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

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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 Analy...
Lab Exercises:
Lab 1: Calculating Areas from Raster Datasets; • Hydrology in ArcGIS Pr...
Lab 2: Problems when creating Watersheds without filling the DEM; • Hydrology in ArcGIS Pr...
Lab 3: Filling Sinks in a DEM; • Hydrology in ArcGIS Pr...
Lab 4: Calculating an Accurate Flow Direction Raster; • Hydrology in ArcGIS Pr...
Lab 5: Generating an Accurate Watershed; • Hydrology in ArcGIS Pr...
Lab 6: Creating a Flow Accumulation Raster; • Hydrology in ArcGIS Pr...
Lab 7: Generating a Stream Network using Strahler Stream Orders; • Hydrology in ArcGIS Pr...
Lab 8: Aspect Statistics within Watershed; • Hydrology in ArcGIS Pr...
Lab 9: Slope Statistics within Watershed; • Hydrology in ArcGIS Pr...
Lab 10: The Topographic Wetness Index (TWI); • Hydrology in ArcGIS Pr...
Lab 11: Finding the Shortest Path Through the Stream Network; • Hydrology in ArcGIS Pr...
Extra Materials:
Word Document with walk-throughs of lab exercises: docs.google.com/document/d/13...
Data for Lab Exercises: drive.google.com/file/d/1i07m...
Manuscripts discussing TWI: drive.google.com/file/d/1W4Ty...

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13 ноя 2022

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Комментарии : 6

@mxzoefelix Год назад

Excellent video and such a patient teacher you are:) Thanks for sharing.

@jennessenterprisesadventur5081 Год назад

Thanks so much for the kind words! Seeing your message was a good way to start the day.

@khalid74316 Год назад

thank you for this wonderful video! I have 2 questions if you don't mind 1- Will it make a difference if my slope was in degrees (some video use degree slop and then convert it to radiance and this confuse me) 2- For the cell size assuming that it is 30 (some videos multiply FlowAccumulation by 30 instead of 30*30 any idea if this is wrong?) Any explanation would be greatly appreciated, thank you for this wonderful channel

@jennessenterprisesadventur5081 Год назад

Hi Khalid, Thank you for the kind words on the video! They are much appreciated. Regarding your questions: 1) It will make a little difference if you use a slope measured in Degrees rather than Radians. Remember that the TWI equation divides by the slope. Slope in Degrees can range from 0 to 90 degrees, while slope in Radians can range from 0 to infinity. Therefore using Radians as the denominator in the equation means to use a variable with a larger possible range, and the consequence is that it will be more influential in the final TWI value. That is not to say that using Radians is "right" and Degrees is "wrong"; it just means that using radians makes slope more important to the final TWI value. I suspect that the main reason so many people use slope in Radians is just because so many people use slope in Radians. That option just has more "momentum", so to speak. "My granddaddy did it that way, and it's good enough for me..." 2) I think you would be better dividing by 30 instead of 30*30. This value is what we're using as the "unit contour interval", and is intended to measure the width of the crossing point that forms the exit for this watershed. The cell width (30) is more in keeping with the spirit of the original equation. I hope this answers it Khalid! Thanks again for the kind words. Take care - Jeff

@khalid74316 Год назад

@@jennessenterprisesadventur5081 Thank you very much for your kind response, I asked because I noticed that you used slop In percent in this video instead of degrees so I wanted to make sure I’m understanding this right, since TWI will be an integral part of my master’s thesis research which I’m currently conducting. Thank you again and please keep up the good work!

@jennessenterprisesadventur5081 Год назад

Glad to help Khalid! Best of luck with your thesis!