Evapotranspiration And Precipitation


Evaporation, along with the less well known process of transpiration, is the means by which water enters the atmosphere in the form of moisture. In evaporation liquid water from nonliving sources, such as the soil and surface waters, is converted into a gas. This conversion is driven by the power of the Sun, whose energy is also behind the process of transpiration, whereby plants lose water through their leaves. As with evaporation, transpiration places water in the atmosphere, and because the two processes work in tandem, they usually are spoken of together under the name evapotranspiration. Both make possible the formation of clouds, which, when they become saturated with moisture, produce the forms of precipitation by which water returns to the solid earth.



The hydrosphere is the sum total of Earth’s water, with the exception of water in the atmosphere. The hydrologic cycle is the continuous circulation between these two earth systems, hydrosphere and atmosphere, as well as the two other principal earth systems, biosphere and geosphere (see Earth Systems). Evapotranspiration and precipitation are principal components of this cycle, which is discussed in depth within the Hydrologic Cycle essay.
In evaporation, heat converts liquid water to a gaseous state, thus allowing It to be transferred to the atmosphere In the form of vapor. Whereas evaporation involves the loss of water to the atmosphere from nonliving sources, transpiration Is the movement of water from living organisms to the atmosphere. This is achieved by the release of water through the plants’ stomata, small openings on the undersides of leaves. In both cases, the Sun’s electromagnetic energy, experienced as heat, is the driving mechanism, and because these phenomena are so closely related, they are normally treated together as evapotranspiration.


Our present concern is primarily with the atmosphere; nonetheless, it is important to consider the other “compartments” in which water is stored. Some of them are regular way stations in the hydrologic cycle, but in the case of groundwater at least, the compartment is just that—a storage area from which it is conceivable that water might not be moved for millions of years.
The vast majority of water on Earth is indeed stored away, deep beneath the planet’s surface in the form of groundwater in the litho-sphere. This accounts for a staggering 94.7% of Earth’s water. (For figures on the mass of this and other components in the water supply, see Hydrologic Cycles.) Much of the remainder Is made up of the oceans, which account for 5,2% of the water on Earth, while glaciers and other forms of permanent and semipermanent ice make up 0.065%.

The last 0.035%.

We have now identified 99.965% of all water, yet we have not even approached any of the forms of water with which most of us typically come into contact. Of the remaining 0,035%, shallow groundwater, the source of most local water supplies, makes up the bulk, 0.30% of the total. Next are the Inland surface waters. AH of them combined) including such vast deposits as the Great Lakes and the Caspian Sea as well as the Mississippi-Missouri, Amazon, and Nile river systems—account for just 0.03% of Earth’s water.
Scanning electron micrograph of the stomata, or pores, on the underside of an apple tree leaf. Leaf pores pull in carbon dioxide for photosynthesis-
Scanning electron micrograph of the stomata, or pores, on the underside of an apple tree leaf. Leaf pores pull in carbon dioxide for photosynthesis-
Now we are left with only 0.02% of the total, which is the proportion occupied by moisture in the atmosphere: clouds, mist, and fog as well as rain, sleet, snow, and hail. While It may seem astounding that atmospheric moisture is such a small portion of the total, this fact says more about the vast amounts of water on Earth than it does about the small amount in the atmosphere. That “small” amount, after all, weighs 1.433 X 1Q13 tons (1,3 X 10IJ tonnes), or 28,659,540,000,000,000 lb.

Water Turnover

The smaller the compartment of water, the greater the amount of turnover—that is, the exchange of “new” water for “old”—and the shorter the turnover time. Groundwater may stay put in aquifers, underground rock formations, for millions of years; on the other hand, atmospheric water experiences an enormous amount of turnover in just a year’s time.
The atmosphere receives vast inputs of evaporation from the oceans as well as evapotranspiration from terrestrial ecosystems, or land-based communities of organisms. In the course of the year, the water in the atmosphere turns over about 34 times. Thus, the Inputs of evaporation and transpiration are balanced almost perfectly by outputs of precipitation, which return more than 75% of atmospheric moisture to the oceans. The remainder falls on the land, where it contributes to brooks, streams, and rivers. The land receives 67% more water as precipitation than it loses through evaporation. The difference is made up by runoff to the oceans, primarily through rivers and in much smaller portions by subterranean channels.

Haw Does the Water Move?

Clearly, water is moving through our atmosphere, but how? The answer is through gradients, or differences, of energy. When you hold a stone over the side of a cliff, preparing to drop it, the stone possesses a large quantity of gravitational potential energy. This potential energy is a function of the large gradient between the position of the stone and that of the ground.
In the same way, rivers and streams are driven by the gravitational potential that exists by virtue of the gradient from their source to the place where they empty into a lake or ocean. By definition, water flows downhill, and therefore the source of the river must be at a higher elevation than its delta, or the place where it discharges.
This matter of elevation is reflected in the confusing names for the two kingdoms that united In about 3100 B.C. to form Egypt: Upper Egypt was actually to the south of Lower Egypt. The names reflect the importance of the Nile, which flowed from the higher elevations of Upper Egypt Into the fertile lowlands of Lower Egypt, home of Egyptian civilization’s greatest farmlands.


Evaporation likewise Is driven by energy gradients; in this case, however, the energy is not gravitational but electromagnetic. Specifically, it Is the energy from the Sun, which we experience In the form of light and heat. (See Sun, Moon, and Earth for more about the Sun’s energy.} As surfaces on Earth absorb solar electromagnetic energy, It increases their heat content and provides a source of energy to drive evaporation.
The second law of thermodynamics, discussed in Energy and Earth, tells us that the flow of heat is always from a high-temperature reservoir or area to a low-temperature one. Thus, if you hold a snowball in your hand, you may think that the snowball is cooling your hand, but, in fact, the opposite is happening: heat is passing from your hand Into the snowball and warming it. As this happens, your hand loses heat, which you perceive as cold, though no coldness has been transferred—only heat.
In the same way, the energy difference between a heated surface and the atmosphere, manifested as a difference in temperature, makes it possible for water to be vaporized and transported into the air. Though the water has risen rather than fallen, its rise is brought about by the same principle that makes it possible for objects to fall from a great height, that is, a difference in potential. Thus it can be said that in evaporating, water “falls” from an area of high heat and high energy to one of low heat and low energy.



When it comes to putting moisture into the atmosphere, transpiration is at least as important as evaporation. In fact, it puts more water into the air than evaporation does: any large area of vegetation tends to evaporate much larger quantities of moisture than an equivalent non-foliated region, such as the surface of a lake or moist soil. Physically, however, the process of transpiration is the same as that of evaporation.
The only difference is that in the case of transpiration, the source of the evaporation is a biological one—leaves, for instance, as well as skin or lungs. From an environmental standpoint, the most important kind of transpiration is that which occurs through leaves. The loss of water from foliage puts an enormous amount of moisture into the atmosphere, and for this reason, areas where foliage appears In high concentration (i.e., forests) are vital to the cycling of water through the atmosphere.
Plants lose their water through moist membranes of a tissue known as .spongy mesophyll, found in the tiny cavities that lie beneath the microscopic leaf pores called stomata. Stomata remain open most of the time, but when they need to be closed, guard cells around their borders push them shut. Because plants depend on them to “breathe” by pulling in carbon dioxide (see Carbon Cycle), however, they keep their stomata open—just as a human’s pores must remain open, or the person would die of suffocation.
Because stomata are exposed in order to receive carbon dioxide for the plant’s photosynthesis, this also means that the stomata are open to allow the loss of moisture to the atmosphere. It can be said, then, that transpiration—vital as it is to the functioning of our atmosphere—is actually an unavoidable consequence of photosynthesis, an unrelated process.

Transpiration and animals

As we have suggested, transpiration in animals (including humans) takes place for much the same reason as it does with plants, as a by-product of breathing. Animals have to keep their moist respiratory surfaces, such as the lungs, open to the atmosphere. We may not think of our own breathing as transferring moisture to the air, but with just a little consideration of the subject it becomes clear that this Is the case. The presence of moisture in our lungs can be proved simply by breathing on a piece of glass and observing the misty cloud that lingers there.
On the one hand, transpiration can cause animals to become dehydrated, but it also can be important in cooling down their bodies. When human bodies become overheated, they produce perspiration, which cools the surface of the skin somewhat. If the air around us is too humid, however, it already is saturated with water, and the perspiration has no place to evaporate. Therefore! instead of continuing to cool our bodies, the perspiration simply forms a sticky film on our skin. Assuming that the air is capable of absorbing more moisture, however, the sweat will evaporate, cooling our bodies considerably.

Heat, Cold, and Evapotranspiration

The preceding discussion brings up several more points about the relationship between heat and evapotranspiration. First of all, everything that is living has some degree of heat; If it did not, it would be at a temperature known as absolute zero, or OK (-459.67T or -273T5°C), which is impossible to reach. Therefore, even in Greenland there is a small amount of molecular motion in plant tissues, even when they appear to be completely frozen.
Naturally, however, there is very little evapotranspiration when temperatures are extremely cold, which is the reason why it can sometimes be “too cold to snow”: temperatures are too low for sufficient moisture to be moved to the atmosphere. Nonetheless, there still can be some physical evaporation as the result of the direct vaporization of solid water, a process called sublimation.

Hot, cry summers

At the height of summer, when air temperatures are warm and the trees are fully foliated (i.e., covered in leaves), a high rate of transpiration occurs. So much water is pumped into the atmosphere through foliage that the rate of evapotranspiration typically exceeds the input of water to the local environment through rainfall. The result is that soil becomes dry, some streams cease to flow, and by late summer, In extremely warm temperate areas such as the southern United States, there is a great threat of drought and related problems, such as forest fires.
Once the trees drop their leaves in the autumn, transpiration rates decline greatly. This makes It possible for the parched soil to become recharged by rainfall and for streams to flow again. Such is the case in a temperate region, which by definition is one that has the four seasons to which most people in the United States (outside Hawaii, Alaska, and extreme southern Florida and Texas) are accustomed. In a tropical region, by contrast, there is a “dry season,” In which transpiration takes place, and a “rainy season” in which moisture from the atmosphere inundates the solid earth. This rainy season may be so intense that It produces floods.


So far, we have talked mostly about the means by which water moves from the solid earth Into the atmosphere. Now let us consider what happens when it gets there, at which point—assuming there is sufficient moisture—It will coalesce to form a cloud. Though most people are inclined to romanticize clouds, seeing in them shapes and colors and sometimes even reflections of their own moods, clouds are nothing more than atmospheric moisture that has condensed to form tiny droplets of water or crystals of ice.
Heated, moist air that rises from the ground is much more dense than the air that lies above It, but as it rises, it expands and becomes less dense. This expansion cools the air, so that the water vapor condenses into tiny droplets. As a result, a cloud forms, and the relative intensity of the energy gradients that brought about the formation of the cloud creates different shapes.
For example, if there is a vigorous uplift of air, resulting from sharp differences in temperature and pressure between the ground and the atmosphere, the resulting clouds will have a tall, stacked appearance. On the other hand, clouds formed by the gentle uplift of air currents have a flat, cottony appearance. Thus, the shapes of the clouds themselves, as well as the ways in which they change, assist meteorologists in predicting the weather.

Cloud Classifications

Thanks to a system developed In 1803 by the English pharmacist and amateur naturalist Luke Howard (1772-1864), it :s possible to classify clouds according to three basic shapes. These shapes are known by the Latin names cumulus (piled heaps and puffs), cirrus (curly, fibrous shapes), and stratus (stretched and layered).
Howard combined these names with adjectival terms, such as alto (“high”) and nimbus (“rain”), to describe variations on the three basic cloud types. Today, the International Cloud Classification used by meteorologists worldwide applies Howard’s system to the identification of ten basic cloud types, or genera. They are divided into three high-altitude genera, two midlevel ones, three low-level genera, and two varieties of rain cloud.

High-level clouds

The three genera of high-level clouds appear at a range between 16,500 ft. and 45,000 ft (5,032-13,725 m), though they usually form in a belt between 20,000 ft. and 25,000 ft. (6,000-7,500 m). All are cirrus clouds, of which the highest are pure cirrus, made of ice crystals.
Then there are clrrocumulus, the least common variety of cloud. Composed of either supercooled water droplets (i.e., water that continues to exist In liquid form even though the temperature has dropped below the freezing point) or ice crystals, these are small and white or pale gray, with a rippled appearance like oatmeal. Finally, clrrostratus clouds—which, like pure cirrus clouds, are made of ice crystals—often form a halo around the Sun or Moon in the wintertime.

Midlevel clouds

The two genera of midlevel clouds, which appear at 6,50023,000 ft, (2,000-7,000 m), are named with the prefix “alto.” Altostratus clouds are usually a uniform bluish or gray sheet covering large portions of the sky and either completely or nearly obscure the Sun and Moon. These complex clouds are composed of layers, with ice crystals at the top, ice and snow in the middle, and water droplets in the lower layers.
Altocumulus are oval-shaped, dense, fluffy balls that may appear either as singular units or as closely bunched groups. When sunlight or moonlight shines through these clouds, the light often is perceived from the ground in the form of rays.

Low-level clouds

There are three genera in the low level, which extends from the surface to 6,500 ft. (2,000 m). Pure stratus clouds, which are generally the lowest, blanket the sky and typically appear gray. They are formed when a large mass of air rises slowly or when cool air moves in over an area close to ground level. Stratus clouds may produce mist or drizzle, or, when they form at ground level, they may appear as fog.
When moisture coalesces in the atmosphere, it forms a cloud. Altocumulus clouds, which look like oval-shaped. dense. puffy balls, are midlevel clouds.
When moisture coalesces in the atmosphere, it forms a cloud. Altocumulus clouds, which look like oval-shaped. dense. puffy balls, are midlevel clouds.
Pure cumulus clouds are flat on the base and vertically thick, with a puffy appearance on top. This puffiness is the result of updrafts. Occurring primarily in warm weather, cumulus clouds are made of water droplets and look brilliant white because of the sunlight’s reflection off the droplets. Last, stratocumulus clouds are large, grayish, and puffy, often looking like dark rolls.

Rain clouds

Stratocumulus clouds can transform into nimbostratus clouds, while cumulus can develop into cumulonimbus clouds. These two—nimbostratus and cumulonimbus—are the last of the ten varieties of cloud, designated not by altitude but by the fact that they give rise to precipitation. (Note that the light, airy variety known as cirrus never portends rain.)
Nimbostratus usually appear at midlevel altitudes. Made of water droplets that produce either rain or snow, they are thick, dark, and gray.
Micrograph of a snowflake. snowflakes are formed by the bonding of solid ice crystals inside a cloud.
Micrograph of a snowflake. snowflakes are formed by the bonding of solid ice crystals inside a cloud.
Sometimes they are seen with virga, which are skirts of rain trailing down their sides.Cumulonimbus, or thunderstorm, clouds arise from cumulus clouds that have reached a great height. At a certain height, the cloud flattens out, and powerful updrafts and downdrafts create a great deal of unrest inside the cloud, which contains all phases of water: gas, liquid, and solid. These clouds can cause violent storms.

Rain, Snow, Hail, and Sleet

Eventually, the amount of moisture in the cloud becomes so great that it has to fall earthward in the form of precipitation—usually rain, snow, hail, or sleet. These types of precipitation are distinguished by the form they take: liquid in the first case, lightly frozen particles in the second case, and hard, frozen nuggets in the latter two cases.
Liquid precipitation includes drizzle and raindrops, which are distinguished from each other on the basis of size. Raindrops have a radius of about 0.04 in. (1 mm), while drops of drizzle are about one-tenth that size. Note the use of the term radius, implying a sphere. Drops of liquid precipitation are spherical, and though a teardrop shape is popularly associated with raindrops, they assume that shape only when they are falling.
Snowflakes are formed by the aggregation, or physical bonding, of solid ice crystals inside a cloud, while hailstones are made of a combination of supercooled water droplets and pellets of ice. Not only are they more dense than snowflakes, but they are also more spherical. Similar to hail is sleet, pellets of pure ice that usually are much smaller than hail.

Forming precipitation

The type of precipitation formed depends on the warmth of the cloud from which it comes. “Warm” clouds are those whose temperatures are above freezing—32°F (0°C)—and “cold” clouds are those that are at least partially below freezing temperature. These temperature values are themselves a function of altitude, since temperature in the lower atmosphere (where all precipitation occurs) decreases by about 5.32aF for every mile (6°C per kilometer) of altitude gained.
As we have suggested earlier, humidity relates to the ability of air to evaporate, and, therefore, when the air has all the water it can hold, :t is said to have a relative humidity of 100%. The cooler air is, the less moisture is required for it to become saturated. Once saturation occurs, moisture molecules form around cloud condensation nuclei, such as nitrates or extremely fine particles of sea salt that have managed to evaporate.
Cold clouds form around Ice crystals. Before freezing, the water in these clouds may be supercooled,. Ice nuclei are much more rare than cloud condensation nuclei and are less well understood by meteorologists.

Mixed clouds

Clouds that contain both liquid water and ice are called mixed clouds. Supercooled water will freeze when it strikes an ice crystal, and if it freezes immediately, it forms what is known as opaque or rime ice. If it freezes slowly, the result is called clear ice. Eventually the ice forms a thick coating, which is the origin of hail.
Not all mixed clouds produce frozen precipitation, however. Thunderstorms, Involving electric charges imparted to precipitation particles, which leads to the eventual discharge of lightning, are also the products of mixed clouds. (For more about storms and precipitation, see Weather and Climate.)


Aquifer: An underground rock formation in which groundwater is stored.
Atmospheres: In general, an atmosphere is a blanket of gases surrounding a planet. Unless otherwise identified, however, the term refers to the atmosphere of Earth, which consists of nitrogen (78%), oxygen (21%), argon (0.93%), and other substances that include water vapor, carbon dioxide, ozone, and noble gases such as neon (0.07%).
Biosphere: A combination of all living things on Earth—plants, mammals, birds, amphibians, reptiles, aquatic life, insects, viruses, single-cell organisms, and so on—as well as all formerly living things that have not yet decomposed.
Ecosystems: A community of Interdependent organisms along with the inorganic components of their environment.
Evaporation: The process whereby liquid water is converted into a gaseous state and transported to the atmosphere. When discussing the atmosphere and precipitation, usually evaporation is distinguished from transpiration. In this context, evaporation refers solely to the transfer of water from nonliving sources, such as the soil or the surface of a lake.
Evapotranspiration: The loss of water to the atmosphere via the combined (and related) processes of evaporation and transpiration.
Geosphere: The upper part of Earth’s continental crust, or that portion of the solid earth on which human beings live and which provides them with most of their food and natural resources.
Groundwater: Underground water resources that occupy the pores in bedrock.
Hydrologic cycle: The continuous circulation of water throughout Earth and between various earth systems.
Hydrology: The study of the hydrosphere, including the distribution of water on Earth, its circulation through the hydrologic cycle, the physical and chemical properties of water, and the interaction between the hydrosphere and other earth systems,
Hydrosphere: The entirety of Earth’s water, excluding water vapor in the atmosphere but including all oceans, lakes, streams, groundwater, snow, and ice.
Meteorology: The study of the atmosphere, weather, and weather prediction.
Potential: Position In a field, such as a gravitational force field.
Potential energy: The energy that an object possesses by virtue of its position or its ability to perform work.
Precipitation: When discussing the hydrologic cycle or meteorology, precipitation refers to the water, in liquid or solid form, that falls to the ground when the atmosphere has become saturated with moisture. In the context of chemistry, precipitation refers to the formation of a solid from a liquid.
Supercooled: A term for water that continues to exist in liquid form even though its temperature has dropped below the freezing point.
System: Any set of interactions that can be set apart mentally from the rest of the universe for the purposes of study, observation, and measurement.
Transpiration: The process whereby moisture is transferred from living organisms to the atmosphere. A major portion of environmental moisture for precipitation comes from plants, which lose water through their stomata, small openings on the undersides of leaves.
Watershed: An area of terrain from which water flows into a stream, river, lake, or other large body.

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