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ArcGIS Pro

Topographic Wetness Index in ArcGIS Pro

A Step-by-Step Guide to Calculating the Topographic Wetness Index in ArcGIS Pro

The Topographic Wetness Index (TWI) is a valuable tool for hydrologists, environmental scientists, and geographers. It quantifies the topographic control on hydrological processes, providing insights into areas that are likely to be wetter due to their landscape position. In this guide, we’ll walk you through the process of calculating the TWI using ArcGIS Pro.

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  • ArcGIS Pro software installed.
  • A Digital Elevation Model (DEM) of your study area.

Step 1: Prepare Your Workspace

  1. Launch ArcGIS Pro and open a new or existing project.
  2. Import your DEM into the project.

Step 2: Fill the DEM

Before calculating the TWI, it’s essential to ensure that the DEM doesn’t have any sinks or depressions that could affect the flow of water.

  1. Navigate to the Geoprocessing pane.
  2. Search for the Fill tool and select it.
  3. Input your DEM into the Input Surface Raster field.
  4. Run the tool.

Step 3: Calculate Flow Direction

This step determines the direction of flow for each cell in the DEM.

  1. Search for the Flow Direction tool in the Geoprocessing pane.
  2. Use the filled DEM from the previous step as the input.
  3. Execute the tool.

Step 4: Compute Flow Accumulation

Flow accumulation calculates the number of cells that contribute flow to each cell in the DEM.

  1. Find the Flow Accumulation tool.
  2. Input the flow direction raster from the previous step.
  3. Run the tool.

Step 5: Determine Slope

The slope tool calculates the maximum rate of change between each cell and its neighbors.

  1. Search for the Slope tool.
  2. Use the filled DEM as the input raster.
  3. Ensure the output measurement is set to “DEGREE.”
  4. Execute the tool.

Step 6: Convert Slope to Radians

TWI calculations require slope values in radians.

  1. Use the Raster Calculator tool.
  2. Convert the slope from degrees to radians using the formula: slope_radians = slope_degrees * (π / 180)

Step 7: Calculate the Tangent of the Slope

  1. Again, use the Raster Calculator.
  2. Compute the tangent of the slope in radians: tan_slope = tan(slope_radians)

Step 8: Adjust Flow Accumulation

  1. In the Raster Calculator, adjust the flow accumulation values: adjusted_flow_accumulation = flow_accumulation + 1

Step 9: Compute Topographic Wetness Index (TWI)

  1. Use the Raster Calculator for this final calculation.
  2. The formula for TWI is: TWI = ln(adjusted_flow_accumulation / tan_slope)

Step 10: Visualize and Interpret the TWI

  1. Add the TWI raster layer to your map.
  2. Use the Symbology pane to classify and color the TWI values as per your preference.


The Topographic Wetness Index is a powerful tool for understanding landscape hydrology. By following this step-by-step guide, you can efficiently compute the TWI for any region using ArcGIS Pro. Whether you’re studying wetland habitats, planning infrastructure, or conducting environmental assessments, the TWI provides valuable insights into the wetness potential of the landscape.

Frequently asked questions about calculating the Topographic Wetness Index (TWI) in ArcGIS Pro

1. What is the Topographic Wetness Index (TWI)?

Answer: The Topographic Wetness Index (TWI) is a terrain attribute that estimates where water will accumulate in an area with elevation differences. It’s a function of slope and the upstream contributing area. The formula for TWI is:
[ TWI = \ln \left( \frac{a}{\tan(b)} \right) ]

  • ( a ) = upslope contributing area (m^2)
  • ( b ) = slope in radians

2. Why is it necessary to fill the DEM before calculating the TWI?

Answer: The DEM, especially when derived from sources like LIDAR, can have voids or pixels with no data. These voids can disrupt the flow of water in hydrological modeling. Filling the DEM ensures that these voids are interpolated, creating a continuous surface for accurate flow direction and accumulation calculations.

3. How does the Flow Direction tool work?

Answer: The Flow Direction tool remains consistent in its function across GIS platforms. It analyzes the elevation of each cell in the DEM and its immediate neighbors to determine the direction of water flow based on the steepest descent.

4. What is the significance of the Flow Accumulation step?

Answer: Flow Accumulation calculates the contributing upslope area for each pixel, representing the number of pixels that are upstream. This value is crucial for the TWI calculation as it represents the ( a ) value in the TWI formula.

5. Why do we need to convert the slope from degrees to radians?

Answer: The TWI formula requires the slope in radians for the tangent function. While some GIS tools calculate slope in degrees, the conversion to radians is essential for accurate TWI calculations.

6. How is the Tangent of Slope used in the TWI calculation?

Answer: The tangent of the slope represents the steepness of the terrain. In the TWI formula, it serves as the denominator, indicating how quickly water can potentially move or flow off a location.

7. What does the final TWI value represent?

Answer: The TWI value for each cell provides an indication of its wetness potential relative to its surroundings. Higher TWI values suggest areas with a greater potential for wetness or water accumulation due to their topographic position.

8. How can I visualize the TWI results effectively?

Answer: In addition to standard 2D visualization techniques, the TWI results can also be visualized in a 3D view, offering a more immersive perspective of the wetness index across the landscape.

9. Can I use TWI for applications other than hydrology?

Answer: The application of TWI remains consistent across platforms, making it valuable for wetland habitat delineation, infrastructure planning, agriculture, and more.

10. Are there any limitations or considerations when calculating TWI in ArcGIS Pro?

Answer: The accuracy of the TWI still depends on the quality and resolution of the DEM. It’s essential to ensure that the DEM accurately represents the study area’s topography and that voids or no-data pixels are appropriately addressed.

About the Author
I'm Daniel O'Donohue, the voice and creator behind The MapScaping Podcast ( A podcast for the geospatial community ). With a professional background as a geospatial specialist, I've spent years harnessing the power of spatial to unravel the complexities of our world, one layer at a time.