Runoffs generated by precipitations start from watersheds. Rain and/or snow falls on higher topography and the distribution of headwaters occurs evenly on both slopes of mountains, hills, etc, conversely to channels generated by aquifer discharge, for which headwaters are located only at the emergence points of water (where water table intersects the surface). The result is that surface runoffs show a high-density, and well-distributed source points following climate patterns, while channels formed from aquifer discharges will show a random distribution and a weak density of source points.

Source: NASA