Mr. José Luis hello back!
I also read that the model works in ArcGIS 8.3 and 9.0 but I desperately needed this script so I decided to try to load it inspite of having a 10.3.1. version of ArcGIS. I had no idea about how to do it because I am pretty new to GIS world so I found this video on youtube:
https://www.youtube.com/watch?v=EVPzPGW7qVo, Add DLL to ArcMap - YouTube https://www.youtube.com/watch?v=EVPzPGW7qVo
Followed the procedure and it worked.
Sorry, I have no knowledge about coding with Phyton...
Actually, I also have a question; does the interface of ArcCN Runoff look the same in your version of ArcGIS? Does Precipitation box have a unit statement as it has in original article? Because as I understand something is wrong with my results.
Best regards.