Unit 8: Water Resources // Section 3: Distribution of Freshwater Resources
Freshwater accounts for only some 6 percent of the world's water supply, but is essential for human uses such as drinking, agriculture, manufacturing, and sanitation. As discussed above, two-thirds of global freshwater is found underground.
If you dig deeply enough anywhere on Earth, you will hit water. Some people picture groundwater as an underground river or lake, but in reality it is rarely a distinct water body (large caves in limestone aquifers are one exception). Rather, groundwater typically fills very small spaces (pores) within rocks and between sediment grains.
The water table is the top of the saturated zone (Fig. 3). It may lie hundreds of meters deep in deserts or near the surface in moist ecosystems. Water tables typically shift from season to season as precipitation and transpiration levels change, moving up during rainy periods or periods of little transpiration and sinking during dry phases when the rate of recharge (precipitation minus evaporation and transpiration that infiltrates from the surface) drops. In temperate regions the water table tends to follow surface topography, rising under hills where there is little discharge to streams and falling under valleys where the water table intersects the surface in the form of streams, lakes, and springs.
Figure 3. Water in the ground
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Source: © United States Geological Survey.
Above the water table lies the unsaturated zone, also referred to as the vadose zone, where the pores (spaces between grains) are not completely filled with water. Water in the vadose zone is referred to as soil moisture. Although air in the vadose zone is at atmospheric pressures, the soil moisture is under tension, with suctions of a magnitude much greater than atmospheric pressure.
This fluid tension is created by strong adhesive forces between the water and the solid grains, and by surface tension at the small interfaces between water and air. The same forces can be seen at work when you insert a thin straw (a capillary) into water: water rises up in the straw, forming a meniscus at the top. When the straw is thinner, water rises higher because the ratio of the surface area of the straw to the volume of the straw is greater, increasing the adhesive force lifting the water relative to the gravitational force pulling it down. This explains why fine-grained soils, such as clay, can hold water under very large suctions.
Water flows upward under suction through small pores from the water table toward plant roots when evapotranspiration is greater than precipitation. After a rainstorm, water may recharge the groundwater by saturating large pores and cracks in the soil and flowing very quickly downward to the water table.
Millions of people worldwide depend on groundwater stocks, which they draw from aquifers—permeable geologic formations through which water flows easily. Very transmissive geologic formations are desirable because water levels in wells decline little even when pumping rates are high, so the wells do not need to be drilled as deeply as in less transmissive formations and the energy costs of lifting water to the surface are not excessive. Under natural conditions many aquifers are artesian: the water they hold is under pressure, so water will flow to the surface from a well without pumping.
Aquifers may be either capped by an impermeable layer (confined) or open to receive water from the surface (unconfined). Confined aquifers are often artesian because the confining layer prevents upward flow of groundwater, but unconfined aquifers are also artesian in the vicinity of discharge areas. This is why groundwater discharges into rivers and streams. Confined aquifers are less likely to be contaminated because the impermeable layers above them prevent surface contaminants from reaching their water, so they provide good-quality water supplies (Fig. 4).
Figure 4. Confined and unconfined aquifer
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Water has an average residence time of thousands to tens of thousands of years in many aquifers, but the actual age of a water sample collected from a particular well will vary tremendously within an aquifer. Shallow groundwater can discharge into streams and rivers in weeks or months, but some deep groundwater is millions of years old—as old as the rocks that hold the water in their pores. Because of this distribution of residence times in aquifers, contaminants that have been introduced at the surface over the last century are only now beginning to reach well depths and contaminate drinking water in many aquifers. Indeed, much of the solute load (salt and other contaminants) that has entered aquifers due to increased agriculture and other land use changes over the last several centuries has yet to reach discharge areas where it will contaminate streams and lakes (footnote 2).
Ice sheets and glaciers are not always thought of as freshwater sources, but they account for a significant fraction of world reserves. Nearly 90 percent of the water in icecaps and glaciers is in Antarctica, with another 10 percent in the Greenland ice sheet and the remainder in tropical and temperate glaciers. As discussed in Unit 1, "Many Planets, One Earth," and Unit 12, "Earth's Changing Climate," Earth's ice sheets constantly expand and contract as the planet's climate fluctuates. During warm periods ice sheets melt and sea levels rise, with the reverse occurring when temperatures fall. Water may remain locked in deep layers of polar ice sheets for hundreds of thousands of years.
Rivers contain a relatively small share of fresh water, but the flux of water down rivers is a large part of the global hydrologic cycle and they are centrally important in shaping landscapes. Their flow erodes solid sediment and carries it toward the sea, along with dissolved minerals. These processes shape land into valleys and ridges and deposit thick layers of sediment in flood plains. Over geologic time the erosion caused by rivers balances the uplift driven by plate tectonics. Much of Earth's freshwater flow passes through several of the planet's largest rivers: the Amazon carries 15 percent of total river flow on Earth, the Congo carries 3.5 percent, and rivers that flow into the Arctic Ocean carry 8 percent. The average residence time of water in rivers is less than a year.