Evaporation

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Evaporation

• Magnus Persson

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Today's topics

• What is evaporation
• Definitions
• Importance for hydrology
• Energy balance
• Penman-Monteith
• Crop coefficient
• Measurements

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Definitions
Evaporation molecules in a liquid state (water)
become gaseous (water vapor). (phase
transition) Transpiration is evaporation from
aerial parts of plants Potential
evapotranspiration (ETp) 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 . Actual evapotranspiration
is the evaporation from a crop or bare soil
surface when the water supply is a limiting
factor.
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The Hydrological cycle
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Aridity index
where ETp is the potential evapotranspiration and
P is the average annual precipitation (UNEP,
1992).
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Interception
Precipitation caught and held by foliage, twigs,
and branches of vegetation. Intercepted
precipitation is lost by evaporation and never
reaches the soil surface. The amount of
interception is depending on the vegetation and
can significantly reduce the precipitation
reaching the soil surface (up to 60 of the
annual precipitation). Some values of
interception storage, up to 6 mm conifers, 2 mm
decidious, 1 mm grass, 8 mm rainforest.
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Interception
Stemflow - precipitation flows down plant
branches and stems (red). Canopy drip - some
plants have an architecture that directs rainfall
or snowfall along the edge of the plant canopy
(purple). Throughfall - the precipitation
passing through the plant canopy (yellow).
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Vapor pressure

• Actual vapor pressure e
• Saturation vapor pressure esat
• Relative humidity e/esat
• Saturation deficit esat - e

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Saturation vapor pressure
e
where t 1 - (373.16/T)
T absolute temperature (K)
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Models

• Mass transfer (aerodynamic models)
• Energy budget
• Combined methods
• Water budget

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Meteorological factors affecting ET

• Solar radiation
• Air temperature
• Air humidity
• Wind speed

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Mass transfer models
Mass transfer models can be written in the
general form E f(u1)(es e2) where f(u1) is
the wind function e is the vapor pressure The
subscripts 1 and 2 refer to measurement levels
above the ground, s refers to the ground surface
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Energy balance

• Continuity
• Energy input - Energy output Energy storage

Energy transport Sensible heat is potential
energy in the form of thermal energy or heat. The
thermal energy can be transported via conduction,
convection, radiation or by a combination
thereof. The heat that causes a change in
temperature is called sensible heat. Latent heat
is the amount of energy in the form of heat
released or absorbed by a substance during a
change of phase state (For water latent heat of
vaporization is 2500 J/g). Heat that causes a
change of state with no change in temperature is
called latent heat.
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Energy balance
Rn LE H G S where Rn net radiation LE
latent heat flux, E evaporative flux, L latent
heat of vaporization H sensible heat flux G
soil heat flux S Heat storage (can often be
neglected)
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Net radiation
Rn Rs(1-a) ?sRld Rlu where Rs is the solar
shortwave radiation a is the albedo of the
surface ?s is the emissivity of the surface Rld
is the downward long wave radiation Rlu is the
upward long wave radiation
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Net radiation
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Soil heat flux
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Albedo and emissivity for different surfaces
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long wave radiation
Rld ?asTa4 where ?a is the atmospheric
emissivity (which is a function of temperature,
vapor pressure and cloudiness) s is
Stefan-Boltzmann constant Ta is the air
temperature
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long wave radiation
Rlu ?ssTa4 where ?s is the surface
emissivity s is Stefan-Boltzmann constant Ts is
the surface temperature
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Bowen ratio
The bowen ratio is the ration between sensible
and latent heat flux
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Penman equation
where ? is slope of the saturation
vapor-pressure curve ? is the psychometric
constant EA is the drying power of the air
(normally some kind of wind function is used)
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Stomatal resistance
Stomata are microscopic pores found on the
underside of a plant leaf and are used for gas
exchange. The stomata is bounded by two half moon
shaped guard cells that function to vary the
width of the pore.
The surface resistance parameter rs describes
the resistance to evaporation, it can be applied
both to crops and bare soil
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Leaf area index
The Leaf Area Index (LAI) is the leaf area (upper
side only) per unit area of soil below it
(m2/m2). A value of 3 5 is common for most
crops. The active LAI is the index of the leaf
area that actively contributes to the surface
heat and vapor transfer. It is generally the
upper, sunlit portion of the canopy. Generally
LAI 0.5LAIactive
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PenmanMonteith
where ? is slope of the saturation
vapor-pressure curve ?a is mean air density at
constant pressure cp is specific heat of air
esat is saturation vapor-pressure of the air e
is actual vapor-pressure of the air ? is the
psychometric constant rs is bulk surface
resistance ra is aerodynamic resistance
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PenmanMonteith
The aerodynamic resistance can be calculated from
the wind speed and the heights of the wind and
humidity measurements The bulk surface resistance
can be calculated from LAI through rs
rl/LAIactive where rl is the bulk stomatal
resistance (crop dependent)
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FAO reference surface
In order to standardize evapotranspiration
calculations, FAO has defined a reference crop "A
hypothetical reference crop with an assumed crop
height of 0.12 m, a fixed surface resistance of
70 s m-1 and an albedo of 0.23. " The reference
surface closely resembles an extensive surface of
green grass of uniform height, actively growing,
completely shading the ground and with adequate
water. Evapotranspiration from this reference
surface is called ETo
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Crop coefficient
The evapotranspiration from a crop (ETc) can be
calculated by ETc Kc ETo
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Crop coefficient
Calculation of evapotranspiration of a given crop
according to FAO
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Crop coefficients for different crops
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Bowen ratio measurements
The bowen ratio is estimated using measurements
of temperature T and vapor pressure e at two
heights 1 and 2
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Bowen ratio measurements
The bowen ratio is then used in the energy
balance to calculate the LE LE (Rn G)/(1
Bo) Using measurements of Bo, Rn, and G
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Eddy covariance
Latent and sensible heat fluxes is calculated by
(1)                                      
where is the covariance between fluctuations of
vertical wind speed w and humidity q'
(kg kg-1) is the covariance between
fluctuations of w and temperature T', ?a the
air density, cp the specific heat of dry air ?
the latent heat of water vaporization
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Eddy covariance
A system typically measures tree-dimensional wind
speed (anemometer), temperature, and relative
humidity. Combined with net radiation, soil heat
flux, soil temperature and soil water content
measurements, the evaporation can be calculated.
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Lysimeters
Measures actual evapotrnspiration using water
bugdet calculation ET P Drain dW/dt
where P is the precipition Drain is the
drainage dW/dt is the change in stored water over
time
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Evaporation Pan
Class A evaporation pan 47.5 in (120.7 cm) in
diameter and 10 in (25 cm) deep. Measures Pan
evaporation Ep 0.75 Epan
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Questions to discuss

• Methods to measure evapotranspiration
• Why is evaporation important for water resources
  or hydrology?
• Difference between evaporation and
  evapotranspiration
• Difference between potential and actual
  evaporation
• How can we roughly guess average evaporation in a
  basin with few stations?
• What problems are caused by evaporation?
• What is the best method for evaporation
  measurements?
• Which are the most important factors determining
  the evaporation rate?
• How can evaporation be diminished in order to
  preserve water?
• Can empirical equations exactly describe the real
  evaporation?
• How do you estimate evaporation during winter in
  Sweden?
• How does evaporation affect snow melt rate?

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Literature

• Brutsaert, W. 2005. Hydrology. An introduction.
  Cambridge University Press, UK.
• Sumner, D.M., and Jacobs, J.M. 2005. Utility of
  PenmanMonteith, PriestleyTaylor, reference
  evapotranspiration, and pan evaporation methods
  to estimate pasture evapotranspiration. Journal
  of Hydrology 308 81104.
• http//www.campbellsci.com/index.cfm (measurement
  systems)
• http//www.fao.org/docrep/X0490E/x0490e00.htmCont
  ents