The earth is often called the blue planet because of its abundance of water in all forms: liquid, solid (ice), and gas (water vapor). However, only a tiny fraction—about 0.024% of the planet's enormous water supply—is readily available to us as liquid freshwater, stored in accessible underground deposits and in lakes, rivers, and streams. The rest is in the salty oceans (about 96.5% of the earth's volume of liquid water), in frozen polar ice caps and glaciers (1.7%), and in underground aquifers (1.7%) (see Figure 25, Supplement 6).
Fortunately, the world's freshwater supply is continually recycled, purified, and distributed in the earth's hydrologic cycle (see Figure 3-15). This irreplaceable water recycling and purification system works well, unless we alter it, overload it with pollutants, or withdraw freshwater from underground and surface water supplies faster than it can be replenished.
Some precipitation infiltrates the ground and percolates downward through spaces in soil, gravel, and rock until an impenetrable layer of rock or clay stops it. The freshwater in these spaces is called groundwater—a key component of the earth's natural capital.
The spaces in soil and rock close to the earth's surface hold little moisture. However, below a certain depth, in the zone of saturation, these spaces are completely filled with freshwater. The top of this groundwater zone is the water table. It falls in dry weather, or when we remove groundwater faster than nature can replenish it, and it rises in wet weather.
Deeper down are geological layers called aquifers, underground caverns and porous layers of sand, gravel, or rock through which groundwater flows. Mostly because of gravity, groundwater normally moves from points of high elevation and pressure to points of lower elevation and pressure. Some caverns have rivers of groundwater flowing through them. However, the porous layers of sand, gravel, or rock in most aquifers are like large, elongated sponges through which groundwater seeps—typically moving only a meter or so (about 3 feet) per year and rarely more than 0.3 meter (1 foot) per day. Watertight layers of rock or clay below such aquifers keep the freshwater from escaping deeper into the earth.
According to hydrologists (scientists who study water and its movements above, on, and below the earth's surface), two-thirds of the annual surface runoff of freshwater into rivers and streams is lost in seasonal floods and is not available for human use. The remaining one-third is reliable surface runoff, which we can generally count on as a source of freshwater from year to year.
During the last century, the human population tripled, global water withdrawals increased sevenfold, and per capita withdrawals quadrupled. As a result, we now withdraw about 34% of the world's reliable runoff. This is a global average. In the arid American Southwest, up to 70% of the reliable runoff is withdrawn for human purposes, mostly for irrigation (Core Case Study and Case Study that follows). Some water experts project that because of a combination of population growth, rising rates of water use per person, longer dry periods, and failure to reduce unnecessary water losses, we may be withdrawing up to 90% of the reliable freshwater runoff by 2025.
Overpumping of aquifers can contribute to limits on food production, rising food prices, and widening gaps between the rich and poor in some areas. As water tables drop, farmers must drill deeper wells, buy larger pumps, and use more electricity to run those pumps. Poor farmers cannot afford to do this and end up losing their land and working for richer farmers, or migrating to cities that are already crowded with poor people struggling to survive.
Withdrawing large amounts of groundwater sometimes causes the sand and rock that is held in place by water pressure in aquifers to collapse. This can cause the land above the aquifer to subside or sink, a phenomenon known as land subsidence. Extreme, sudden subsidence is sometimes referred to as a sinkhole. Once an aquifer becomes compressed by subsidence, recharge is impossible. In addition, land subsidence can damage roadways, water and sewer lines, and building foundations. Since 1925, overpumping of an aquifer to irrigate crops in California's San Joaquin Valley has caused half of the valley's land to subside by more than 0.3 meter (1 foot) and, in one area, by more than 8.5 meters (28 feet) (Figure 13-15).
Producers of chemicals, paper, oil, coal, primary metals, and processed foods consume almost 90% of the freshwater used by industries in the United States. Some of these industries recapture, purify, and recycle water to reduce their water use and water treatment costs. For example, more than 95% of the freshwater used to make steel can be recycled. Even so, most industrial processes could be redesigned to use much less freshwater. GREEN CAREER: water conservation specialist
Flushing toilets with freshwater (most of it clean enough to drink) is the single largest use of domestic freshwater in the United States and accounts for about one-fourth of home water use. Since 1992, U.S. government standards have required that new toilets use no more than 6.1 liters (1.6 gallons) of freshwater per flush. Even at this rate, just two flushes of such a toilet require more than the daily amount of freshwater available for all uses to many of the world's poor living in arid regions (see chapter-opening photo).
Some areas have too little freshwater, but others sometimes have too much because of natural flooding by streams, caused mostly by heavy rain or rapidly melting snow. A flood happens when freshwater in a stream overflows its normal channel and spills into the adjacent area, called the floodplain.
People settle on floodplains to take advantage of their many assets, including fertile soil on flat land suitable for crops, ample freshwater for irrigation, and availability of nearby rivers for transportation and recreation. In efforts to reduce the threat of flooding on floodplains, rivers have been narrowed and straightened (or channelized), surrounded by protective dikes and levees (long mounds of earth along their banks), and dammed to create reservoirs that store and release water as needed (Figure 13-2). However, in the long run, such measures can lead to greatly increased flood damage when heavy snowmelt or prolonged rains overwhelm them.