Evapotranspiration (ET) is a term used to describe the sum of evaporation and plant transpiration from the
Earth's land surface to atmosphere. Evaporation accounts for the movement of water to the air from sources
such as the soil, canopy interception, and waterbodies. Transpiration accounts for the movement of water within
a plant and the subsequent loss of water as vapor through stomata in its leaves. Evapotranspiration is an
important part of the water cycle. 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.
Evapotranspiration is said to equal potential evapotranspiration when there is ample water.
Evapotranspiration and the water cycle
Evapotranspiration is a significant water loss from drainage basins. Types of vegetation and land use
significantly affect evapotranspiration, and therefore the amount of water leaving a drainage basin. Because
water transpired through leaves comes from the roots, plants with deep reaching roots can more constantly
transpire water. Herbaceous plants generally transpire less than woody plants because they usually have less
extensive foliage. Conifer forests tend to have higher rates of evapotranspiration than deciduous forests,
particularly in the dormant and early spring seasons. This is primarily due to the enhanced amount of
precipitation intercepted and evaporated by conifer foliage during these periods [2]. Factors that affect
evapotranspiration include the plant's growth stage or level of maturity, percentage of soil cover, solar radiation,
humidity, temperature, and wind.
Through evapotranspiration, forests reduce water yield, except in unique ecosystems called cloud forests. 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. It will usually be less because some water will be
lost due to percolation or surface runoff. 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. If potential
evapotranspiration is greater than actual precipitation, then soil will dry out, unless irrigation is used.
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 transpire readily.
Estimating evapotranspiration
Evapotranspiration can 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. 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 drainage basin. 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). 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-
Monteith variation is recommended by the Food and Agriculture Organization [3]. 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. 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 coefficient must be used. Crop coefficients referred to in many hydrological models are
themselves the result of equations that describe predictable variation in coefficient values depending upon plant
conditions that change during periods for which the model is used. This is because crops are seasonal, perennial
plants mature over multiple seasons, and stress responses can significantly depend upon many aspects of plant
condition.
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.
The SEBAL algorithm solves the energy balance at the earth surface using satellite imagery. This allows for
both actual and potential evapotranspiration to be calculated on a pixel-by-pixel basis. Evapotranspiration is a
key indicator for water management and irrigation performance. SEBAL can map these key indicators in time
and space, for days, weeks or years [4].
Direct method
The only direct method for measuring ET is through a lysimeter.
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. 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.
Potential evapotranspiration (PET) is the amount of water that could be evaporated and transpired if there were
sufficient water available. 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.
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. 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.
Average annual PET is often compared to average annual precipitation, P. The ratio of the two, P/PET, is the
aridity index.
Reference: Wikipedia
Introduction
Evapotranspiration (ET) is a term describing the transport of water into the atmosphere from surfaces, including
soil (soil evaporation), and from vegetation (transpiration). The latter two are often the most important
contributors to evapotranspiration. Other contributors to evapotranspiration may include evaporation from wet
canopy surface (wet-canopy evaporation), and evaporation from vegetation-covered water surface in wetlands.
The process of evapotranspiration is one of the main consumers of solar energy at the Earth's surface. Energy
used for evapotranspiration is generally referred to as latent heat flux; however, the term latent heat flux is
broad, and includes other related processes unrelated to transpiration including condensation (e.g., fog, dew),
and snow and ice sublimation. Apart from precipitation, evapotranspiration is one of the most significant
components of the water cycle.
The evaporation component of ET is comprised of the return of water back to the atmosphere through direct
evaporative loss from the soil surface, standing water (depression storage), and water on surfaces (intercepted
water) such as leaves and/or roofs. Transpired water is that which is used by vegetation and subsequently lost to
the atmosphere as vapor. The water generally enters the plant through the root zone, is used for various
biophysiological functions including photosynthesis, and then passes back to the atmosphere through the leaf
stomates. Transpiration will stop if the vegetation becomes stressed to the wilting point, which is the point in
which there is insufficient water left in the soil for a plant to transpire, or if the plant to atmosphere vapor
concentration gradient becomes prohibitive to plant physiological processes (e.g. photosynthesis).
Factors affecting evapotranspiration
The rate of evapotranspiration at any location on the Earth's surface is controlled by several factors:
Energy availability. The more energy available, the greater the rate of Evapotranspiration. It takes about
600 calories of heat energy to change 1 gram of liquid water into a gas.
The humidity gradient away from the surface. The rate and quantity of water vapor entering into the
atmosphere both become higher in drier air.
The wind speed immediately above the surface. The process of evapotranspiration moves water vapor
from ground or water surfaces to an adjacent shallow layer that is only a few centimeters thick. When
this layer becomes saturated evapotranspiration stops. However, wind can remove this layer replacing it
with drier air which increases the potential for Evapotranspiration. Winds also affect evapotranspiration
by bringing heat energy into an area. A 5-mile-per-hour wind will increase still-air evapotranspiration by
20 percent; a 15-mile-per-hour wind will increase still-air evapotranspiration by 50 percent
Water availability. Evapotranspiration cannot occur if water is not available.
Physical attributes of the vegetation. Such factors as vegetative cover,plant height, leaf area index and
leaf shape and the reflectivity of plant surfaces can affect rates of evapotranspiration. For example
coniferous forests and alfalfa fields reflect only about 25 percent of solar energy, thus retaining
substantial thermal energy to promote transpiration; in contrast, deserts reflect as much as 50 percent of
the solar energy, depending on the density of vegetation.
[Stomatal resistance]]. Plants regulate transpiration through adjustment of small openings in the leaves
called stomata. As stomata close, the resistance of the leaf to loss of water vapor increases, decreasing to
the diffusion of water vapor from plant to the atmosphere.
Soil characteristics. Soil characteristics that can affect evapotranspiration include its heat capacity, and
soil chemistry and albedo.
Seasonal trends of evapotranspiration within a given climatic region follow the seasonal declination of solar
radiation and the resulting air temperatures. Minimum evapotranspiration rates generally occur during the
coldest months of the year. Maximum rates generally coincide with the summer season. However since
evapotranspiration depends on both solar energy and the availability of soil moisture and plant maturity the
seasonal maximum evapotranspiration actually may precede or follow the seasonal maximum solar radiation
and air temperature by several weeks.
Geographical patterns of evapotranspiration
Assuming that moisture is available, evapotranspiration is dependent primarily on the availability of solar
energy to vaporize water. Evapotranspiration therefore varies with latitude, season of year, time of day, and
cloud cover. Most of the evapotranspiration of water on the Earth's surface occurs in the subtropical oceans
(Figure 1). In these areas, high quantities of solar radiation provide the energy required to convert liquid water
into a gas. Evapotranspiration generally exceeds precipitation on middle and high latitude landmass areas
during the summer season. Once again, the greater availability of solar radiation during this time enhances the
evapotranspiration process.
Figure 1. Mean Annual Potential Evapotranspiration. Source: UNEP World Atlas of Desertification
Estimates of average nationwide evapotranspiration for the conterminous United States range from about 40
percent of the average annual precipitation in the Northwest and Northeast to close to 100 percent in the
Southwest. During a drought, the significance of evapotranspiration is magnified, because evapotranspiration
continues to deplete the limited remaining water supplies in lakes and streams and the soil.
The lower 5 miles of the atmosphere transports an average of about 40,000 billion gallons of water vapor over
the conterminous United States each day. Slightly more than 10 percent of this moisture, however, is
precipitated as rain, sleet, hail, or snow. The greatest proportion, about 67 percent, is returned to the atmosphere
through evapotranspiration. About 29 percent is discharged from the conterminous United States as surface-
water flowing into the Pacific and Atlantic Oceans and across the borders into Canada and Mexico, about 2
percent is discharged as groundwater outflow, and about 2 percent is consumed by people, animals, plants, and
used for industrial and commercial processes. For most of the United States, evaporation returns less moisture
to the atmosphere than does transpiration. Globally, evaporation processes are resposible for an overwhelming
majority of the water returned to the atmosphere.
Related terms
Actual evapotranspiration (AE or AET) is the quantity of water that is actually removed from a surface due to
the processes of evaporation and transpiration.
Potential evapotranspiration or PE is a measure of the ability of the atmosphere to remove water from the
surface through the processes of evaporation and transpiration assuming no control on water supply. Since PET
assumes that water availability is unlimited, vegetation would never reach the wilting point (the point in which
there is not enough water left in the soil for a plant to transpire). Therefore, the only limit to the transpiration
rate of the plant is due to the physiology of the plant and not due to any atmospheric or soil moisture
restrictions. Therefore, PET is considered the maximum ET rate possible with a given set of meteorological and
physical parameters. On this basis, any irrigation that supplies more water than PET can accommodate could be
viewed as wasted water.
Evapotranspiration (ET) is also sometimes discussed in terms of consumptive water use.
The overall resistance to water vapor transport due to soil, plant and surface factors is often termed bulk surface
resistance.
Reference: http://www.eoearth.org/article/Evapotranspiration
What is Evapotranspiration?
The term evapotranspiration combines two words: evaporation of water from the soil, and transpiration of
water from plants into the air. Evapotranspiration means the total loss of water from a crop into the air. Water
evaporates from any moist surface into the air unless the air is saturated. Water surfaces in contact with air, such
as lakes, plant leaves, and moist soils, all evaporate water.
Plant leaves lose water though small openings called stomata that are found on the leaf surface. The water
moves from the moist soil into the plant roots, up through the plant, and leaves through the stomata.
Evapotranspiration is loosely called crop water use.
Crop water is important because it determines how much water must be provided by irrigation or rain. If there is
too little water, the crop yield can diminish. If there is too much irrigation, then it will waste energy, water, and
nutrients and unnecessarily deplete the aquifer.
Water has three functions in plants. It cools and hydrates them, and is also essential for the transport of
nutrients. Less than 1% of water remains in the plant tissue. Considering water makes up 90% of the weight of
most crops, this may seem surprising, but plants use the water for other purposes.
Weather is a major factor in evapotranspiration. The surface temperature of plants and soil is almost that of the
air temperature. Brighter sunlight means that plants need to evaporate more water through evapotranspiration to
keep their temperature near normal. If the air is dry and hot, with strong winds, then the crops will lose water at
a faster rate. More water will evaporate from plants if the air is a higher temperature, if there is more solar
energy and lower humidity, and if there is a faster windspeed.
Evapotranspiration is estimated by the use of many formulas. There are computer software programs available
to help people estimate evapotranspiration. Radio, newspapers, and network services often give out reports on
potential evapotranspiration figures. There is also crop referencing, which compares the evapotranspiration rate
of a reference crop of plants to the same types of crops grown by other individuals.
What Is a Lysimeter?
A lysimeter is a device used to measure evapotranspiration, the rate at which plants and soil release moisture
into the atmosphere. Strictly speaking, a lysimeter only collects or removes water or soil samples, rather than
performing any calculations. There are a range of designs used in lysimeters, though there is some debate about
exactly which designs should be given the name.
Evapotranspiration is part of the water cycle and is a combination of evaporation and plant transpiration.
Evaporation is the same process as the way sweat disappears from our skin on a hot day. It refers specifically to
liquid which evaporates from the soil, plus rainfall which lands on leaves and then evaporates before falling to
the ground. Transpiration refers to moisture which is absorbed by a plant, usually through its roots, and then
released into the air, mainly through leaves but also through branches or stalks.
One way of measuring evapotranspiration is through a pan lysimeter. This is simply a bucket-style container
which is placed in the soil with its rim level with the ground surface. Ideally the container will be filled with the
soil which was displaced as part of its installation. Knowing how much soil is in the container, a researcher can
measure the water content of the soil, compare it with the rainfall levels, and calculate the rate of
evapotranspiration.
A modern version of the lysimeter combines a porous ceramic cup and a tube to which allows samples to be
collected from beneath the surface. This version of the lysimeter uses a vacuum to create the suction needed to
withdraw the water. The pores on the cup will be small enough that only the water is removed, rather than any
soil itself.
One interpretation of the word lysimeter, which is common in Europe, only refers to models which hold soil
flush to the ground surface. Somebody using this strict definition would therefore not consider a suction cap-
based model to be a lysimeter. This means there may be some circumstances when users need to clarify exactly
what they mean by the term.
Reference: http://www.wisegeek.com/what-is-evapotranspiration.htm
Potential evaporation or potential evapotranspiration (PET) is defined as the amount of evaporation that
would occur if a sufficient water source were available. If the actual evapotranspiration is considered the net
result of atmospheric demand for moisture from a surface and the ability of the surface to supply moisture, then
PET is a measure of the demand side. Surface and air temperatures, insolation, and wind all affect this. A
dryland is a place where annual potential evaporation exceeds annual precipitation.
Estimates of potential evaporation (mm)
Thornthwaite equation (1948)
Where,
Ta is the average daily temperature of the month being calculated.
N is the number of days in the month being calculated
L is the average day length of the month being calculated
Reference: http://en.wikipedia.org/wiki/Potential_evaporation
Penman equation
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The Penman equation describes evaporation (E) from an open water surface, and was developed by Howard
Penman in 1948. Penman's equation requires daily mean temperature, wind speed, relative humidity, and solar
radiation to predict E. Simpler Hydrometeorological equations continue to be used where obtaining such data is
impractical, to give comparable results within specific contexts, e.g. humid vs arid climates.
Details
Numerous variations of the Penman equation are used to estimate evaporation from water, and land.
Specifically the Penman-Monteith equation refines weather based potential evapotranspiration (PET) estimates
of vegetated land areas.[1] It is widely regarded as one of the most accurate models, in terms of estimates.[citation
needed]
The original equation was developed by Howard Penman at the Rothamsted Experimental Station, Harpenden,
UK.
The equation for evaporation given by Penman is:
where:
m = Slope of the saturation vapor pressure curve (Pa K-1)
Rn = Net irradiance (W m-2)
ρa = density of air (kg m-3)
cp = heat capacity of air (J kg-1 K-1)
ga = momentum surface aerodynamic conductance (m s -1)
δe = vapor pressure deficit (Pa)
λv = latent heat of vaporization (J kg-1)
γ = psychrometric constant (Pa K-1)
which (if the SI units in parentheses are used) will give the evaporation Emass in units of kg/(m²·s), kilograms of
water evaporated every second for each square meter of area.
Remove λ to obviate that this is fundamentally an energy balance. Replace λv with L to get familiar precipitation
units ETvol, where Lv=λvρwater. This has units of m/s, or more commonly mm/day, because it is flux m3/s per
m2=m/s.
This equation assumes a daily time step so that net heat exchange with the ground is insignificant, and a unit
area surrounded by similar open water or vegetation so that net heat & vapor exchange with the surrounding
area cancels out. Some times people replace Rn with and A for total net available energy when a situation
warrants account of additional heat fluxes.
temperature, wind speed, relative humidity impact the values of m, g, cp, ρ, and δe.
Shuttleworth (1993)
In 1993, W.Jim Shuttleworth modified and adapted the Penman equation to use SI, which made calculating
evaporation simpler[2]. The resultant equation is:
where:
Emass = Evaporation rate (mm day-1)
m = Slope of the saturation vapor pressure curve (kPa K-1)
Rn = Net irradiance (MJ m-2 day-1)
γ = psychrometric constant = (kPa K-1)
U2 = wind speed (m s-1)
δe = vapor pressure deficit (kPa)
λv = latent heat of vaporization (MJ kg-1)
Some useful relationships
δe = (es - ea) = (1-relative humidity)es
es = saturated vapor pressure of air, as is found inside plant stoma.
ea = vapor pressure of free flowing air.
es, mmHg = exp(21.07-5336/Ta), approximation by Merva, 1975[3]
Therefore , mmHg/K
Ta = air temperature in kelvins
Reference: http://en.wikipedia.org/wiki/Penman_equation
Penman-Monteith
Like the Penman equation, the Penman-Monteith equation requires daily mean temperature, wind speed,
relative humidity, and solar radiation to predict net evapotranspiration. Other than radiation, these parameter are
implicit in the derivation of Δ, cp, and δq, if not conductances below.
The United Nations Food & Agriculture Organization (FAO) standard methods for modeling
Evapotranspiration (ET) use a Penman-Monteith equation[1]. The ASCE standard methods modify that Penman-
Monteith equation for use with an hourly time step. The SWAT model is one of many GIS integrated
hydrologic models[2] estimating ET using Penman-Monteith equations.
Evapotranspiration contributions are very significant in a watershed's water balance, yet are often not
emphasized in results because the precision of this component is often weak relative to more directly measured
phenomena, e.g. rain and stream flow. In addition to weather uncertainties, the Penman-Monteith equation is
sensitive to vegetation specific parameters, e.g. stomatal resistance or conductance[3]. Gaps in knowledge of
such are filled by educated assumptions, until more specific data accumulates.
Various forms of crop coefficients (Kc) account for differences between specific vegetation modeled and a
Reference Evapotranspiration (RET or ET0) standard. Stress coefficients (Ks) account for reductions in ET due
to environmental stress (e.g. Soil saturation reduces root zone O2, low soil moisture induces wilt, air pollution
effects, and salinity). Models of native vegetation cannot assume crop management to avoid recurring stress.
Equation
λv = Latent heat of vaporization. Energy required per unit mass of water vaporized. (J/g)
Lv = Volumetric latent heat of vaporization. Energy required per water volume vaporized. (Lv = 2453 MJ
m-3)
E = Mass water evapotranspiration rate (g s-1 m-2)
ETo = Water volume evapotranspired (m3 s-1 m-2)
Δ = Rate of change of saturation specific humidity with air temperature. (Pa K-1)
Rn = Net irradiance (W m-2), the external source of energy flux
cp = Specific heat capacity of air (J kg-1 K-1)
ρa = dry air density (kg m-3)
δe = vapor pressure deficit, or specific humidity (Pa)
ga = Conductivity of air, atmospheric conductance (m s-1)
gs = Conductivity of stoma, surface conductance (m s-1)
γ = Psychrometric constant (γ ≈ 66 Pa K-1)
(Monteith, 1965)[4]:
Note: Often resistances are used rather than conductivities.
where rc refers to the resistance to flux from a vegetation canopy to the extent of some defined boundary layer.
Also note that gs varies over each day, and in response to conditions as plants adjust such traits as stoma
openings. Being sensitive to this parameter value, the Penman-Monteith equation obviates the need for more
rigorous treatment of gs perhaps varying within each day. Penman's equation was derived to estimate daily ET
from daily averages.
A derivation of this equation may be found at
http://biomet.ucdavis.edu/Evapotranspiration/PMDerivation/PMD.htm
This also explains relations used to obtain δq & Δ in addition to assumptions key to reaching this simplified
equation.
Reference: http://en.wikipedia.org/wiki/Penman-Monteith
Evapotranspiration (ET) is the sum of evaporation and plant transpiration. Potential evapotranspiration (PET)
is the amount of water that could be evaporated and transpired if there was plenty of water available. Actual
evapotranspiration is usually no greater than precipitation, with some buffer in time depending on the soils
ability to hold water. It will usually be less because some water will be lost due to percolation or surface run off.
An exception is areas with high water tables, where capillary rise can cause water from the groundwater to rise
through the soil matrix to the surface. If potential evapotranspiration is greater than actual precipitation, then
soil will dry out, unless irrigation is used.
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. 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.
The potential evapotranspiration can be graphed during the year. It follows an approximate bell curve around
the seasons. Factors that affect evapotranspiration include the plant's growth stage or level of maturity,
percentage of soil cover, solar radiation, humidity, temperature, and wind.
The most general and widely used equation for calculating ET is the Penman equation. The Penman-Monteith
variation is the FAO recommendation. 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. Other solutions used includes
Makkink, which is simple but must be calibrated to a specific locale, and Hargreaves.
Reference: http://www.wordiq.com/definition/Evapotranspiration
Evapotranspiration: Potential or Reference? 1
Suat Irmak and Dorota Z. Haman2
The objective of this article is to define two commonly used evapotranspiration (ET) concepts: potential
evapotranspiration (ETp) and reference evapotranspiration (ETo); and to provide insight on the differences
between the two terms. A common understanding of these widely used concepts in agricultural communities
will help to make communication easier between farmers/growers, extension agents, and researchers in the
academic environment.
The process known as evapotranspiration (ET) is of great importance in many disciplines, including irrigation
system design, irrigation scheduling, and hydrologic and drainage studies. In a broad definition, the
evapotranspiration is a combined process of both evaporation from soil and plant surfaces and transpiration
through plant canopies. In the evapotranspiration process, the water is transferred from the soil and plant
surfaces into the atmosphere in the form of water vapor. In practice, the estimation of the evapotranspiration
rate for a specific crop requires first calculating potential or reference evapotranspiration and then applying the
proper crop coefficients (Kc) to estimate actual crop evapotranspiration (ETa).
The objective of defining “potential” or “reference” evapotranspiration is to eliminate the crop specific changes
in the evapotranspiration process. In the "potential" evapotranspiration definition this is attemped by assuming
the constant crop conditions. However, in this definition, the reference crop is not very well specified and this
may create a problem in total elimination of crop component. Since "reference" evapotranspiration is based on
hypothetical crop, the process of elimination of crop specific changes is much easier.
Potential evapotranspiration (ETp): The potential evapotranspiration concept was first introduced in the late
1940s and 50s by Penman and it is defined as “the amount of water transpired in a given time by a short green
crop, completely shading the ground, of uniform height and with adequate water status in the soil profile”. Note
that in the definition of potential evapotranspiration, the evapotranspiration rate is not related to a specific crop.
The main confusion with the potential evapotranspiration definition is that there are many types of horticultural
and agronomic crops that fit into the description of short green crop. So, scientists may be confused as to which
crop should be selected to be used as a short green crop because the evapotranspiration rates from well-watered
agricultural crops may be as much as 10 to 30% greater than that occurring from short green grass.
Reference evapotranspiration (ETo): Reference evapotranspiration is defined as "the rate of evapotranspiration
from a hypothetical reference crop with an assumed crop height of 0.12 m (4.72 in), a fixed surface resistance of
70 sec m-1 (70 sec 3.2ft-1) and an albedo of 0.23, closely resembling the evapotranspiration from an extensive
surface of green grass of uniform height, actively growing, well-watered, and completely shading the ground".
In the reference evapotranspiration definition, the grass is specifically defined as the reference crop and this
crop is assumed to be free of water stress and diseases. In the literature, the terms “reference
evapotranspiration” and “reference crop evapotranspiration” have been used interchangeably and they both
represent the same evapotranspiration rate from a short, green grass surface.
The reference evapotranspiration concept was introduced by irrigation engineers and researchers in the late
1970s and early 80s to avoid ambiguities that existed in the definition of potential evapotranspiration. By
adopting a reference crop (grass), it has become easier and more practical to select consistent crop coefficients
and to make reliable actual crop evapotranspiration (ETa) estimates in new areas. Introduction of the reference
evapotranspiration concept also helped to enhance the transferability of the crop coefficients from one location
to another. In addition, with using reference evapotranspiration, it is easier to select consistent crop coefficients
and to calibrate evapotranspiration equations for a given local climate.
Historically two main crops have been used as the reference crop, grass and alfalfa. In Florida, the reference
crop is grass since alfalfa is not commonly grown. It is generally accepted that the grass reference crop is the
type of grass with physiological and structural characteristics similar to perennial ryegrass (Lolium perenne L.)
or alta fescue (Festuca arundinacea Schreb. Alta). Although alfalfa has the physical characteristics (leaf area
index, roughness, etc.) closer to many agronomic crops than the grass, researchers generally agree that a clipped
grass provides a better representation of reference evapotranspiration than does alfalfa. This is mainly because
of the two reasons: (1) the characteristics of the grass are better known and defined, (2) the grass crop has more
planting areas than alfalfa throughout the world and the measured evapotranspiration rates of the grass are more
readily available and accessible as compared to the measured alfalfa evapotranspiration rates.
One of the other important differences between the potential and reference evapotranspiration is that the
weather data collection site is well defined in the reference evapotranspiration definition. It is important to note
in the reference evapotranspiration definition that the climate data that are used to estimate reference
evapotranspiration need to be collected in a well-defined (reference) environment. Therefore, based on the
definition, the weather data for the reference evapotranspiration estimations should be collected in a well-
irrigated and well-maintained grass area. The irrigated grass area of the weather data collection site should be
fairly large [(approximately two hectares) (4.94 acres)] because the quality of the weather data will ultimately
affect the final estimated reference evapotranspiration value. For example, in a hot, dry month the average air
temperature may be as much as 5 to 6 oC (9 to 10.8 oF) higher in a dryland (non-irrigated) than for a well-
irrigated land. The differences in the air temperature will also affect the relative humidity and vapor pressure
deficit values and these differences will ultimately cause differences in the reference evapotranspiration
calculated using the weather data collected from the two sites (dry versus well-irrigated).
The reference evapotranspiration concept has been gaining significant acceptance by the engineers and
scientists throughout the world since its introduction. Specific equations and standardized procedures are being
recommended for reference evapotranspiration estimates. The International Commission for Irrigation and
Drainage (ICID) and the Food and Agriculture Organization of the United Nations (FAO) Expert Consultation
on Revision of FAO Methodologies for Crop Water Requirements recommended that the Food and Agriculture
Organization of the United Nations Paper No. 56 Penman-Monteith equation (FAO56-PM) be used as the
standard method to estimate ETo. This equation has been increasingly gaining acceptance and used throughout
the world for reference evapotranspiration estimations. It is recommended that the grass-reference
evapotranspiration concept be used for irrigation scheduling and water management, hydrologic studies, and
drainage researches in Florida in order to establish a common and standard ground between the growers/farmers
and their advisors and between the researchers in Florida and other states.
References
Penman, H.L. 1948. Natural evaporation from open water, bare soil and grass. Proceedings of the Royal Society
of London, A193: 120-146.
Allen, R.G., Pereiro, L.S., Raes, D. and Smith, M. 1998. Crop evapotranspirtion: Guidelines for computing crop
requirements. Irrigation and Drainage paper No. 56. FAO, Rome.
http://edis.ifas.ufl.edu/ae256
