The water table is an underground boundary between the soil surface and the area where groundwater saturates spaces between sediments and cracks in rock. Water pressure and atmospheric pressure are equal at this boundary.

The soil surface above the water table is called the unsaturated zone, where both oxygen and water fill the spaces between sediments. The unsaturated zone is also called the zone of aeration due to the presence of oxygen in the soil. Underneath the water table is the saturated zone, where water fills all spaces between sediments. The saturated zone is bounded at the bottom by impenetrable rock.

The shape and height of the water table is influenced by the land surface that lies above it; it curves up under hills and drops under valleys. The groundwater found below the water table comes from precipitation that has seeped through surface soil. Springs are formed where the water table naturally meets the land surface, causing groundwater to flow from the surface and eventually into a stream, river, or lake.

The water table level can vary in different areas and even within the same area. Fluctuations in the water table level are caused by changes in precipitation between seasons and years. During late winter and spring, when snow melts and precipitation is high, the water table rises. There is a lag, however, between when precipitation infiltrates the saturated zone and when the water table rises. This is because it takes time for water to trickle through spaces between sediments to reach the saturated zone, although the process is helped by gravity. Irrigation of crops can also cause the water table to rise as excess water seeps into the ground.

During the summer months, the water table tends to fall, due in part to plants taking up water from the soil surface before it can reach the water table. The water table level is also influenced by human extraction of groundwater using wells; groundwater is pumped out for drinking water and to irrigate farmland. The depth of the water table can be measured in existing wells to determine the effects of season, climate, or human impact on groundwater. The water table can actually be mapped across regions using measurements taken from wells.

If water is not extracted through a well in a sustainable manner, the water table may drop permanently. This is starting to be the case around the world. Some of the largest sources of groundwater are being depleted in India, China, and the United States to the point where they cannot be replenished. Groundwater depletion occurs when the rate of groundwater extraction through wells is higher than the rate of replenishment from precipitation.