Page 139 - Environment: The Science Behind the Stories

Atmosphere
12,700

Evaporation 63,000 Precipitation
Precipitation
Evaporation
63,000
413,000 113,000 Ice caps, glaciers,
413,000
113,000
and snowfields
26,350,000
Precipitation
Precipitation Rivers and Lakes
Evaporation
373,000
373,000 178,000 Evaporation
Transpiration
Extraction Transpiration
Extraction
7000
Runoff 7000
Runoff
40,000
40,000
Land plants
Human use
Human use Uptake Infiltration
Infiltration
12,600 Water
12,600
table
Extraction
Extraction Soil water 122,000
Oceans
1,335,000,000
Groundwater flow
Groundwater flow
2000
2000 Aquifer
Groundwater
15,300,000
Figure 5.16 The water cycle, or hydrologic cycle, summarizes the many routes that water molecules
take as they move through the environment. Gray arrows represent fluxes among reservoirs, or pools, for
water. Oceans hold 97% of our planet’s water, whereas most fresh water resides in groundwater and ice caps.
Water vapor in the atmosphere condenses and falls to the surface as precipitation, then evaporates from land
and transpires from plants to return to the atmosphere. Water flows downhill into rivers, eventually reaching
the oceans. In the figure, pool names are printed in black type, and numbers in black type represent pool sizes
3
expressed in units of cubic kilometers (km ). Processes, printed in italic red type, give rise to fluxes, printed in
3
italic red type and expressed in km per year. Data from Schlesinger, W.H., 2013. Biogeochemistry: An analysis of global
change, 3rd ed. Academic Press, London.
Human activity has influenced certain fluxes. We have The oceans are the main reservoir in the water cycle,
increased the flux of nitrogen from the atmosphere to reser- holding 97% of all water on Earth. The fresh water we depend
voirs on Earth’s surface, and we have shifted the flux of carbon on for our survival accounts for less than 3%, and two-thirds
in the opposite direction. As we discuss biogeochemical cycles, of this small amount is tied up in glaciers, snowfields, and ice
think about how they involve negative feedback loops that pro- caps (p. 409). Thus, considerably less than 1% of the planet’s
mote dynamic equilibrium, and also consider how some human water is in forms that we can readily use—groundwater, sur-
actions can generate destabilizing positive feedback loops. face fresh water, and rain from atmospheric water vapor.

The water cycle affects all other cycles Evaporation and transpiration Water moves from
oceans, lakes, ponds, rivers, and moist soil into the atmos-
Water is so integral to life and to Earth’s fundamental processes phere by evaporation, the conversion of a liquid to gase-
that we frequently take it for granted. The essential medium for ous form. Warm temperatures and strong winds speed rates
all manner of biochemical reactions (pp. 44–45), water plays of evaporation. A greater degree of exposure has the same
key roles in nearly every environmental system, including each effect; an area logged of its forest or converted to agricul-
of the nutrient cycles we are about to discuss. Water carries ture or residential use will lose water more readily than a
nutrients, sediments, and pollutants from the continents to the comparable area that remains vegetated. Water also enters the
oceans via surface runoff, streams, and rivers. These materi- atmosphere by transpiration, the release of water vapor by
als can then be carried thousands of miles on ocean currents. plants through their leaves, or by evaporation from the sur-
Water also carries atmospheric pollutants to the surface when faces of organisms (such as sweating in humans). Transpira-
they dissolve in falling rain or snow. The water cycle, or hydro- tion and evaporation act as natural processes of distillation,
logic cycle (Figure 5.16), summarizes how water—in liquid, because water escaping into the air as a gas leaves behind its
138 gaseous, and solid forms—flows through our environment. dissolved substances.

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