├── README.md
└── TWI-filled-plus0.1constant.py


/README.md:
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 1 | Topographic Wetness Index 
 2 | 2013-11-12
 3 | Jeffrey Wolf (EEB); G. Andrew Fricker (GEOG)
 4 | UCLA
 5 | 
 6 | This script was written to be used as a tool in ArcGIS.  This python script can be imported to create a TWI tool.  Inputs are the workspace and the input DEM, output is the TWI layer
 7 | We chose to fill all sinks due to some small sinks in a lidar redived DEM.  We also add a small constant to the denominator to avoid dividng by zero.  
 8 | 
 9 | The original script was based off the arcpy script written by Prasad Pathak.
10 | http://arcscripts.esri.com/details.asp?dbid=16750
11 | 
12 | This revised script converts the terrain slope in degrees to radians
13 | This script also uses the default settings for the flow accumulation raster
14 | however different methods to calculate flow accumulation can dramtically change the results of the TWI
15 | 


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/TWI-filled-plus0.1constant.py:
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 1 | """
 2 | TWI-filled-plus0.1constant.py
 3 | Topographic Wetness Index 
 4 | 2013-11-12
 5 | Jeffrey Wolf (EEB); G. Andrew Fricker (GEOG)
 6 | UCLA
 7 | 
 8 | This script was written to be used as a tool in ArcGIS.  This python script can be imported to create a TWI tool.  Inputs are the workspace and the input DEM, output is the TWI layer
 9 | We chose to fill all sinks due to some small sinks in a lidar redived DEM.  We also add a small constant to the denominator to avoid dividng by zero.  
10 | 
11 | The original script was based off the arcpy script written by Prasad Pathak.
12 | http://arcscripts.esri.com/details.asp?dbid=16750
13 | 
14 | This revised script converts the terrain slope in degrees to radians
15 | This script also uses the default settings for the flow accumulation raster
16 | however different methods to calculate flow accumulation can dramtically change the results of the TWI
17 | 
18 | """
19 | 
20 | import arcpy, math
21 | 
22 | 
23 | if __name__ == '__main__':
24 | 	arcpy.CheckOutExtension("Spatial")
25 | 	
26 | 	# Define workspace and set input and output files
27 | 	arcpy.env.workspace = arcpy.GetParameterAsText(0)
28 | 	inDEM = arcpy.GetParameterAsText(1)
29 | 	outTWI = arcpy.GetParameterAsText(2)
30 | 
31 | 	# Intermediates
32 | 	arcpy.AddMessage("Filling DEM.\n")
33 | 	DEM_filled = arcpy.sa.Fill(inDEM)
34 | 	
35 | 	arcpy.AddMessage("Creating flow direction.\n")
36 | 	outFlowDirection = arcpy.sa.FlowDirection(DEM_filled, "FORCE")
37 | 	
38 | 	
39 | 	arcpy.AddMessage("Creating flow accumulation.\n")
40 | 	#outFlowAccumulation = arcpy.sa.FlowAccumulation(outFlowDirection, "", "FLOAT") + 1 
41 | 	outFlowAccumulation = arcpy.sa.FlowAccumulation(outFlowDirection, "", "INTEGER") + 1 
42 | 	
43 | 	arcpy.AddMessage("Creating slope.\n")
44 | 	slope = arcpy.sa.Slope(DEM_filled)
45 | 	
46 | 
47 | 	arcpy.AddMessage("Converting slope in degrees to slope in radians")
48 | 	# 2Pi radians = 360 degrees
49 | 	# Pi radians = 180 degrees
50 | 	# conversion: Pi radians/180 degress
51 | 	slope_radians = slope * math.pi/180.0
52 | 	
53 | 	# Output
54 | 	arcpy.AddMessage("Creating TWI\n")
55 | 	TWI = arcpy.sa.Ln(outFlowAccumulation / (arcpy.sa.Tan(slope_radians)+.01))
56 | 	TWI.save(outTWI)
57 | 	arcpy.AddMessage("Saved TWI. Done.")
58 | 	
59 | 


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