Visualizing Environmental Science

PROCESS DIAGRAM

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distributed among the ocean, the land, and the atmo-

sphere (ˆ}ÕÀiÊx°£ä).

Water may evaporate from land and reenter the at-

mosphere directly. Alternatively, it may flow in rivers

and streams to lakes or the ocean. The movement of

water from land to rivers, lakes, wetlands, and the ocean

is runoff, and the area of land where runoff drains is a

watershed. Regardless of its physical form—solid, liquid,

Water in

atmosphere

13,000

71,000*

Evaporation from soil,

streams, rivers, and lakes

to form clouds in atmosphere

Percolation

through soil and

porous rock to

become

groundwater

Transpiration from vegetation

adds water to atmosphere

Runoff

to ocean

40,000

Ocean

1,350,000,000

25 percent of

water in

atmosphere

falls on land as

precipitation

111,000

Evaporation from

ocean surface to

form clouds in

atmosphere

425,000

Groundwater supplies

water to soil, streams,

rivers, and ocean

15,300,000

Movement of

moist air

40,000

75 percent of

water in

atmosphere

reenters ocean as

precipitation

385,000

Condensation

(cloud formation) 75 7757575 perpercceent oof

water inin

ttttttiioniionioon

aaatiotiotiooon)n)nnn)n

In the hydrologic cycle, water moves among the ocean, the atmosphere, the land, and

back to the ocean in a continuous process that supports life. Estimated values for pools in

the global water budget are expressed as cubic km, and the values for movements

(associated with arrows) are in cubic km per year. *The starred value (71,000 cubic km per

year) includes both transpiration from plants and evaporation from soil, streams, rivers,

and lakes.

Values are from Schlesinger, W. H.

Biogeochemistry: An Analysis of Global Change

, 2nd edition. Academic

Press, San Diego (1997) and based on several sources.

108 CHAPTER 5 How Ecosystems Work

or vapor—or its location, every molecule of water moves

through the hydrologic cycle repeatedly.

Some research suggests that air pollutants known

as aerosols (see Chapter 9) may weaken the global hy-

drologic cycle. Aerosols are believed to cause clouds to

form that release less precipitation, potentially affecting

the availability and quality of water in some regions. Cli-

mate change is also altering the global hydrologic cycle

How might the

components and quantities in the hydrologic

cycle change during drought conditions?

Think Critically