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Water and the Hydrologic Cycle

Water is one of the central materials that determines the conditions of life on Earth. Water makes up eighty to ninety percent of all living organisms. Most organisms can survive only in a limited range of temperatures. Water provides one of the main buffers between the living entity and the environment, preventing sudden changes in temperature.

The ancient Greek philosophers considered water to be one of the four elements along with earth, wind, and fire. At the end of the eighteenth century, chemists were able to decompose water and show that it is made of oxygen and hydrogen. As it is a vital compound in life—our bodies are 70% water—and as the compound that keeps the Earth's environment fit for life, it is indeed an "element" of what makes up the conditions on Earth.

Water appears to have been on the Earth throughout its 4.5 billion year history. The Earth is the only planet of the triplet of "identical" planets—Mars, Earth and Venus—with surface oceans. Mars and Venus have dry surfaces. The clouds on Venus are not made up of H2O like the Earth's clouds, rather they are made up of CO2. Most of the Earth's water resides in the oceans.

We believe that water was part of the original primordial soup in which complex molecules, including proteins, were first formed—eventually leading to life.

The Water Molecule

Water has several properties that make it a unique compound in its ability to support life. The properties are: its latent heat, its density, and its ability to dissolve so many substances. All of these properties come from the peculiar molecular composition and the geometry of the water molecule. Because of the importance of the structure in determining the properties of water as liquid water and as ice, we describe the molecular structure and the liquid state in some detail.

The peculiar geometry of water with an angle of approximately 105° (104.5° to be precise) between the hydrogen bonds turns out to be the basis of the miracle of our planet. First, let us look at how the molecule is formed when two hydrogen atoms and an oxygen atom combine. Figure W1 shows the valence electrons of H and O atoms, and the covalent bonding of H20.

Figure W1. Scheme of formation of covalent bonds between H and O to form water. The two "open" pairs of electrons in oxygen are called the non-bonding pairs.


Note that with the sharing shown, H and O atoms complete their shells for part of the time. However, as oxygen has a higher positive charge in the nucleus the shared electron in each bond spend a larger fraction of time in the oxygen orbital. This makes the oxygen side of the molecule more negative than the hydrogen sides. The actual reason that the molecule is bent rather than linear has to do with the mutual repulsion of the unshared pair of electrons in oxygen and is beyond our scope here. The molecule has a shape shown schematically and in terms of the actual spatial configuration in Figure W2 a and b.

Figure W2 a
Figure W2 b

The shape of the water molecule is an isosceles triangle in which the H—O—H bond angle is approximately 105°. The illustration on the right shows the scheme of covalent bonds in water. Altogether, there are eight valence electrons in the water molecule: six originally belonging to the oxygen atom and one each to hydrogen. Four are involved in the O—H bonds, two in each. The remaining four belong to oxygen, and are in nonbonding orbitals. This gives water its peculiar geometry and charge character.

The molecule is electrically polar, that is, it has a net positive charge at the hydrogen end of the triangle and a net negative charge at the oxygen atom. In a group of water molecules clustered together, a positively charged region in one molecule tends to be attracted to the negatively charged region in another. There are two positive regions in each water molecule (the two hydrogen atoms). There are two negative regions projected by the nonbonding electrons. Each of the nonbonding pairs of electrons attract a positive hydrogen atom on a neighboring water molecule, and each of the hydrogen ends attracts the oxygen end of a neighboring water molecule. This results in each water molecule having four nearest neighbors. This type of bonding between molecules, due to hydrogen atoms forming a "bridge," is called a hydrogen bond.

Figures W3 a and b show the scheme of a hydrogen bond, and the spatial configuration of water. Figure W3c shows how the continuously moving liquid water molecules make transient hydrogen bonds with one another, forming a fluid network.

Figure W3 (a). The hydrogen bond is the weak covalent bond between the hydrogen in one water molecule with the two electrons in the non-bonding orbital of oxygen in another water molecule. The hydrogen bond is very weak (about 5 kcal/mole) compared to the H-O bond in its original molecule (about 110 kcal/mole).


Figure W3b. Space configuration results from the hydrogen bonding in liquid water, magnified about a million times the original size. The shadows suggest the constant motion of the molecules.


Figure W3 (c). Models of the three-dimensional network formed by the liquid water molecules, creating a fluid network.


At low temperatures, this structure is highly geometrical and coordinated. This ordered structure is the structure of ice that in effect gives the hexagonal shape to snowflakes. At a temperature close to the freezing point, the kinetic energy of the water molecules is small and the hydrogen bonds keep the molecules in place but still not very closely packed. This is sketched in Figure W4.

Figure W4. The structure of ice, magnified a million times. The atoms come together in a hexagonal pattern.


The weak hydrogen bonding means that ice has a lot of empty space. When ice melts, the "frozen" geometry is removed, but not all the hydrogen bonds are broken. The molecules begin to pack more closely together so as to fill some of the empty space. Thus liquid water is denser than ice. Water has its greatest density at 4°C. This is why the top of a lake freezes first. The cooler part freezes and the more dense water at slightly higher temperature sinks to the bottom. The bottom freezing last helps protect fresh water organisms that live in the bottom. The empty space also means that ice does not conduct heat very well. So, the frozen top of the lake keeps the heat from the water below it from escaping too readily, maintaining it as liquid. This characteristic is central to maintaining aquatic life during winter.

Even when ice converts to water at 0°C, only about 15% of the bonds are broken. So, cold water molecules are still relatively bound together. Although molecules are constantly in motion, local order is still mostly maintained, as molecules remain bound to one another even as they move fast. Thus, it is able to absorb a lot of heat without significant change in temperature. That is, the heat capacity of water is higher than most substances including air. Oceans do not suffer from sudden changes in temperature, which is important for living systems in the oceans. The ocean also buffers the climate on its shore. The large quantity of water on Earth serves to prevent sudden rises of temperature, between night and day for example .

It takes a lot of heat and thermal motion of the molecules to finally break all the bonds, and to vaporize the water. All of this means that it takes high amounts of energy to change the states of water from solid to liquid and liquid to gas - water has a high latent heat of fusion (energy required for melting) and a high latent heat of vaporization (energy to vaporize). Land plants and animals are able to dissipate a lot of heat simply by evaporation of water through transpiration or by sweating, in the case of animals. Under extreme conditions such as in a desert a human body may evaporate as much as one liter of water per hour by sweating to rid itself of heat.

The polar nature and empty spaces in water also make it a good solvent. The polar nature gives rise to the high surface tension of water. This high surface tension makes water capable of rising in capillary structures of roots and stems. It also gives firmness to the surface of lakes so that light insects can actually sit on the surface.

Water vapor is a greenhouse gas. Both the capability to keep heat in, and to transfer heat from the tropics serve to buffer temperatures on Earth. For example, as one half of the Earth rotates away from the sun, the fall in temperature is much more gradual than it would have been if there were no water vapor in the atmosphere. Thus water has a combination of properties that accounts for its central role in preserving life on Earth: liquid denser than solid; high surface tension; high heat capacity; and high solubility.

The Hydrologic Cycle and Water Balance

Water is a fundamental necessity for all ecological systems, as it is a cornerstone of life. Estuaries (where river and sea meet) are an aquatic environment that is important to the life cycle of many species. Water is cycled through evaporation and transpiration from plants into the atmosphere, and precipitation back to Earth. Four-fifths of the water in the global water cycle comes from the oceans. Of a total of about 1.36 billion cubic kilometers of water, about 97% is in oceans. An additional 2% is locked up in glaciers and icecaps, and 0.31% is stored in deep groundwater reserves.

This leaves only about 4.2 million cubic kilometers of relatively accessible fresh water. Water evaporates from the large surfaces of the Earth's oceans. It is estimated that 41,000 cubic kilometers of water returns to the sea from the land per year, balancing the transport of water from sea to land through the atmosphere as precipitation. About 32,000 cubic kilometers return to the sea as runoff that cannot be captured. The remaining 9,000 km3, is potentially available as water supply for people. This could theoretically supply 20 billion people. During the past 300 years, human water use has increased 35-fold. But the availability is far from uniform on the Earth's continents. There are places in the Middle East and Africa which have no access to natural fresh water. In addition, some people have lifestyles that consume much more water than others. The average U.S. resident consumes 70 times as much water per year as an average resident of Ghana.

The United States Geological Survey website has a section completely devoted to the Water Resources of the United States. The water withdrawn for public supply during 1995 was an estimated 40,200 Mgal/d. Public suppliers served about 225 million people during 1995. Total public supply withdrawals in 1995 averaged 1979 gal/d for each person served. The pie charts below show the amount of surface water used and ground water used for public consumption. Public supply refers to water withdrawn by public and private water suppliers and delivered to multiple users for domestic, commercial, industrial, and thermoelectric power uses.

insert pie charts

The hydrological cycle also cleans the environment. Clouds and run off transport and deposit pollutants into lakes and oceans. The transport and deposition of SO2 and NOx in pollution is the environmental problem known as acid rain. SO2 and NOx come from sources such as fossil fuel burning as well as from some natural sources such as volcanoes. The amounts of water cycled per day is enormous and highly variable. One thousand gigatonnes of water evaporate from the oceans each day. In some places like the southern coastal regions of Peru, decades may pass with no rain at all. The water cycle is one cycle that has been manipulated by technology through building dams and hydroelectric energy plants.

Figure W5: Hydrologic Cycle in Quantities

Availability and Pollution of Water

Although water is so abundant on Earth that we call the Earth a "water planet," 97% of this water is salt water. Only about 3% of water is fresh water, and less than 0.01% is readily available from rivers and lakes.

Problems of availability of fresh water arise from agriculture--both the vast amounts used in irrigation and the pollution arising from pesticides and fertilizers--, industrial pollution, and pollution from sewage. Some types of industrial pollution of water (like heavy metal pollution) have been realized only relatively recently. Water had historically been thought of a "bottomless" sink for pollutants. A statement often used by industry as recently as a couple of decades ago was "dilution is the solution to pollution." These attitudes and the fact that water is relatively plenty in highly industrialized nations delayed our recognition and addressing of water pollution.

While the hydrologic cycle is continuously at work globally, local conditions of rain and fresh water supply vary tremendously. This uneven distribution of water determines the nature of many of the problems related to fresh water management and use. The average residence time of a molecule of water in the atmosphere is about eight days. The residence time of water in deep ground water aquifers, or large glaciers, may be hundreds, thousands, or hundreds of thousands of years.

Table 3 shows the major salt and freshwater stocks on Earth, and the small amount of freshwater available. Less than 1% of this is actually usable.

Table 3. Volume (million km3) % of total water
Saltwater Stocks
  Oceans 1,338,000 96.54
  Salty ground water 12,870 .0.93
  Saltwater lakes 85 0.006
Freshwater Stocks
  Glaciers, permanent snow 24,064 1.24
  Fresh ground water 10,530 0.76
  ground ice 300 0.022
  Freshwater lakes 91 0.007
  Soil moisture 16.5 0.001
Atmospheric water vapor
  Marshes, wetlands 11.5 0.001
  Rivers 2.12 0.0002
  In live organisms 1.12 0.0001
Table 3: Water stocks on Earth.

Figure W6 shows the annual fresh water availability for selected countries.

Figure W6. Average Annual Water Availability for Selected Countries, 1921-1985. The average amount of fresh water available for various countries is shown here, in cubic kilometers per year, as measured over the period 1921 to 1985. This figure shows the vast differences in the natural distribution of fresh water among different regions. Source: The World's Water 2000-2001: The Biennial Report on Freshwater Resources.

These figures speak for themselves. While these amounts themselves are low, we also add to the problem by polluting our groundwater. Groundwater lies mainly deep underground in aquifers which are geological niches of porous materials or space between rocks. Farms, cities, and factories all have run-off that can get deep into the ground and pollute even aquifers. Industrialized agriculture has also been responsible for digging deep into the water resources.

Pollution of Ground Water

Fertilizers and pesticides applied to cropland, organic wastes from farmlands, and sewage from cities all pollute ground water. Nitrate pollution of groundwater due to these sources have become very severe. In California's central valley, nitrate level in ground water almost tripled between 1895 and 1980. One of the effects of nitrates in groundwater is the so-called blue-baby syndrome or methenoglobinemia in which the oxygen-carrying capacity of the baby's blood decreases.

Pesticides of various kinds have entered many aquifers and are one pathway for organochlorine compounds--also known as endocrine disrupters--to enter our systems. The effects of organochlorines are discussed in the unit on Risk & Human Health.

Even when the problem of water pollution was first realized, the remedies sought were end-of-pipe--cleaning up polluted water--rather than conservation of fresh water or prevention of pollution. Water purification technologies were developed. Creative redesign of industrial processes and water conservation technologies like low-flush toilets have begun to get serious attention only recently. Industry is just beginning to design and implement methods that reduce water pollution. The use of "grey water" (water that has only been partially re-cleaned) for high water use applications such as agriculture has not received much attention in the U.S.

To add:

water resources in the US

water quality regulations

Safe Drinking Water Act (1974) - national interim primary drinking water regulations - established by EPA in 1977.
1986, with the passage of the amendments to the Safe Drinking Water Act, (EPA was mandated specifically to regulate microbiological constituents, inorganic and organic compounds, and radioactivity) 83 contaminants were to be regulated in the initial stage with an additional 25 contaminants to be added every 3 years. In 1988, regulatory efforts focused on lead. The latest amendments to the Safe Drinking Water Act occurred in 1996. The criteria for the selection and regulation of contaminants was a key amended item. EPA no longer has to set 25 new standards every three years. Regulated contaminants either need to have adverse health effects or are present at levels sufficiently high to warrant public concern. Development of regulatory levels are to be based on risk assessment, cost-benefit analysis, and minimizing overall risk.

Dr. John Snow

consumer uses

water pollution

oxygen demand *make link with oxygen cycle see last paragraph*

teacher's note = Hydrologic Cycle = "natural model" for scientific model




  ©Copyright 2003 Carnegie Mellon University
This material is based upon work supported by the National Science Foundation under Grant Number 9653194. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.