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 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.
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.
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