Figure 1 – Illustration of the water cycle (https://www.usgs.gov/media/images/water-cycle-natural-water-cycle).

Movement of Water in the Water Cycle

The figure above illustrates components of the water cycle in detail.  Most of Earth's water is contained in the oceans and seas. The sun, which drives the water cycle, heats water at the earth’s surface. Some of it evaporates as vapor into the air. Ice and snow can sublimate directly into water vapor. Evapotranspiration, which is water transpired from plants and evaporated from the soil, also adds water to the atmosphere. The vapor rises into the air where cooler temperatures cause it to condense into clouds.

Air currents move clouds around the globe and, as droplets grow, fall as precipitation. Some precipitation falls as snow and can accumulate as ice caps and glaciers, which can store frozen water for thousands of years. Snowpacks in warmer climates often thaw and melt when spring arrives, and the melted water flows overland as snowmelt.

Most precipitation falls back into the oceans or onto land, where, due to gravity, the precipitation flows over the ground as surface runoff.  Depending on the soil or rock type at the surface, some of it soaks into the ground as infiltration. Some infiltrated water moves deep into the ground and fills pores and fractures of subsurface unconsolidated material and rock.  Except for the water held in glaciers and ice caps, this is the largest store of freshwater on earth. Some of the groundwater flows underground to join with surface runoff as it enters rivers in valleys in the landscape.  Much of the water in streams that flow during periods of little or no precipitation is derived from the seepage of groundwater into the stream.  Some groundwater moves underground and finds openings in the land surface and emerges as springs. Surface runoff and groundwater seepage accumulate and are stored as freshwater in lakes and wetlands which constitute the third largest store of freshwater on earth.

Using the Water Cycle to Illustrate Global Water Distribution

In the figure below, notice that, of the world's total water supply (about 333 million cubic miles of water), over 97 percent is saline. Approximately half the groundwater and lake water in storage is saline. 

Of the total freshwater, over 68 percent is locked up in ice and glaciers. Another 30 percent of freshwater is in the ground. Worldwide, there are 28 lakes that each contain more than 25 cubic miles of freshwater.  Of these, Lake Baikal and the North American Great Lakes contain about 45 percent of the total lake water.  The many smaller lakes that dot the landscape and the water in rivers and streams that dissect it contain a tiny fraction of total freshwater.  Yet, rivers and lakes are the sources of most of the water people use every day. 

Figure 2 – Pie chart showing the overall distribution of Earth’s water from Alberta Environment (http://ygraph.com/graphs/waterchart-20120826T171009-2q2m6fa.jpeg ).

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. The pie chart shows that about 1.7 percent of all of Earth's water is groundwater and about 30 percent of freshwater on Earth occurs as groundwater. As the bar chart shows, about 5,614,000 cubic miles, or 23,400,000 cubic kilometers, of groundwater exist on Earth. About 54 percent is saline, with the remaining 2,526,000 cubic miles, about 46 percent, being freshwater.

Water in aquifers below the oceans is generally saline, while the water below the land surface (where freshwater, which fell as precipitation, infiltrates into the ground) is generally freshwater. There is a fairly 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 mostly contain freshwater.

Table 1 – Table showing estimate of global groundwater distribution (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.)

More about Groundwater Storage

Large amounts of water are stored in the ground. The water is still moving, mostly 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, where all the pores, cracks, and spaces between rock particles are saturated with water. The term “groundwater” is used to describe this area. When the characteristics of the rock units or unconsolidated material are porous and permeable enough to allow water to be extracted, the rock or unconsolidated unit is called an “aquifer.”  Aquifers are a storehouse of Earth's water and people all over the world depend on groundwater in their daily lives.

Figure 3 – Illustration of groundwater to unsaturated zone (https://www.usgs.gov/media/images/groundwater-area-underground-where-openings-are-full-water).

The top of the surface where groundwater occurs is called the water table. In Figure 3, you can see how the ground below the water table is saturated with water (the saturated zone). Aquifers are replenished by the seepage of rain and snowmelt that falls on the land, but there are many geologic, meteorological, 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 amounts of groundwater recharge vary all over the world.

For more information on aquifers see the lesson on “Groundwater Availability” in this lesson series and two other lesson series titled “Aquifers and Geology” and “Aquifer Properties and Heterogeneities.”