Not sure if this is helpful, but might inspire some ideas:
I recently re-wrote a simple “particle descending down a surface/mesh drainage”-type script (that looks like a similar approach to your first example) for use on a project, and extended it with what is essentially a heat map visualisation of the drain polylines density (second image in the Instagram post):
I basically snap the drainage polyline vertices to the nearest face center in the heat map mesh, and then count how many points are at each face (i.e. how many drain paths flowed through it). Obviously this doesn’t take volume, physics etc into account, but might still be used to differentiate between places of high and low runoff (though in my case, the particles doesn’t actually flow off the terrain, need to add that come to think of it).
My colleague is currently working on implementing the Catchment Modelling Framework within Grasshopper/GHPython. So hopefully there’ll soon be some physically accurate hydrological models as well (I believe this will be made open to the public).