Water and Water Pollution - Environmental Science - page 238

All water Fresh water Accessible fresh water

Biota
0.0001%
Rivers
0.0001%

Groundwater
0.592%

Lakes
0.007%

0.014%

Fresh water
2.6%

Atmospheric
water vapor
0.001%

Oceans and
saline lakes
97.4%

Ice caps
and glaciers
1.984%

Soil
moisture
0.005%

Figure 11-2 Natural capital: the planet's water budget. Only a tiny fraction by volume of the world's water
supply is fresh water available for human use.

Deeper down are geological layers called aqui-
fers: porous, water-saturated layers of sand, gravel, or
bedrock through which groundwater flows. They are
like large elongated sponges through which ground-
water seeps. Fairly watertight layers of rock or clay be-
low an aquifer keep the water from escaping. About
one of every three people on the earth depends on wa-
ter pumped out of aquifers for drinking and other
uses.
Most aquifers are replenished naturally by precip-
itation that percolates downward through soil and
rock, a process called natural recharge. Others are
recharged from the side by lateral recharge from nearby
streams. Most aquifers recharge extremely slowly.
Groundwater normally moves from points of high
elevation and pressure to points of lower elevation
and pressure. This movement is quite slow, typically
only a meter or so (about 3 feet) per year and rarely
more than 0.3 meter (1 foot) per day.
Some aquifers get very little, if any, recharge and
on a human time scale are nonrenewable resources.
They are found fairly deep underground and were
formed tens of thousands of years ago. Withdrawals
from them amount to water mining. If kept up, they
will deplete these ancient deposits of natural capital.

Explore, through a microscope, how water molecules bond,
why ice floats, and how water and salt mix together at Envi-
ronmental ScienceNow.

Science: Surface Water
Water that does not evaporate into the air or sink
into the ground runs off the land into bodies
of water.
One of our most precious resources is fresh water that
flows across the earth's land surface and into rivers,
streams, lakes, wetlands, and estuaries. The precipita-
tion that does not return to the atmosphere by evapo-
ration or infiltrate the ground is called surface runoff.
The region from which surface water drains into a
river, lake, wetland, or other body of water is called its
watershed or drainage basin.
Two-thirds of the world's annual runoff is lost by
seasonal floods and is not available for human use. The
remaining one-third is reliable runoff: the amount of
runoff that we can generally count on as a stable source
of fresh water from year to year.

Science: Groundwater
Some precipitation infiltrates the ground and is stored
in spaces in soil and rock.
Some precipitation infiltrates the ground and perco-
lates downward through voids (pores, fractures,
crevices, and other spaces) in soil and rock (Figure 11-3).
The water in these spaces is called groundwater —and
it is one of our most important sources of fresh water.
Close to the surface, the spaces in soil and rock
hold little moisture. Below a certain depth, in the zone
of saturation, these spaces are completely filled with
water. The water table is located at the top of the zone
of saturation. It falls in dry weather or when we re-
move groundwater faster than it is replenished, and it
rises in wet weather.

Tapping the World's Reliable Surface
Water Supply
We currently use more than half of the world's reliable
runoff of surface water and could be using 70-90% by
2025.
During the last century, the human population tripled,
global water withdrawal increased sevenfold, and per
capita withdrawal quadrupled. As a result, we now
withdraw about 34% of the world's reliable runoff. We
leave another 20% of this runoff in streams to transport
goods by boats, dilute pollution, and sustain fisheries
and wildlife. In total, we directly or indirectly use about
54% of the world's reliable runoff of surface water.