Geology Reference
In-Depth Information
due to global environmental change, and increased con-
sideration of the environmental benefits and needs of
other components of the hydrologic cycle such as wet-
lands. We also must remember that there is a connection
between surface water and groundwater in this cycle;
most rivers flow because of groundwater discharging
into them.
Society's sources of water include rivers, lakes,
groundwater (fresh and saline), and the oceans. Some
cultures collect precipitation directly; precipitation also
is used directly by agriculture. Most of the water that is
withdrawn for use is freshwater (<1000 mg/L of dis-
solved solids) obtained from surface water (75%) and
groundwater (25%). Some saline water (>1000 mg/L
dissolved solids) is also withdrawn. Treated water
released from wastewater treatment facilities is a
source for irrigation of parks and golf courses and
groundwater replenishment in a growing number of
aquifer storage and recovery systems. In regions where
water supply is a problem because of climate and/or
increasing population, pollution, environmental con-
cern, and capital shortages, the traditional "supply
management" approach to managing water resources
is changing to "water-demand management," which
includes conservation programs.
the importance of total freshwater runoff and how
much there is, we begin to focus on our options for
water management. Do we increase conservation and
wastewater reuse, increase desalination (with associ-
ated increases in costs), move people to environments
with surplus water, increase interbasin water transfers,
increase domestic precipitation collection and storage
systems, or all of these? Maybe we will also begin to
reflect on a possible planetary capacity for humans.
On a global basis currently the per capita use of water
is about 700 m
3
per year; for the United States it is
about 1800 m
3
per year.
HUMAN USE
Humans only require a gallon of water a day for
drinking, but domestic use in most developed areas
of the world includes drinking, cooking, bathing
(30% of household use), washing dishes, laundry
(10%), flushing toilets (40%), watering lawns and
gardens, and sometimes washing cars and filling
swimming pools. In the United States, this house-
hold total averages about 80 gallons per person per
day. Water delivered by public water supply sys-
tems, which includes household (about 60%), com-
mercial, and industrial deliveries, and supplier uses
and losses in delivery is 180 gallons per person per
day (Solley et al., 1993). However, per capita public-
supply use varies with region. For example, when
measured in gallons per day (gpd), Nevada uses 344
gpd/person and Utah 308 gpd/person, while Mass-
achusetts uses 130 gpd/person and Vermont 116
gpd/person. Groundwater accounts for about 40
percent and surface water 60 percent of water in
public supply systems.
Private water supply systems also withdraw
water from surface and groundwater sources for com-
mercial, industrial, irrigation, livestock, mining, and
thermoelectric plant cooling uses. Most is freshwater,
but for some industrial, mining, and thermoelectric
needs it is saline. Irrigation is the largest category of
freshwater use.
If we apportion all the freshwater withdrawals
(not just for public water supply) to individuals in the
nation, then the United States withdraws about 1,340
gpd/person for all these uses. (If we include both fresh
and saline water it is 1,620 gpd/person.) The range of
use by state or district is as low as 256 gpd/person in
Washington, DC, where use is primarily domestic and
commercial, to 19,600 gpd/person in Idaho, where
large quantities are used for irrigation and there is a
small population.
All of the above uses are known as
offstream
use
because the water was withdrawn or diverted from
the river (or lake, reservoir, or groundwater reservoir!)
to a place of use.
Instream
use of water refers to uses
within the channel. This includes hydroelectric gener-
ation and such difficult to quantify uses as navigation,
AVAILABILITY OF WATER
On a global scale water abundance is not a problem. It is
the availability of water in the right form in the right
place at the right time that is a problem. Thus, the water
available for human use is the key. Although more than
97 percent of the total water on Earth is in the oceans,
this water is too salty for most uses. Less than 1 percent
of the Earth's water is available for human use. The
other 99 percent is unavailable or unsuitable because of
its salinity (oceans and deeper groundwater) or its loca-
tion and form (glaciers).
The annual runoff (surface and groundwater) per
person has been used as a measure of the availability
of water resources for a region or country. Although
there may be local shortages and excesses of freshwa-
ter, the annual
water yield or runoff
from rivers and
groundwater on a per capita basis is the critical figure
in determining the water status in a region. According
to Falkenmark and Widstrand (1992), when annual
runoff is less than 1600 m
3
per person water stress
begins, and when it reaches 1000 m
3
per person,
chronic shortages occur. At less than 500 m
3
/person,
there is extreme scarcity. As population increases or
runoff decreases in countries and regions, more areas
with some level of water stress are to be expected.
In 2000, the global total offstream use of water
was about 6000 km
3
/yr. This figure is significant in
comparison to the average annual global runoff of
47,000 km
3
, which includes water that cannot be
diverted for human use because of location or envi-
ronmental needs or lack of storage. When we realize
