Oceanography in the 21st Century - An Online Textbook
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The Hydrological Cycle

We read in Oceans and Climate that:

  1. Sunlight reaching earth's surface is absorbed mostly in the tropics, mostly in the tropical ocean.
  2. 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.

  1. 74% of all water evaporated into the atmosphere falls as precipitation on the ocean, mostly in the tropics.
  2. 26% falls on the land. But the distribution of rainfall is very uneven.
    1. Some of the water runs into streams, lakes, and rivers, which return the water to the ocean.
    2. 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:
      1. The type of soil. Sandy soils absorb water faster than clay soils.
      2. Vegetation, which tends to delay runoff.
      3. Water content of the soil. Soils saturated with water absorb little more.
      4. Rainfall rate.
    3. 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 hydrological cycle or water cycle. Here are the major parts of the cycle.

global water cycle
World water cycle and estimated residence times.
From United Nations Environmental Programme: Vital Water Graphics.

global rain map
Global average rainfall map. Notice the importance of tropical rain. From Negri et al (2004).

Water Distribution

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.

earth's water distribution
From US Geological Survey Earth's water distribution.

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.
From United 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 (1), 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.


Water Withdrawals

Water Consumption


66 %

93 %


20 %

4 %

Domestic use

10 %

3 %

Evaporation from reservoirs

4 %

Table from World Water Council, Water At A Glance. Original data from Shiklomanov, 1999

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:

(millions of acre-feet)

Category of UseTotalGround Water UseSurface Water Use% of Total
Steam Electric Power0.430.060.382.8%

texas historical water use
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 crops.
From Texas 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.

household water usage by country
From: Manitoba State of the Environment Report 1997: Issues and Trends

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.

American Water Works Association Fact Sheet.

water use
From: American Water Works Association: Residential End Use of Water, cited by State of Washington Water Conservation Tips.
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.

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 cases.

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.

Surface Water
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 Law.


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 (5720): 405-408.

Steffen, W., A. Sanderson, et al. (2003). Global Change and the Earth System, Springer.

Revised on: 23 December, 2008

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