Calculate Topographic Wetness Index (TWI) using Surface Parameters
Description
Calculate the topographic wetness index (TWI) from a DEM.
Usage
This tool calculates the topographic wetness index (TWI) from DEM derivatives slope and flow accumulation. The TWI output can be used as a predictor variable for wetland identification with the WIM workflow. TWI has been shown to be useful for mapping saturated areas (e.g., Agren et al., 2014; Lang et al., 2013; Millard & Richardson, 2015; Murphy et al., 2009; O'Neil et al., 2018; O'Neil et al., 2019). Developed by Beven and Kirkby (1979), TWI relates the tendency of an area to receive water to its tendency to drain water. This tool uses the Spatial Analyst Surface Parameters tool to calculate the slope component. Like Surface Parameters, this tool gives users the option to implement an adaptive neighborhood size to compute slope. To calculate the flow accumulation component, the tool uses the Spatial Analyst Fill tool or Spatial Analyst Derive Continuous Flow tool. Users should derive TWI from a smoothed, high resolution DEM to avoid microtopographic noise.
Parameters
| Parameter Name | Type | Direction | Data Type | Dialog Reference |
|---|---|---|---|---|
| Input Smoothed DEM | Required | Input | Raster Layer | Full pathname to the input DEM. If both TWI Slope and TWI Flow Accumulation rasters are given below, this input is ignored. |
| Hydrocondition Method | Required | Input | String | Hydroconditioning method that will be used when deriving TWI Flow Accumulation. If "Fill" is selected, the tool will execute Spatial Analyst tools: Fill, Flow Direction (D-infinity), and Flow Accumulation. If "Continuous Flow" is selected, the tool will execute Spatial Analyst Derive Continuous Flow to derive flow accumulation using MFD flow direction. |
| Slope Neighborhood Distance (meters) | Required | Input | Double | The maximum neighborhood length (meters) that will be used by the adaptive neighborhood process for the Surface Parameters method. The adaptive neighborhood process optimizes the window size for which slope is calculated based on local DEM variability. If the neighborhood distance is equal to the cell size of the input DEM, adaptive neighborhoods will not be used. See surface parameters tool documentation. Optimal maximum neighborhood length will vary by use case. Users may begin with a neighborhood length that corresponds to the minimum size of wetlands of interest. |
| Output TWI Raster (*.tif) | Required | Output | Raster Dataset | Name of the resulting TWI raster. Must be in TIFF format. The output directory will be created if it does not exist. |
| Save Intermediate Rasters | Required | Input | Boolean | Option to save intermediate outputs from the TWI calculation: TWI Slope and TWI Flow Accumulation. |
| Output TWI Slope Raster (*.tif) | Optional | Output | Raster Dataset | Output slope raster used for TWI calculation. The output directory will be created if it does not exist. |
| Output TWI Flow Accumulation Raster (*.tif) | Optional | Output | Raster Dataset | Output flow accumulation raster used for TWI calculation. The output directory will be created if it does not exist. |
| Input TWI Slope Raster | Optional | Input | Raster Layer | Input slope raster for the TWI calculation. If not given, the tool calculates the slope raster directly from the input DEM. The input slope raster must be in units of m/m (rise/run) and have a very small constant (i.e., 0.001) added to all values to avoid undefined cells in the output TWI raster. |
| Input TWI Flow Accumulation Raster | Optional | Input | Raster Layer | Input flow accumulation for the TWI calculation. If not given, the tool calculates the flow accumulation raster directly from the input DEM. Internally, flow accumulation is converted to specific catchment area (contributing area per unit contour width). |