Your going to need multi layer texturing and use (painted) blend layers, etc.
(https://www.youtube.com/results?search_query=multilayer+landscape+texture+in+blender)
(https://www.youtube.com/results?search_query=projecktion+mapping+in+blender)
Camera/projection mapping can be a way to do “terrain decals”, that's something often used in games to add details.
Hope this points you into right direction?