< 1% of the world's fresh water (~0.007% of all water on earth) is accessible for direct human uses. This is the water found in
lakes, rivers, reservoirs and those underground sources that are shallow enough to be tapped at an affordable cost. Only this amount
is regularly renewed by rain and snowfall, and is therefore available on a sustainable basis.
Water as a Resource
Since antiquity,
irrigation, drainage, and impoundment have been the three types of water control having a major impact on landscapes and water flows.
Since the dawn of irrigated agriculture at least 5,000 years ago, controlling water to grow crops has been the primary motivation
for human alteration of freshwater supplies. Today, principal demands for fresh water are for irrigation, household and municipal
water use, and industrial uses. Most supplies come from surface runoff, although mining of "fossil water" from underground aquifers
is an important source in some areas. The pattern of water withdrawal over the past 300 years shows the dramatic increases in this
century.
A timeline of human water use:
12,000 yrs. ago: hunter-gatherers continually return to fertile river valleys
7,000 yrs. ago: water shortages spur humans to invent irrigation
1,100 yrs ago: collapse of Mayan civilization due to
drought
Mid 1800's: fecal contamination of surface water causes severe health problems (typhoid, cholera) in some major North
American cities, notably Chicago
1858: "Year of the Great Stink" in London, due to sewage and wastes in Thames
Late 1800s-early
1900: Dams became popular as a water management tool
1900s: The green revolution strengthens human dependency on irrigation
for agriculture
World War II: water quality impacted by industrial and agricultural chemicals
1972: Clean Water Act passed;
humans recognize need to protect water.
--Source: the University of Michigan
The Water Resources of the Earth
Over 70% of our Earth's surface is covered by water (we should really call our planet "Ocean"
instead of "Earth"). Although water is seemingly abundant, the real issue is the amount of fresh water available.
97.5% of all water
on Earth is salt water, leaving only 2.5% as fresh water
Nearly 70% of that fresh water is frozen in the icecaps of Antarctica
and Greenland; most of the remainder is present as soil moisture, or lies in deep underground aquifers as groundwater not accessible
to human use.
