Hydrology in ArcGIS Pro, Lab Exercise 10: The Topographic Wetness Index (TWI)

Topographic Wetness Index (TWI) is a landscape index that combines two topographic phenomena, slope and catchment area, to describe the tendency of water to collect in an area. Areas with high TWI values may be more likely to have higher soil moistures, different soil pH values and more likely to support water- or moisture-dependent species.

We will look at two ways to calculate TWI in ArcGIS Pro, first using the Raster Calculator tool to perform all calculations in a single function. This method is generally the quickest and easiest way to run the analysis. Then we will use ModelBuilder to build a model linking several Spatial Analyst Math tools together to run the analysis. This method has the advantage of producing a flexible tool that can be rerun in different areas. This model will use the Spatial Analyst Math tools Plus, Times, Divide and NL (for Natural Log).

In both cases we will modify the standard TWI equation to avoid division-by-zero errors and log-of-zero errors.

Basic Hydrologic Analysis in ArcGIS Pro Lecture:    • Basic Hydrologic Analysis in ArcGIS Pro  

Lab Exercises:
Lab 1: Calculating Areas from Raster Datasets;    • Hydrology in ArcGIS Pro, Lab Exercise...  
Lab 2: Problems when creating Watersheds without filling the DEM;    • Hydrology in ArcGIS Pro, Lab Exercise...  
Lab 3: Filling Sinks in a DEM;    • Hydrology in ArcGIS Pro, Lab Exercise...  
Lab 4: Calculating an Accurate Flow Direction Raster;    • Hydrology in ArcGIS Pro, Lab Exercise...  
Lab 5: Generating an Accurate Watershed;    • Hydrology in ArcGIS Pro, Lab Exercise...  
Lab 6: Creating a Flow Accumulation Raster;    • Hydrology in ArcGIS Pro, Lab Exercise...  
Lab 7: Generating a Stream Network using Strahler Stream Orders;    • Hydrology in ArcGIS Pro, Lab Exercise...  
Lab 8: Aspect Statistics within Watershed;    • Hydrology in ArcGIS Pro, Lab Exercise...  
Lab 9: Slope Statistics within Watershed;    • Hydrology in ArcGIS Pro, Lab Exercise...  
Lab 10: The Topographic Wetness Index (TWI);    • Hydrology in ArcGIS Pro, Lab Exercise...  
Lab 11: Finding the Shortest Path Through the Stream Network;    • Hydrology in ArcGIS Pro, Lab Exercise...  

Extra Materials:
Word Document with walk-throughs of lab exercises: https://docs.google.com/document/d/13...

Data for Lab Exercises: https://drive.google.com/file/d/1i07m...

Manuscripts discussing TWI: https://drive.google.com/file/d/1W4Ty...