The Water Cycle
We read in Oceans and Climate that:
- Sunlight reaching earth's surface is absorbed mostly in the tropics,
mostly in the tropical ocean.
- The absorbed sunlight warms the ocean, which cools mainly by evaporation
from the surface (think of it as the ocean sweating to keep cool).
Water evaporated from the ocean eventually condenses as water droplets
in clouds. If the cloud grows large enough, the droplets coalesce and
fall as precipitation, mostly as rain, sometimes as snow or ice.
- 74% of all water evaporated into the atmosphere falls as precipitation
on the ocean, mostly in the tropics.
- 26% falls on the land. But the distribution of rainfall is very uneven.
- Some of the water runs into streams, lakes, and rivers, which
return the water to the ocean.
- Some soaks into the ground (infiltrates) and becomes groundwater.
The water then can percolate deeper into the ground supplying water
to subsurface reservoirs. The rate of infiltration depends on:
- The type of soil. Sandy soils absorb water faster than
- Vegetation, which tends to delay runoff.
- Water content of the soil. Soils saturated with water absorb
- Rainfall rate.
- Some evaporates back into the air, or it is absorbed by plants,
which transpire the water into the air. This is called evapotranspiration.
The cycling of water molecules from the ocean to the atmosphere to the
land and back to the ocean, and the storage in various reservoirs, is
called the water cycle or hydrological cycle. Here
are the major parts of the cycle.
World water cycle and estimated residence times.
From United Nations Environmental Programme: Vital
Global average rainfall map. Notice the importance of tropical rain.
From Negri et al (2004).
Most of earth's water is in the oceans, and most of the fresh water
is in ice and below ground (groundwater).
Very little water is available for human use. For example, only 0.91%
of all earth's water is available as fresh ground water or surface water.
Only 0.009% (3% times 0.3%) is available in lakes, rivers, and swamps.
Most of the fresh water available for human use is ground water.
From US Geological Survey Earth's
The problem with water is, it is not uniformly distributed. It is not
often available where it is needed. Globally, there is enough precipitation
to serve 6.5 billion people. But many people live in desert regions or
in densely populated regions, leading to water shortages in these regions.
The U.S. receives enough annual precipitation to
cover the entire country to a depth of 30 inches. This 30 inches is
known as the U.S. water budget. The eastern half of the country receives
more rainfall than the western half. Most of this precipitation returns
to the water cycle through evapotranspiration. Of the 30 inches of
rainfall, 21 inches returns to the atmosphere in this manner. Water
loss by plants, the transpiration portion of evapotranspiration, is
most significant. One tree transpires approximately 50 gallons of water
a day. Approximately 8.9 inches of annual precipitation flows over
the land in rivers and returns to the ocean. Only 0.1 of an inch of
precipitation infiltrates into the ground water zone by gravity percolation.
The actual amount of water that enters the ground water zone for any
specific area depends upon the annual rainfall in that area.
States Water Budget, Purdue University.
Therefore if we want to understand water use on land, we must focus
on groundwater, even though rivers and lakes are much more visible. Most
people get water from wells. Roughly half the population served by public
water systems use ground water.
Human Influence On The Water Cycle
We read in the Anthropocene that
human activity has a significant influence on the hydrological cycle
at the global level. About 40% of the total global runoff to the oceans
has been captured for human use (Steffan et al, 2004: 113). Groundwater
is being used faster than it is replenished in most dry areas of the
world. We have extensively altered river systems through impoundments
and diversions to meet their water, energy, and transportation needs.
There are >45,000 dams above 15 m high, capable of holding back >6500
km3 of water, or about 15% of the total annual river runoff
globally (Nillson et al, 2005).
Overall Water Use
We use water in households, to grow crops, to manufacture goods, and
to carry off waste and sewage. Most uses require clean, unpolluted
water free of harmful molecules, yet the very use of the water tends
to add pollution. As a result, clean water is often scarce, and most
easily accessible sources of water have been developed. In some regions,
clean water is not available.
Most water is consumed by agriculture. In the following table of global
water use, note that some uses withdraw water from reservoirs, but the
water is returned. The difference between what is withdrawn and what
is returned is consumption. Most domestic water is returned to streams
via city sewage systems.
Evaporation from reservoirs
Table from World Water Council, Water
At A Glance. Original data from Shiklomanov,
For information on water use in the US, read Estimated
Use of Water in the United States in 2000 (Hutson, 2005). Here
is the breakdown of use in Texas from the Texas Environmental Almanac:
Water Quantity: Chapter 1:
1990 WATER USE IN TEXAS
(millions of acre-feet)
|Category of Use
||Ground Water Use
||Surface Water Use
||% of Total
|Steam Electric Power
Changes in Texas water use from 1974 to 2001. Although total water
use (in millions of acre-feet) has not changed much, the distribution
of use has changed. More water is being used by cities, less by irrigated
Historical Water Use Data, Texas Water Development Board.
Household Water Use
The amount of water used by each household varies between countries,
with households in the US using the most.
From: Manitoba State
of the Environment Report 1997: Issues
The American Water Works Association has studied the use of water by
households in the US. They found
The North American households
included in this study use approximately 146,000 gallons annually.
Of this amount, 42 percent (61,300 gallons) is used indoors. The
remaining 58 percent (84,700 gallons) is used outdoors.
In households that utilized water-efficient fixtures, Clothes washers
assume the role of top water user (15 gallons per capita per day),
followed by faucets (10.9 gallons per capita per day), showers (10
gallons per capita per day) and toilets (9.6 gallons per capita per
day). NOTE: The REUWS study group did not contain a significant number
of homes with water conserving clothes washers.
Water Works Association Fact Sheet.
From: American Water Works Association: Residential
End Use of Water, cited by State of Washington Water
Click on figure for a zoom.
In Texas, household use averages 167 gallons per person per day. Of
this, about 25% is used for lawns and outdoors in Spring and Summer.
Who Owns Water
The ownership of water and water rights has a long and complex history.
Rights vary from country to country, and from state to state in the US.
In general, states own surface water, but the federal government exercises
its right to control use of rivers and pollutants dumped into rivers.
Texas Water Law Synopsis
Here is a brief summary of Texas water law from the Texas Water Foundation
and the Texas water Resources Institute (tx H2O
Texas water law is - in a word - complex. It has
its roots in Hispanic law and in English common law and has been hammered
into its current form by more than 200 years of legislation and court
Basically, water rights in Texas are divided into
two categories: groundwater and surface water.
Groundwater law, which pertains to any water
that is underground, is fairly limited. Groundwater includes water
percolating through soil and rock, underground flow in confined channels,
artesian water, and stream underflow.
In Texas, groundwater is considered the property
of the owner of the surface property from which it is pumped - much
like a mineral or oil and gas.
The English common law of "rule of capture" is
in force, allowing landowners to pump as much as they want without
regard to how such action might affect a neighbor's water supply.
[Recent legislation and court rulings are beginning
to restrict the use of groundwater as more and more users pump from
aquifers. Groundwater Conservation Districts are authorized by the
legislature to modify the rule of capture by regulating groundwater
production through permitting of non-exempt water wells, well spacing
requirements, and through other rules as deemed necessary to conserve,
protect, recharge, prevent waste of groundwater, and control subsidence.]
Texas courts have limited the rule of capture
to prohibit a landowner from:
- pumping water for the purpose of maliciously harming an adjoining
- pumping water for a wasteful purpose.
- causing land subsidence (sinking) on adjoining
land from negligent pumping.
- drilling a slant well that crosses the adjoining property line.
The Texas legislature have passed several
laws that curtail groundwater pumping. Three major restrictions to
prevent unlimited pumping of groundwater govern:
- pumping water that comes from the underflow of a river.
- pumping groundwater without a permit from
an aquifer within the jurisdiction of a groundwater conservation
- pumping groundwater from the Edwards Aquifer
within the jurisdiction ofthe Edwards Aquifer Authority without
Generally, surface water is owned by the
state. All natural streams, rivers, lakes, watersheds and bays of
the Gulf of Mexico are considered property of the state. There are
exceptions, however. Surface water can be used for domestic purposes
and for livestock.
Texans who own property next to a body of water
are free to make reasonable use of it.
For more information consult the Texas
Water Resources Education web page on Water
Law at Texas A&M University, and in the Handbook of Texas Online
article on Water
Various units are used to measure water volume:
1 gallon (US liquid) = 3.785 411 8 liters
1 acre-foot = 325,851 gallons = an area about the size of a football
field covered with one foot of water = 1,233,480 liters
Hutson,Susan S.; Nancy L. Barber, Joan F. Kenny, Kristin S. Linsey,
Deborah S. Lumia, and Molly A. Maupin (2005) Estimated
Use of Water in the United States in 2000. U.S. Geological
Survey Circular 1268, 15 figures, 14 tables, with revisions.
Negri, A. J., R. F. Adler, et al. (2004). A 16-year climatology of global
rainfall from SSM/I highlighting morning versus evening differences. 13th
Conference on Satellite Meteorology and Oceanography, Norfolk,
VA, American Meteorological Society.
Nilsson, C., C. A. Reidy, et al. (2005). "Fragmentation and Flow
Regulation of the World's Large River Systems." Science 308
Steffen, W., A. Sanderson, et al. (2003). Global
Change and the Earth System, Springer.
10 July, 2009