Groundwater Storage and the Water Cycle Completed

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There's a whole lot of water beneath your feet

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Large amounts of water are stored in the ground. The water is still moving, possibly very slowly, and it is still part of the water cycle. Most of the water in the ground comes from precipitation that infiltrates downward from the land surface. The upper layer of the soil is the unsaturated zone, where water is present in varying amounts that change over time, but does not saturate the soil. Below this layer is the saturated zone, called groundwater, where liquid water is stored in all the pores, cracks, and spaces between rock particles. The term aquifer refers to water-bearing foundations or rocks that can hold large amounts of water. Aquifers are a huge storehouse of Earth's water and people all over the world depend on groundwater in their daily lives.

The top of the surface where groundwater occurs is called the water table. In the diagram, you can see how the ground below the water table is saturated with water (the saturated zone). Aquifers are replenished by the seepage of precipitation that falls on the land, but there are many geologic, meteorologic, topographic, and human factors that determine the extent and rate to which aquifers are refilled with water. Rocks have different porosity and permeability characteristics, which means that water does not move around the same way in all rocks. Thus, the characteristics of groundwater recharge vary all over the world.

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To find water underground, look under the (water) table

This beach photo is a great way to illustrate the concept of how the ground is saturated with water at a certain depth if it is permeable enough to allow water to move through it. The top of the pool of water in this hole is the water table. The breaking waves of the ocean are just beyond this hole, and the water level in the hole is the same as the level of the ocean. Of course, the water level here changes by the minute due to the movement of the tides, and as the tide goes up and down, the water level in the hole moves, too. Just as with this hole, the level of the water table is affected by other environmental conditions.

The hole in the photo to the right is like a dug well used to access groundwater, probably saline in this case. But if this was freshwater, people could grab a bucket and supply themselves with the water they need to live their daily lives. This might sound familiar: If you took a bucket and tried to empty this hole, it would refill immediately. Why is that? It's because the sand is so permeable that water flows easily through it, meaning our "well" is very "high-yielding" (too bad the water is saline). To access freshwater, people must drill wells deep enough to tap into an aquifer. The well might have to be dozens or thousands of feet deep. But the concept is the same as the "well" in the photo — we must access the water in the saturated zone where the voids in the rock are full of water.

Pumping can affect the level of the water table

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In an aquifer, the soil and rock are saturated with water. If the aquifer is shallow enough and permeable enough to allow water to move through it at a rapid-enough rate, then people can drill wells into it and withdraw water. The level of the water table can naturally change over time due to changes in weather cycles and precipitation patterns, streamflow and geologic changes, and even human-induced changes, such as the increase in impervious surfaces, such as roads and paved areas, on the landscape.

The pumping of wells can have a great deal of influence on water levels below ground, especially in the vicinity of the well, as this diagram shows. Depending on geologic and hydrologic conditions of the aquifer, the impact on the level of the water table can be short-lived or last for decades, and the water level can fall a small amount or many hundreds of feet. Excessive pumping can lower the water table so much that the wells no longer supply water—they can "go dry."

Groundwater and global water distribution

As these charts show, even though the amount of water locked up in groundwater is a small percentage of all of Earth's water, it represents a large percentage of total freshwater on Earth. About 1.7 percent of all of Earth's water is groundwater and about 30.1 percent of freshwater on Earth occurs as groundwater. As the bar chart shows, about 5,614,000 cubic miles (mi³), or 23,400,000 cubic kilometers (km³), of groundwater exist on Earth. About 54 percent is saline, with the remaining 2,526,000 mi³ (10,530,000 km³) , about 46 percent, being freshwater.

Water in aquifers below the oceans is generally saline, while the water below the land surfaces (where freshwater, which fell as precipitation, infiltrates into the ground) is generally freshwater. There is a stable transition zone that separates saline water and freshwater below ground. It is fortunate for us that the relatively shallow aquifers that people tap with wells contain freshwater, since if we tried to irrigate crops with saline water, they would not grow well.

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Source: Gleick, P. H., 1996: Water resources. In Encyclopedia of Climate and Weather, ed. by S. H. Schneider, Oxford University Press, New York, vol. 2, pp.817-823.

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Sources and more information:

• NASA Earth Observatory - The Water Cycle