Yes! Sorry I didnt know you internalised the geometry. Ok so,
I reduced the size of the terrain by 0.01 factor so that I could run the drop simulation with the script I sent (smaller = faster). I let it run for about 45 mins. This is important so that you can see longer particle flows as seen in the immage Anders shared. If I reduced the geometry further and let it run a little longer the results would just get better and better. After this, I simply took the endpoints of all the resulting flow curves and added a sphere. This way you can see where water accumulates, both when flowing down a slope, and when standing still on flat surface. Essentially, where there is no slope, water accumulates. See below:
This is the easiest/simplest way i can think of portraying what youre looking for, and then simply overlay this with a graphic interpretation, or coloring the spheres, etc.