Water cycle of the earth's surface, showing the individual components of transpiration and evaporation that make up evapotranspiration. Other closely related processes shown are runoff and groundwater recharge. RUNOFF was the first Computer Text formatting program to see significant use Groundwater recharge or deep drainage or deep percolation is a hydrologic Process where Water moves downward from Surface water

Evapotranspiration (ET) is a term used to describe the sum of evaporation and plant transpiration from the earth's land surface to atmosphere. Evaporation is the process by which Molecules in a Liquid state (e Plants are living Organisms belonging to the kingdom Plantae. Transpiration is the Evaporation of water from the aerial parts of Plants especially leaves but also stems Flowers and Roots An atmosphere (from Greek ατμός - atmos, " Vapor " + σφαίρα - sphaira, " Sphere " Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and waterbodies. Soil, often typeset as SOiL, is a four piece rock band from Chicago Illinois United States founded by Shaun Glass Tom Schofield Tim King and Adam Zadel Interception, or canopy interception, refers to precipitation that does not reach the soil but is instead intercepted by the leaves and branches of plants A body of water is any significant accumulation of Water, usually covering the Earth or another planet Transpiration accounts for the movement of water within a plant and the subsequent loss of water as vapor through stomata in its leaves. Plants are living Organisms belonging to the kingdom Plantae. In Botany, a stoma (also stomate; plural stomata) is a tiny opening or pore found mostly on the underside of a Plant Leaf In Botany, a leaf is an above-ground Plant organ specialized for Photosynthesis. Evapotranspiration is an important part of the water cycle. The Earth 's Water is always in movement and the water cycle, also known as the hydrologic cycle, describes the continuous movement of water on above An element (such as a tree) that contributes to evapotranspiration can be called an evapotranspirator. ^[1]

Potential evapotranspiration (PET) is a representation of the environmental demand for evapotranspiration and represents the evapotranspiration rate of a short green crop, completely shading the ground, of uniform height and with adequate water status in the soil profile. It is a reflection of the energy available to evaporate water, and of the wind available to transport the water vapour from the ground up into the lower atmosphere. In Physics and other Sciences energy (from the Greek grc ἐνέργεια - Energeia, "activity operation" from grc ἐνεργός Wind is the flow of Air or other Gases that compose an Atmosphere (including but not limited to the Earth's) Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five Evapotranspiration is said to equal potential evapotranspiration when there is ample water.

Evapotranspiration and the water cycle

Evapotranspiration is a significant water loss from a watershed. Types of vegetation and land use significantly affect evapotranspiration, and therefore the amount of water leaving a watershed. Because water transpired through leaves comes from the roots, plants with deep reaching roots can more constantly transpire water. Thus herbaceous plants transpire less than woody plants because herbaceous plants usually lack a deep taproot. A herbaceous plant (or in botanical use a Herb) is a Plant that has leaves and stems that die down at the end of Dicotyledons, or "dicots", is a name for a group of Flowering plants whose Seed typically has two embryonic leaves or Cotyledons There Also, woody plants keep their structure over long winters while herbaceous plants must grow up from seed in the spring in seasonal climates, and will contribute almost nothing to evapotranspiration in the spring. Conifer forests tend to have much higher rates of evapotranspiration than deciduous forests. Botany Autumn leaf color. See --> In Botany and Horticulture, deciduous Plants, including This is because their needles give them superior surface area, resulting in more pores for transpiration, and allowing for more droplets of rain to be suspended in and around the needles and branches, where some of the droplets can then be evaporated. Factors that affect evapotranspiration include the plant's growth stage or level of maturity, percentage of soil cover, solar radiation, humidity, temperature, and wind. Humidity is the amount of water vapor in the air In daily language the term "humidity" is normally taken to mean Relative humidity. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature Wind is the flow of Air or other Gases that compose an Atmosphere (including but not limited to the Earth's)

Through evapotranspiration, forests reduce water yield, except for in unique ecosystems called cloud forests. A cloud forest, also called a fog forest, is a generally Tropical or Subtropical evergreen Montane moist forest characterized by a Trees in cloud forests condense fog or low clouds into liquid water on their surface, which drips down to the ground. These trees still contribute to evapotranspiration, but often condense more water than they evaporate or transpire.

In areas that are not irrigated, actual evapotranspiration is usually no greater than precipitation, with some buffer in time depending on the soil's ability to hold water. In Meteorology, precipitation (also known as one class of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric It will usually be less because some water will be lost due to percolation or surface runoff. In Physics, Chemistry and Materials science, percolation concerns also the movement and filtering of fluids through porous materials An exception is areas with high water tables, where capillary action can cause water from the groundwater to rise through the soil matrix to the surface. The water table is the level at which the ground water pressure is equal to Atmospheric pressure. Capillary action, capillarity, capillary motion, or wicking is the ability of a substance to draw another substance into it If potential evapotranspiration is greater than actual precipitation, then soil will dry out, unless irrigation is used. Irrigation is an artificial application of water to the soil usually for assisting in growing crops

Evapotranspiration can never be greater than PET, but can be lower if there is not enough water to be evaporated or plants are unable to readily transpire.

Estimating evapotranspiration

Evapotranspiration be measured or estimated using several methods.

Indirect methods

Pan evaporation data can be used to estimate lake evaporation, but transpiration and evaporation of intercepted rain on vegetation are unknown. Pan evaporation is a measurement that combines or integrates the effects of several climate elements temperature humidity solar radiation and wind There are three general approaches to estimate evapotranspiration indirectly.

Catchment water balance

Evapotranspiration may be estimated by creating an equation of the water balance of a catchment (or watershed). The equation balances the change in water stored within the basin (S) with inputs and exports:

The input is precipitation (P), and the exports are evapotranspiration (which is to be estimated), streamflow (Q), and groundwater recharge (D). Groundwater recharge or deep drainage or deep percolation is a hydrologic Process where Water moves downward from Surface water If the change in storage, precipitation, streamflow, and groundwater recharge are all estimated, the missing flux, ET, can be estimated by rearranging the above equation as follows:

Hydrometeorological equations

The most general and widely used equation for calculating reference ET is the Penman equation. The Penman equation describes Evaporation ( E) from an open water surface and was developed by Howard Penman in 1948 The Penman-Monteith variation is recommended by the Food and Agriculture Organization. Like the Penman equation, the Penman-Monteith equation requires daily mean temperature wind speed relative humidity and solar radiation to predict net Evapotranspiration. ^[2] The simpler Blaney-Criddle equation was popular in the Western United States for many years but it is not as accurate in regions with higher humidities. The Blaney-Cridle equation is a method for estimating reference crop Evapotranspiration: ETo = p ·(0 The United States of America —commonly referred to as the Other solutions used includes Makkink, which is simple but must be calibrated to a specific location, and Hargreaves. To convert the reference evapotranspiration to actual crop evapotranspiration, a crop coefficient and a stress coeficient must be used.

Energy balance

A third methodology to estimate the actual evapotranspiration is the use of the energy balance.

where λE is the energy needed to change the phase of water from liquid to gas, R_n is the net radiation, G is the soil heat flux and H is the sensible heat flux. Using instruments like a scintillometer, soil heat flux plates or radiation meters, the components of the energy balance can be calculated and the energy available for actual evapotranspiration can be solved. A Scintillometer is a Scientific device used to measure small fluctuations of the Refractive index of Air caused by variations in temperature humidity

Eddy covariance

Main article: Eddy covariance

The most direct method of measuring evapotranspiration is with the eddy covariance technique in which fast fluctuations of vertical wind speed are correlated with fast fluctuations in atmospheric water vapor density. The eddy covariance ( eddy correlation, eddy flux) technique is a prime atmospheric flux measurement technique to measure and calculate vertical turbulent fluxes This directly estimates the transfer of water vapor (evapotranspiration) from the land (or canopy) surface to the atmosphere.

Potential evapotranspiration

Monthly estimated potential evapotranspiration and measured pan evaporation for two locations in Hawaii, Hilo and Pahala. The State of Hawaii ( or həˈwaɪʔiː Hawaiian: Mokuāina o Hawaii) is a state in the United States located on an Archipelago in the

Potential evapotranspiration (PET) is the amount of water that could be evaporated and transpired if there was sufficient water available. Water is a common Chemical substance that is essential for the survival of all known forms of Life. This demand incorporates the energy available for evaporation and the ability of the lower atmosphere to transport evaporated moisture away from the land surface. PET is higher in the summer, on less cloudy days, and closer to the equator, because of the higher levels of solar radiation that provides the energy for evaporation. PET is also higher on windy days because the evaporated moisture can be quickly moved from the ground of plants, allowing more evaporation to fill its place.

PET is expressed in terms of a depth of water, and can be graphed during the year (see figure). There is usually a pronounced peak in summer, which results from higher temperatures. Summer is one of the four Temperate Seasons Summer marks the warmest time of year with the longest days

Potential evapotranspiration is usually measured indirectly, from other climatic factors, but also depends on the surface type, such free water (for lakes and oceans), the soil type for bare soil, and the vegetation. A lake (from Latin lacus) is a Terrain feature (or Physical feature) a body of Liquid on the surface of a world that is localized to the An ocean (from Greek, ''Okeanos'' (Oceanus) is a major body of saline water, and a principal component of the Hydrosphere. Soil, often typeset as SOiL, is a four piece rock band from Chicago Illinois United States founded by Shaun Glass Tom Schofield Tim King and Adam Zadel Vegetation is a general term for the plant life of a region it refers to the Ground cover provided by plants Often a value for the potential evapotranspiration is calculated at a nearby climate station on a reference surface, conventionally short grass. This value is called the reference evapotranspiration, and can be converted to a potential evapotranspiration by multiplying with a surface coefficient. In agriculture, this is called a crop coefficient. The difference between potential evapotranspiration and precipitation is used in irrigation scheduling. Irrigation scheduling is the process used by Irrigation system managers to determine the correct frequency and duration of watering

Average annual PET is often compared to average annual precipitation, P. The ratio of the two, P/PET, is the aridity index. An aridity index (AI is a numerical indicator of the degree of dryness of the climate at a given location

References

1. ^ http://www.oslpr.org/download/en/2000/0031.pdf
2. ^ Allen, R. G. ; Pereira, L. S. ; Raes, D. ; Smith, M. (1998). Crop Evapotranspiration—Guidelines for Computing Crop Water Requirements, FAO Irrigation and drainage paper 56. Rome, Italy: Food and Agriculture Organization of the United Nations. ISBN 92-5-104219-5. Retrieved on 2007-10-08. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 314 - Roman Emperor Licinius is defeated by his colleague Constantine I at the Battle of Cibalae, and loses  

See also

• Hydrologic Evaluation of Landfill Performance (HELP)
• Soil plant atmosphere continuum

External links

• New Mexico Eddy Covariance Flux Network (Rio-ET)
• California's Irrigation Management Information System (CIMIS)
• Texas Evapotranspiration Network
• Use and Construction of a Lysimeter to Measure Evapotranspiration
• Evapotranspiration, from the U.S. Geological Survey's Water Cycle Web site
• Washoe County (NV) Et Project
• Irrigation and Training Research Center at Cal Poly San Luis Obispo
• Transpiration Benefits For Urban Catchment Management