Click for larger view: How groundwater moves.

Courtesy of US Geological Survey.

Among the rights you may acquire when you purchase a piece of land in California is a right to groundwater. Groundwater rights are called "overlying rights," and like riparian rights they attach to your land.

Also like riparian rights they are "correlative"; you own a percentage of the percolating water that exists under your land in common with the other landowners in your neighborhood.

Except in areas where water rights in a groundwater basin were settled (or "adjudicated"), groundwater pumping is at present unregulated in California.

Water Rights Primer