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

[@mxzoefelix]

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

[@JennessEnterprises]

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

[@khalid74316]

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

[@JennessEnterprises]

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]

@@JennessEnterprises 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!

[@JennessEnterprises]

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