This is especially important when an aquifer dips beneath another layer that is much less permeable. As usual, rainwater fills up the aquifer and water flows out from it where it meets the surface. Below the impermeable layer, water is trapped in the aquifer, which is said to be confined. The water presses on the less-permeable confining layers above and below it.
If one drills a borehole into a confined aquifer, water will rise up the borehole until the column of water is enough to balance the pressure in the aquifer. Most major aquifers have a confined portion, where the aquifer is covered by impermeable rock layers and an unconfined portion, where the aquifer covered simply by soil or river or glacial deposits. Rainwater enters the aquifer through the unconfined part.
If many boreholes are drilled into the aquifer and found the level of water in all of them, it could be imagined a surface made by joining all the individual levels. In the unconfined part of the aquifer this surface would be the water table. It separates the saturated part of the aquifer from the unsaturated zone above it. But in the confined part of the aquifer, the spaces between mineral grains, every pore in fact, is filled with water: there is no unsaturated zone. Here the surface is an imaginary one called the pressure surface, or potentiometric surface. It passes through the confining layer somewhere above the aquifer.
If the unconfined part of the aquifer is beneath high ground, and the confined part beneath low ground, then the potentiometric surface may be above ground level. If a borehole is drilled into the aquifer the groundwater will be under sufficient pressure to overflow from the borehole. Such a borehole is called artesian well. But the borehole must go deep enough to reach the aquifer, even if this means drilling far below the potentiometric surface. Otherwise it will yield little or no water from the impermeable confining layer.
In many parts of world, the availability of water form artesian wells makes agriculture possible.