Sheri Hunt, age 12, of Des Moines, Iowa, for her question:
IS THE WATER TABLE THE SAME AS SEA LEVEL?
The water table belongs to the land‑‑to the continents and islands. Usually this water is fairly fresh, though sometimes it is saturated with strong chemicals dissolved from the rocks underground. Sea level belongs to the salty ocean. The two are not the same, but both take part in the global water cycle.
The amount of water on earth is astounding. It is measured by the cubic mile, which is enough water to fill a huge tank one mile square and one mile high. The global total is estimated to be 326 million cubic miles‑‑and about 97.2$ of this is in the sea.
The surface of the worldwide ocean is called sea level. Its waves are tossed by the winds, and twice each day its mighty waters are heaved up and down by the tides. Nevertheless, the sea level stays more or less the same around the globe. To get a fair estimate, it is measured by the midway point between high tides and low tides.
Every year the beaming sun evaporates about 95,000 cubic miles of water from the earth's surface, mostly from the seas. This water vapor becomes involved in the never‑ending water cycle. It forms misty clouds that travel far and wide on the winds. Sooner or later they shed their moisture as rain or snow. Every year the same amount of moisture is evaporated, circulated and dropped back to the earth.
Some of the fallen moisture drains away in rivers that empty into the sea. Some sinks into the soil to feed the plants. Some sinks down into the ground where it is trapped for a while in porous rocks. This is the ground water, and some of it goes a mile or more below the surface.
The top level of the ground water may be a few inches or several hundred feet below the surface. This is called the water table. In rainy seasons more water sinks down and the water table rises. In dry seasons it sinks lower as ground water is used by reservoirs and irrigation systems.
Near the surface ground water often escapes in springs that stream down the slopes to join the sea. At deeper levels it percolates and circulates, sometimes for many years. But eventually it also escapes and rejoins the seas.