Drainage basin = Root system

Above is an image of a drainage basin that acts like a funnel, collecting all the water within the area covered by the basin and channeling it into a waterway. Each drainage basin is separated topographically from adjacent basins by a geographical barrier such as a ridge, hill or mountain, which is known as a water divide. Other terms that are used to describe the a drainage basin are catchment, catchment area, catchment basin, drainage area, river basin, water basin and watershed.

Drainage basins are important elements to consider also in ecology. As water flows over the ground and along rivers it can pick up nutrients, sediment, and pollutants. Like the water, they get transported towards the outlet of the basin, and can affect the ecological processes along the way as well as in the receiving water source. (As a note, may be there is a way of designing an underground drainage basin piping system with its own filtration system...???)

Modern usage of artificial fertilizers, containing nitrogen, phosphorus, and potassium, has affected the mouths of watersheds. The minerals will be carried by the watershed to the mouth and accumulate there, disturbing the natural mineral balance. (back to my red note?)

Because drainage basins are coherent entities in a hydrological sense, it has become common to manage water resources on the basis of individual basins. In the U.S. state of Minnesota, governmental entities that perform this function are called watershed districts. In New Zealand, they are called catchment boards. Comparable community groups based in Ontario, Canada, are called conservation authorities.

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Here's an idea:

Underground drainage basin / reservoir. Basically, all the wastewater from homes and businesses from the neighbourhood drains down to one big underground reservoir (where all water gets stored). On its way, it gets its preliminary treatment, i.e. some nutrients/minerals/chemicals go back to the soil, the rest remain in the water. Then, it gets further filtered (possibly above ground, using any or all of the proposed filtration systems) and after that it then gets distributed to homes.

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