Generating Wetness Indices for Watersheds in GRASS - Revision history

Vincentribberink: /* DEM Filtering: */

2025-10-01T22:13:54Z

DEM Filtering:

2025-10-01T22:01:09Z

DEM Filtering:

2025-10-01T22:00:19Z

Displaying Maps:

2025-10-01T21:59:29Z

Wetness Index:

2025-10-01T21:59:08Z

Wetness Index:

2025-10-01T21:55:57Z

Displaying Maps:

2025-10-01T21:41:59Z

Wetness Index:

2025-10-01T21:34:56Z

2025-10-01T21:32:33Z

Slope

2025-10-01T21:31:37Z

Generating Wetness Indices:

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 =DEM Filtering:=
 So far, all sample analysis images that are illustrated were generated from an unfiltered DEM. However, in order to achieve a specific level of detail in final wetness index images, filtering the DEM prior to generating the slope and flow accumulation maps may be required. Filtering can also reduce errors in future output maps that are caused by the DEM, or how the DEM was created. The DEM should be the only image that is filtered throughout the analysis. This ensures that all output images are derivatives of a specific elevation model. However, filtering final output maps '''for visual display''' is acceptable.
+NOTE: This section of the tutorial was completed with a different DEM dataset on an older version of GRASS, so outputs may look slightly different.

← Older revision		Revision as of 22:01, 1 October 2025
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	* In the sample analysis, the filter file contained a single 5x5 average filter; however, multiple filters can be included in one filter file.		* In the sample analysis, the filter file contained a single 5x5 average filter; however, multiple filters can be included in one filter file.
	* Refer to [~~http~~://grass.osgeo.org/~~gdp~~/~~html_grass64~~/r.mfilter.html r.mfilter] for the syntax of filter files.		* Refer to [https://grass.osgeo.org/grass-stable/manuals/r.mfilter.html r.mfilter] for the syntax of filter files.

← Older revision		Revision as of 22:00, 1 October 2025
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	3. Create a text file (.txt) containing the color rules for the image.		3. Create a text file (.txt) containing the color rules for the image.

	Refer to [~~http~~://grass.~~fbk~~.eu/~~grass62~~/manuals~~/html62_user~~/r.colors.html r.colors] for syntax of color rules.		Refer to [https://grass.osgeo.org/grass-stable/manuals/r.colors.html r.colors] for syntax of color rules.

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 . Select the slope image as ‘B’.
-. Enter ‘log(A/tan(B), 2.718282)’ as the ‘Formula’.
+. Enter ‘log(A/tan(B), 2.718282)’ as the ‘Formula’ and specify a name for ‘Name for Output Raster Map’.
-   Specify a name for ‘Name for Output Raster Map’.

← Older revision		Revision as of 21:59, 1 October 2025
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	4. Enter ‘log(A/tan(B), 2.718282)’ as the ‘Formula’.		4. Enter ‘log(A/tan(B), 2.718282)’ as the ‘Formula’.

	4. Specify a name for ‘Name for Output Raster Map’.		Specify a name for ‘Name for Output Raster Map’.

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 To display images based on user-defined color rules:
-. Generate distributions for an image using the ''r.stats –c '' module in GRASS or the ''Histogram'' function in QGIS.
+. Generate distributions for an image using the ''r.stats –c '' module in GRASS
 . Based on the distribution and your research needs, determine suitable color breaks for displaying the image.

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 To generate a wetness index map:
-. Open the r.mapcalculator module.
+. Open the r.mapcalc.simple module in Raster -> Raster Calculator
-[[File:MapcalcAF.PNG|400px]]
 . Select the upslope contributing area image as ‘A’.

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 '''Watershed Masks:'''
-Since this method produces a watershed map consisting of numerous watersheds, if analysis on a particular watershed is required, a watershed mask will also be required. To find the unique value of a watershed, the "Query Raster Map(s)" tool in the Map Display toolbar can be used.
+Since this method produces a watershed map consisting of numerous watersheds, if analysis on a particular watershed is required, a watershed mask will also be required. To find the unique value of a watershed, the "Query Raster Map(s)" tool in the Map Display toolbar can be used. This mask is created using the r.mapcalc tool located in Raster -> Raster Map Calculator.
-[[File:Findingmapcalc.png]]
 [[File:Rmapcalc1.png]]
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 To generate an upslope contributing area map:
-. Open the r.mapcalculator module.
+. Open the r.mapcalc.simple module in Raster -> Raster Map Calculator
-[[File:Findingmapcalc.png|400px]]
 . Select the flow accumulation image as ‘A’.

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 . Open the r.mapcalc.simple module in Raster -> Map Calculator
-[[File:Findingmapcalc.png]]
 . Select the output slope map as ‘A’.

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 To eliminate zero slope areas from slope maps:
-. Open the r.mapcalculator module.
+. Open the r.mapcalc.simple module in Raster -> Map Calculator
 [[File:Findingmapcalc.png]]
 . Select the output slope map as ‘A’.
+[[File:elimzero2.png]]
 . Enter ‘A+0.0001’ as the ‘Formula’.
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 . Specify a name for ‘Name for Output Raster Map’.
+[[File:elimzero1.png]]
 Alternatively, this can be done in command line based on [r.mapcalc Slope_NoZeros = Slope + 0.0001]
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 . Select the DEM as the input map.
-. Specify a name for 'Output Map: Number of Cells that Drain through Each Cell'.
+. Specify a name for 'Output accumulation raster map'.