If you’re gonna adapt your locality and all, you better know where you are in relation to how water drains into, through and beyond your community. This is your watershed. For some of us it’s a lot more recognizable than for others.
Thanks to WorldChanging I’ve been turned on to Inforain, a source of topgraphical watershed mapping for the Pacific Northwest. I was actually surprised a bit by how their map of my local watershed wrapped around the topography of the Marin Headlands.