Several years ago, a few days after Hurricane Sandy struck NYC, a picture was shown of either the 86th Street Station or the Avenue U Station in Brooklyn on the "N" Line ( Sea Beach ) showing the open cut completely covered with water almost up to the platform level. This water came from the bay I thought because the open cut elevations at track level was probably near sea level values. I was introduced recently to ARCGIS on line that has a nice analysis section dealing with various operations such as view shed analysis and watershed analysis. What would happen if I would enter a point at the Avenue U or 86th Street Station on the N Line? First, I would need to bring up a base map that shows subway stops and lines. Surprisingly, the Watershed Analyzer here is very simple to use. If you do not have a point shape file, you can add a point by "hand" and just select a range, let us say, 2000 feet. To my surprise, the resultant layer was not south of the 86th Street Station but Northwest. And now I can see why. A watershed is an area that provides water to a selected point. I believe that for this to work, the watershed has to be at higher elevation than the selected point. A very interesting resultant shows that the watershed for the 86th Station (theoretically), if there were no buildings and sewers in Brooklyn, would be along the "N" train all the way to Sunset Park!
Water analysis in GRASS is more difficult to use. The resultant "layer" can be zipped to ARCGIS maps and used in your favorite style maps, as shown below.