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The resulting raster assigns each cell a value of 1 when it intersects with a flowline and a value of NoData when it does not intersect a flowline.

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Reclassifying a raster

Though you now have a uniform value of 1 to flag the presence of a flowline cell, the remaining cells have a value of NoData, which is not useful for performing mathematical operations across all the cells, so you would like to reassign all cells with a value of NoData to a value of 0.

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  1. In the New values column, replace the “NODATA” value with a value of “0”.
  2. For ‘Output raster’, rename the raster from “Reclass_Flow1” to “Reclassify”.
  3. At the top of the Geoprocessing window, click the Environments tab on the right.
  4. Under the 'Processing Extent' section, for 'Extent', select Same As layer: DEMft.
  5. Under the 'Raster Analysis' section, for ‘Cell Size’, select Same as layer DEMft.
  6. For ‘Snap Raster’, select the DEMft raster.
  7. Ensure your ‘Reclassify’ window Geoprocessing pane appears as shown below and click Run.

  

You will notice that the output raster is a rectangle that contains the geometry of the subbasin – but does not match it exactly.  In the next section (Calculating Euclidean Distances) you will assign a “mask” that will define a more specific geometry.  The null values in this raster prevent this from being an option - so we will retroactively clip the reclassified raster.

  1. At the top left of the Geoprocessing pane, click the Back button.
  2. In the search box, type "clip".
  3. Click the Clip Raster tool.
  4. For ‘Input Raster’, drag in select the Reclassify layer.
  5. Use the ‘Output Extent’ drop-down menu to select the Subbasin_StatePlane layer
  6. Click ‘Use Use Input Features for Clipping Geometry’Geometry.
  7. For ‘Output raster’, rename the raster to “FlowlineReclass”“FlowlineReclass”.
  8. Ensure that your ‘Clip’ window your Geoprocessing pane appears as shown below and click Run.



  9. Open the attribute table for the “Reclassify” layer and note the ‘Count’ of cells with a value of 0.
  10. Now open the attribute table for the “FlowlineReclass” layer and note the ‘Count’ of cells with a value of 0.

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Now all cells that were previously assigned a value of NoData within the subbasin geometry have been assigned a value of 0.  This will allow you to perform uniform mathematical operations across the entire subbasin in the future.

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Calculating Euclidean distances

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  1. At the top left of the Geoprocessing pane, click the Back button.
  2. In the search box, type "euclidean".
  3. Click the Euclidean Distance tool.
  4. For ‘Input raster or feature source data’, drag in the select the Flowlines_StatePlane layer from the Contents pane.
  5. For ‘Output distance raster’, rename the raster from “EucDist“EucDist_Flow1” to “FlowlineDistance”Flow1” to “FlowlineDistance”.
  6. Click the Environments… button.
  7. For Processing Extent, use the ‘Extent’ drop-down menu to select Same as layer DEMft.
  8. Use the ‘Snap Raster’ drop-down menu to select the DEMft raster.

In order to output a raster with the desired geometry (in this case the same as DEMft or subbasin) you must apply a mask.  Without a mask, the output raster will be a rectangle that contains the geometry.

  1. Use the ‘Mask’ drop-down menu to select the Subbasin_StatePlane layer.
  2. Go back to Parameters, ensure your ‘Euclidean Distance’ window your Geoprocessing pane appears as shown below and click Run

  3. Right-click FlowlineDistance layer and click Symbology.
  4. Select Quantile for Method and 5 for classes.

You should now see cells that vary in color the farther away they are from the nearest flowline.

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Performing map algebra

Finally, you are ready to perform the reconditioning step. You can think of this as digging a trench with a funnel-shaped cross-section along each flowline to ensure that water will flow in the intended direction. The math behind this step is explained in the diagram below and all units are in feet, since the State Plane Texas South Central project stores XY coordinates in feet and since you also recalculated the Z elevations in Lab 2 in feet.

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  1. By double-clicking the layer names and single-clicking the calculator buttons, enter the following equation: "DEMft" - 30 * "FlowlineReclass" - 0.02 * (1500 - "FlowlineDistance") * ("FlowlineDistance" < 1500).
  2. For ‘Output raster’, rename the raster from “demft_raster” to “DEMRecon”.
  3. Ensure your ‘Raster Calculator’ window your Geoprocessing pane appears as shown below and click Run.

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  1. Click the Tools tab.
  2. Double-click the Raster Calculator tool again.
  3. Using the same clicking technique as before, enter the following equation: "DEMft” – “DEMRecon”DEMft” – “DEMRecon”.
  4. For ‘Output raster’, rename the raster to “DEMDiff”.
  5. Ensure your ‘Raster Calculator’ window your Geoprocessing pane appears as shown below and click Run.

The DEMDiff layer displays the amount of earth that has been removed through the reconditioning process.

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Part 3: Analyzing Hydrologic Terrain

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The Fill tool fills sinks in the DEM raster to remove small imperfections in the data. If cells with higher elevations surround a cell, the water gets trapped in that cell and cannot flow. The Fill tool modifies the elevation of the original DEM to eliminate these problems.

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  1. At the top left of the Geoprocessing pane, click the Back button.
  2. In the search box, type "fill".
  3. Click the Fill tool.
  4. For ‘Input surface raster’, select the DEMRecon layer.
  5. For ‘Output surface raster’, rename the raster from “Fill_DEMReco1” to “FIL”.
  6. Ensure your ‘Fill’ window your Geoprocessing pane appears as shown below and click Run.

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Now you will calculate the cardinal direction in which water from each cell is expected to flow based on the elevations of surrounding cells. Each direction is assigned one of 8 integer values between 1 and 128 as indicated in the illustration below.

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  1. At the top left of the Geoprocessing pane, click the Back button.
  2. In the search box, type "flow".
  3. Click the Flow Direction tool.
  4. For ‘Input surface raster’, select the FIL layer.
  5. For ‘Output flow direction raster’, rename the raster from “FlowDir_FiIL1” to “FDR”.
  6. Ensure your ‘Flow Direction’ window your Geoprocessing pane appears as shown below and click Run.

The result may appear a bit strange at first, due to the color scheme assigned to each cardinal direction.

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  1. Symbolize the FDR layer using Unique Values with a smooth black and white gradient for the Color Scheme.

You will notice the layer now looks similar to a hillshade.

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Calculating flow accumulation

Finally, you will calculate flow accumulation in each cell as illustrated below. Flow accumulation is dependent on the flow direction you calculated previously.

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  1. At the top left of the Geoprocessing pane, click the Back button.
  2. In the search box, type "flow".
  3. Click the Flow Accumulation tool.
  4. For ‘Input flow direction raster’, select the FDR layer.
  5. For ‘Output accumulation raster’, rename the raster from “FlowAcc_FDR1” to “FAC”.
  6. Ensure your ‘Flow Accumulation’ window your Geoprocessing pane appears as shown below and click Run.



  7. Turn off the Flowlines_StatePlane layer to better see the FAC layer.
  8. Right-click the FAC layer and click the Symbology.
  9. For ‘Symbology’, use drop-down menu to select Classify.
  10. In the ‘Method’ drop-down menu to select Equal Interval.
  11. Use the ‘Classes’ drop-down menu to select 8 classes.
  12. For ‘Class Breaks’, double-click the numbers in the Upper Value column  and typethe following list: 100, 300, 1000, 3000, 10000, 30000, 100000. (Leave the final max value, 351408 as is.)
  13. Use the ‘Color Ramp’ drop-down menu to select the multipart color scheme.



Visually trace along the streams until you find the darkest path exiting the watershed. Notice that, according to the model you have just generated, the Buffalo-San Jacinto basin drains north into Lake Houston, instead of south into Galveston Bay. Look for the exact point on the map where this divergence is created. It is circled in the image below. A problem is that the topography in that area is extremely flat and, rather than a single well-defined channel, there are wide bodies of water connecting the bayous to Galveston Bay. Calculating the flow accumulation more accurately would require further editing the base raster so that the wide flat bodies of water are not treated as narrow flowlines. This example also illustrates how you must always double-check the results of your models to ensure they make sense. For the purposes of learning the remaining geoprocessing tools in this lab, you will continue with the existing flow accumulation model, as if it was correct.

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Zoom into the northern outlet point, as indicated on the image above, so that you can see the individual pixels.

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Creating and editing feature classes

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  1. Click the Catalog tab.
  2. Right-click the HydrologyLab geodatabase and select New > Feature Class…Class….
  3. For ‘Feature Class Name’, type “Outlet” “Outlet”.
  4. Use the ‘Geometry Type’ drop-down menu to select Point.
  5. For Coordinate System, select Current Map.
  6. Click Run.
  7. On the Standard toolbar, click the Edit tab and click the Create button.

  8. On the Create Features pane, select Outlet layer and click Point. Click ->.

  9. In the Map Display, click the outlet pixel to draw a point there.

  10. On the Editor toolbar, click Save.
  11. When asked if you want to save your edits, click Yes.
  12. Close the Editor toolbar and the Create Features tab.

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  1. Click Tools from Analysis tab.
  2. In the Hydrology toolset, double-click the Watershed tool.
  3. For ‘Input flow direction raster’, select the FDR layer from the drop-down menu.
  4. For ‘Input raster or feature pour point data’ select the Outlet layer from the drop-down menu.
  5. For ‘Output raster’, rename the raster from “Watersh“Watersh_FDR1” to “Watershed”FDR1” to “Watershed”.
  6. Ensure your ‘Watershed’ window your Geoprocessing pane appears as shown below and click Run.



 

Notice that this is very close, but does not exactly match the boundaries of the subbasin you were originally provided from NHDPlus.

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  1. Click Tools from Analysis tab.
  2. In the Map Algebra toolset, double-click the Raster Calculator tool.
  3. Using the same clicking technique as before, enter the following equation:    ("FAC" > 3000) & ("Watershed" > 0).
  4. For ‘Output raster’, rename the raster from “fac“fac_rasterca” to “Streams”rasterca” to “Streams”.
  5. Ensure your ‘Raster Calculator’ window your Geoprocessing pane appears as shown below and click Run.

 

Again you should have a binary raster, where flowline cells are assigned a value of 1 and all other cells are assigned a value of 0.

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  1. Click Tools from Analysis tab.
  2. In the Hydrology toolset, double-click the Stream Link tool.
  3. For ‘Input stream raster’, select the Streams layer from the drop-down menu.
  4. For ‘Input flow direction raster’, select the FDR layer from the drop-down menu.
  5. For ‘Output raster’, rename “StreamL “StreamL_Stre1” to “StreamLinks”Stre1” to “StreamLinks”.
  6. Ensure your ‘Stream Links’ window your Geoprocessing pane appears as shown below and click Run.

  7. Symbolize the StreamLinks layer using unique values.
  8. In the Contents pane, collapse the StreamLinks symbology.

Delineating catchment basins

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  1. Click Tools from Analysis tab.
  2. In the Hydrology toolset, double-click the Watershed tool.
  3. For ‘Input D8 flow direction raster’, select the FDR layer from the drop-down menu.
  4. For ‘Input raster or feature pour point data’ select the StreamLinks layer from the drop-down menu.
  5. For ‘Output raster’, rename “Watersh “Watersh_FDR2” to “Catchments”FDR2” to “Catchments”.
  6. Ensure your ‘Watershed’ window your Geoprocessing pane appears as shown below and click Run.

  7. Symbolize the Catchments layer using unique values.

  8. In the Contents pane, collapse the Catchments symbology.

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  1. Click Tools from Analysis tab.
  2. Double-click the Conversion Tools toolbox à From Raster toolset à Raster to Polygon tool.
  3. For ‘Input raster’, select the Catchments layer from the drop-down menu.
  4. For ‘Output polyline features’, rename “RatsterT “RatsterT_Catchme1” to “CatchmentPoly”Catchme1” to “CatchmentPoly”.
  5. Uncheck Simplify polylines.
  6. Ensure your ‘Raster to Polygon’ window your Geoprocessing pane appears as shown below and click Run.

  7. Symbolize the CatchmentPoly layer using unique values with ID as the value field.

  8. In the Contents pane, collapse the CatchmentPoly symbology.

Now you will also convert the raster StreamLinks into a polyline feature class. While you could use the Raster to Polyline tool, you will instead use a tool in the Hydrology toolset, designed specifically for converting raster stream links into polyline features. The difference between the two methods is illustrated below.

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  1. Click Tools from Analysis tab.
  2. Collapse the Conversion Tools toolbox.
  3. In the Hydrology toolset, double-click the Stream to Feature tool.
  4. For ‘Input stream raster’, drag in the select the StreamLinks layer from the Contents pane.
  5. For ‘Input flow direction raster’, drag in the select the FDR layer from the Contents pane.
  6. For ‘Output polyline features’, rename the feature class from “StreamT“StreamT_StreamL1” to “DrainageLines”StreamL1” to “DrainageLines”.
  7. Uncheck Simplify polylines.
  8. Ensure your ‘Stream to Feature’ window your Geoprocessing pane appears as shown below and click Run.

Calculating Strahler stream order

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  1. Click Tools from Analysis tab.
  2. In the Hydrology toolset, double-click the Stream Order tool.
  3. For ‘Input stream raster’, select the Streams layer from the drop-down menu.
  4. For ‘Input flow direction raster’, select the FDR layer from the drop-down menu.
  5. For ‘Output raster’, rename the raster from “StreamO“StreamO_Stre1” to “StrahlerOrder”Stre1” to “StrahlerOrder”.
  6. Ensure your ‘Stream Order’ window your Geoprocessing pane appears as shown below and click Run.

  7. In the Contents pane, turn off all layers except for the new StrahlerOrder layer, so that its symbology is more apparent.

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  1. Click Tools from Analysis tab.
  2. In the Spatial Analyst Tools toolbox, double-click the Zonal toolbox > Zonal Statistics as Table.
  3. For ‘Input raster or feature zone data’, select the StreamLinks layer from the drop-down menu.
  4. Use the ‘Zone field’ drop-down menu to select the Value field.
  5. For ‘Input value raster’, select the StrahlerOrder layer from the drop-down menu.
  6. For ‘Output table’, rename the table from “ZonalSt“ZonalSt_StreamL1” to “OrderTable”StreamL1” to “OrderTable”.
  7. Ensure your ‘Zonal Statistics as Table’ window your Geoprocessing pane appears as shown below and click Run.

Adding a new field to an attribute table

Now you will create a new field in the DrainageLines attribute table to store the Strahler order number.

  1. Open the DrainageLines attribute table.
  2. At the top of the DrainageLines attribute table, click the Add Field button.

  3. For ‘Name:’, type “StrahlerOrder”.
  4. Use the ‘Type:’ drop-down menu to select Short Integer.
  5. Ensure your ‘Add Field’ window appears as shown below and click Save on top of  the main tab.




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  1. Click the Table Options button and select Joins and Relates > Add Join….
  2. For ‘Input Join Field’, select the grid_code field.
  3. For ‘Join Table’, select the OrderTable table.
  4. For ‘Output Join Field’, select the VALUE field.
  5. Ensure that the ‘Join Data’ window your Geoprocessing pane appears as shown below and click Run.

Scroll across the table and notice that most of the statistics (Min, Max, Mean, Majority, Minority, Median) are the same and all contain the Strahler order integer, so it doesn’t matter which field you copy, though you will choose the MIN field for this lab.

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