more direct methods. First, you need to recode the NLCD data to the four broad classes listed above. The original codes are: 11-water, 21-developed, open space (e.g., golf courses), 22-developed, low intensity, 23-developed, medium, 24-developed, high intensity 31-barren land, 41-deciduous forests, 42-evergreen forests, 43-mixed forests, 52-shrub/scrub, 71-grassland/herbaceous 81-pasture/hay, 82-cultivated crops, 90-woody wetland. Use the Reclass function in the ArcToolbox (see lab 10) to assign these categories to the 4 broad classes noted two paragraphs up: 1-Urban/developed, 2-agricultural, 3-forest&shrub, 4-water&wetlands. Name the reclassified raster reclassnlcd. Next, you need “masks” or “zones” for each watershed. If you saved the raster version of the watersheds you created in Part 3, and then use this. If not, then create one (ArcToolboxConversion ToolsTo RasterPolygon to Raster) with the same spatial resolution as the NLCD data. The raster values should be different for the two watersheds, e.g., the Cartoogechaye with the number 1, and the Nantahala with the number 2.

GIS Fundamentals: Supplementary Lessons with ArcGIS Pro Watershed Analysis

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Now, summarize areas by watershed. Display the toolbox, then select ArcToolboxSpatial Analysis ToolsZonalTabulate (see at right): Note you should use the watershed grids for the input raster zone data, VALUE for the zone field, the NLCD data for the input raster, VALUE for the class field, some output table name and 30 for the cell size. Next This should create a table that shows the zone (watershed) data organized in a row, with columns for land classes. The columns report number of cells in each zone/class combination. You can use the measure tool and knowledge of the grid cell size (about 30 meters) to make sure the areas are reasonable (e.g., so you don’t report they are unimaginably large or small). Map 1, Part 4: Create and export a pdf output map that shows the watersheds in their four classes and turn this in via Moodle. Include a table in the map that shows the summary land classes area. Report the class area as a percent of the total area for each watershed. The map portion of the output should appear similar to the figure to the right, and along with the tables of area, be sure to include the usual north arrow, name, scale bar, legend, and description. Now you will perform a buffer analysis to identify the total length of roads that are close to streams in each watershed. If you haven’t already, add the roads and streams data to the data frame. Note that you should use a merged roads layer that includes both the ClayRD.shp and the MaconRD.shp layers (you may need to use ArcToolboxData Management ToolsGeneral Merge). Clip the roads data to the extent of the watersheds (remember, ArcToolboxAnalysis Tools Extract Clip). You should also clip the streams to the extent of the watersheds.

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Now, buffer the clipped streams to 150 meters (ArcToolboxAnalysis ToolsProximity Buffer). Select a simple buffer, at a fixed distance of 150 meters, dissolve based on ALL attributes, and save to a file. Now, intersect the clipped roads with the stream buffer (ArcToolboxAnalysis ToolsOverlay Intersect). This should generate an output set of roads. Finally, calculate the total road length (kilometers) in each watershed, the road length within 150 meters of a stream (kilometers), and the percentage of roads within 150 meters of a stream for each watershed. Map 2, Part 4: Create a map with the watershed boundaries, watershed streams (clipped), watershed roads, and the stream buffers. Include text or a table on the map that the total and percentage of roads close to streams for each watershed. Note that your stream network may be a bit different, as we used a different threshold for these examples (more than one student in times past have scanned our example, pasted it into a pdf, and tried to pass it off as their own work).

Create a pdf of this map composition, including as always, the title, legend, north arrow, scale bar, and your name, and submit it for grading via Moodle.