Groundwater is the water beneath the Earth’s surface largely in pore spaces, little holes within rocks, located in the strata zone. One part of the strata zone is the zone of saturation. Groundwater is withdrawn from the zone of saturation. Below the zone of saturation is solid bedrock that acts as a container for the groundwater. These containers are called aquifers and are typically rock formations that yield large amounts of water. Hydrostatic pressure, indicated by arrows in the graphic below, cause groundwater to flow from higher pressure to lower pressure. 
The term Groundwater Basin is used to describe a unit usually consisting of one large aquifer and one or more smaller aquifers.
If an aquifer is hydraulically connected to a stream so that groundwater withdrawals impact a stream or other surface water, the groundwater can be considered a tributary. When pumping groundwater, a cone of influence forms around the pump extraction point when a pump withdraws water from the aquifer. The cone of influence is inverted and expands as more water is extracted, as depicted in the graphic below. When there is more pumping than the aquifer can replenish, an overdraft condition exists causing some wells to go dry. In addition to dry wells, the lack of water can permanently impact the balance of pressure within a groundwater basin which could cause a decrease in water quality and ability to utilize for agricultural purposes. 
The right to pump groundwater is exercised by the overlying landowner, which means if you own the property above the aquifer, you can pump the water. Under the Sustainable Groundwater Management Act (SGMA), The local Groundwater Sustainability Agency (GSA) will restrict the right of overlying landowners to to pump groundwater in non-adjudicated basins. (adjudicated basins are governed by a court approved settlement). The goal of these limitations under SGMA is to bring many of California’s struggling groundwater basins back into “sustainable yield,” which is a term created by SGMA and for you water veterans, is different than “safe yield.”
