Evaporation, Transpiration,Sublimation

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Evaporation, Transpiration,Sublimation

• Processes by which water changes phase-
• Liquid or solid to gas vapor

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• Learning Objectives Evapotranspiration (ET)
• Learn what conditions are necessary for
  evaporation to occur
• Learn what factors control evaporation rates
• Learn how to measure ET
• Learn where to find or how to compute variables
  needed to
• estimate ET
• Understand the difference between
• potential evapotranspiration (PET) and
• actual evapotranspiration (AET)
• Understand the difference between evaporation and
• transpiration
• Learn what factors control transpiration

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Evaporation

• Phase change liquid to gas
• Hydrogen bonds broken vapor diffuses from
  higher to lower vapor pressure
• At an open water surface, net evaporation 0-
  bonds constantly forming and breaking
• Most takes place over open water surfaces such as
  lakes and oceans

weather.cod.edu/karl/Unit2_Lecture1.ppt
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What controls evaporation?

• Energy inputs
• Temperature
• Humidity
• Wind
• Water availability

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What controls evaporation?

• Evaporation is energy intensive- latent heat of
  vaporization is 540 cal/gram
• Provided mainly by
• Solar energy - radiation
• Sensible heat temperature transferred via
  conduction and convection
• kinetic energy of water internal energy, heat

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Energy Budget

• L is latent heat of vaporization, E evaporation,
  H energy flux that heats the air or sensible
  heat, G is heat of conduction to ground and Ps is
  energy of photosynthesis.
• LE represents energy available for evaporating
  water
• Rnet is the primary source for ET snow melt.

• Net radiation Rnet is determined by measuring
  incoming outgoing short- long-wave rad. over
  a surface.
• Rnet can or
• If Rnet gt 0 then can be allocated at a surface as
  follows
• Rnet (L)(E) H G Ps

http//www.ctahr.hawaii.edu/faresa/courses/nrem600
/10-0220Lecture.ppt
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• Advection is movement of warm air to cooler
  plant-soil-water surfaces.
• Convection is the vertical component of
  sensible-heat transfer.

• In a watershed Rnet, (LE) latent heat and
  sensible heat (H) are of interest.
• Sensible heat can be substantial in a watershed,
  Oasis effect where a well-watered plant community
  can receive large amounts of sensible heat from
  the surrounding dry, hot desert.

http//www.ctahr.hawaii.edu/faresa/courses/nrem600
/10-0220Lecture.ppt
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What controls evaporation?

• Energy inputs
• Temperature
• Vapor content
• Wind
• Water availability

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Temperature

• Measure of heat energy
• Affects vapor pressure- Saturation vapor pressure
  increases with air temperature
• Can compute with an equation if know temperature
• Saturation vapor pressure minus actual vapor
  pressure saturation deficit
• The amount of additional water vapor that air can
  hold at a given temperature

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What controls evaporation?

• Energy inputs
• Temperature
• Vapor content
• Wind
• Water availability

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Measuring the Vapor Content

• There are a number of ways that we can measure
  and express the amount of water vapor content in
  the atmosphere
• Vapor Pressure
• Mixing Ratio
• Relative Humidity
• Dew Point
• Precipitable Water Vapor
• Others (absolute humidity, specific humidity)

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Humidity can be describe in many ways, for
example, Measure symbol units Volumetric
concentration cwv mol m-3 Vapor pressure
ea, also pH2O kPa (the partial pressure
of H2O vapor) Relative humidity RH
(ea/es) 100, where es is saturation vapor
pressure Vapor pressure deficit VPD kPa es
ea
www.fsl.orst.edu/bond/fs561/lectures/humidity20a
nd20transpiration.ppt
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Vapor Pressure (e)

• Vapor pressure (e) is simply the amount of
  pressure exerted only by the water vapor in the
  air
• The pressures exerted by all the other gases are
  not considered
• The unit for vapor pressure will be in units of
  pressure (millibars and hectopascals are the same
  value with a different name)

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Relative Humidity (RH)

• The relative humidity (RH) is calculated using
  the actual water vapor content in the air (mixing
  ratio) and the amount of water vapor that could
  be present in the air if it were saturated
  (saturation mixing ratio)
• RH w/ws x 100
• The relative humidity is simply what percentage
  the atmosphere is towards being saturated
• Relative humidity is not a good measure of
  exactly how much water vapor is present (50
  relative humidity at a temperature of 80 degrees
  Fahrenheit will involve more water vapor than 50
  relative humidity at -40 degrees)
• Relative humidity can change even when the amount
  of water vapor has not changed (when the
  temperature changes and the saturation mixing
  ratio changes as a result)

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Dew Point (Td)

• The dew point temperature is the temperature at
  which the air will become saturated if the
  pressure and water vapor content remain the same
• The higher the dew point, the more water vapor
  that is present in the atmosphere
• The temperature is always greater than the dew
  point unless the air is saturated (when the
  temperature and dew point are equal)

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Precipitable Water Vapor (PWV)

• Precipitable water vapor (PWV) is the amount of
  water vapor present in a column above the surface
  of the Earth
• Measured in units of inches or millimeters
• It represents the maximum amount of water that
  could fall down to the surface as precipitation
  if all the water vapor converted into a liquid or
  a solid
• Can be measured easily by weather balloons or
  satellites

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What controls evaporation?

• Energy inputs
• Temperature
• Vapor content
• Wind
• Water Availability

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Wind

• Creates turbulent diffusion and maintains vapor
  pressure gradient
• Turbulence a function of wind velocity and
  surface roughness
• Evaporation can increase substantially with
  turbulence up to some limit that is a function of
  energy, temperature and humidity

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Additional factors affecting evaporation from
free water surface

• Water quality
• More salinity means less evaporation
• Depth of water body
• Deep lakes have more evap in winter
• High heat capacity means lake water warmer that
  air temperature
• Shallow lakes cool fast in fall and freeze
• No evap in winter

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Additional factors affecting evaporation from
free water surface

• Area of water body
• More evap from larger surface area but rate
  decreases upwind as air picks up vapor
• Maximum rates from small, shallow lakes in dry
  climates

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Evaporation from soil

• Same factors drive the process as in open water
• 1. Soil moisture also important
• Evap rates decrease as surface dries
• 2. Soil texture affects soil moisture content
  and capillary forces
• E.g., Fine soil- retains moisture, rates high at
  first but then depends on capillary forces

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Evaporation from soil

• Soil color affects albedo and thus energy
  inputs
• Depth to water table
• If shallow such as wetlands, almost unlimited
  evaporation
• Vegetation
• - provides shade- limits insolation (energy and
  heat)
• - reduces windspeed at ground level
• - increase vapor pressure through
  transpiration

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How do we measure/estimate evaporation?

• Direct measurement
• Pans
• Lake water balance
• Lysimeters

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Pan evaporation

• Class A pan 4 feet diameter, 10 inches deep-
  galvanized steel measure daily water loss by
  adding water to same level
• Evap change in water level - precipitation
• Pan evap gt lake evap why?
• Use a pan coefficient (usually 0.6-0.8)
• Map of pan evap

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http//fr.cfans.umn.edu/courses/FR3114/FieldMeas2
0-20Transpir_10_03_06.pdf
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http//fr.cfans.umn.edu/courses/FR3114/FieldMeas2
0-20Transpir_10_03_06.pdf
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http//fr.cfans.umn.edu/courses/FR3114/FieldMeas2
0-20Transpir_10_03_06.pdf
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Soil lysimeter

• Water tight box on a scale or pressure transducer
• If only soil and water, loss of weight is due to
  evaporation of water
• Evap change in weight precipitation
• Either prevent seepage or collect and measure

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Transpiration

• Evaporation from plants
• Water vapor escapes when stomata open for
  photosynthesis, need carbon dioxide
• Related to density and size of vegetation, soil
  moisture, depth to water, soil structure
• Of the water taken up by plants, 95 is returned
  to the atmosphere through their stomata (only 5
  is turned into biomass!)

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Water Availability

• An open water surface provides a continuous water
  source
• Transpiration can provide water up until a
  certain limit based upon the plants ability to
  pull water up through its roots and out its
  stomatae (rate of transpiration)

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Water movement in plants

• Illustration of the energy differentials which
  drive the water movement from the soil, into the
  roots, up the stalk, into the leaves and out into
  the atmosphere. The water moves from a less
  negative soil moisture tension to a more negative
  tension in the atmosphere.

http//www.ctahr.hawaii.edu/faresa/courses/nrem600
/10-0220Lecture.ppt
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The driving force of transpiration is the vapor
pressure gradient. This is the difference in
vapor pressure between the internal spaces in the
leaf and the atmosphere around the leaf
www.fsl.orst.edu/bond/fs561/lectures/humidity20a
nd20transpiration.ppt
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Stomatal conductance balances the atmospheric
demand for evaporation with the hydraulic
capacity to supply water
DEMAND VPD
Transpiration
?VPD LAI leaf conductance
SUPPLY Flow of liquid water (Yleaf Ysoil)
K
www.fsl.orst.edu/bond/fs561/lectures/humidity20a
nd20transpiration.ppt
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Leaf Conductance

• Ease of water loss affected by leaf conductance
• Conductance a function of
• light,
• carbon dioxide concentration,
• vapor pressure deficit,
• leaf temperature and
• leaf water content

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Effects of Vegetative Cover
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Evapotranspiration - ET

• Hard to separate evaporative loss from
  transpiration loss in wildland situations
• Look at ET (evapotranspiration)
• AET Actual ET
• PET Potential ET

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PET Potential Evapotranspiration

• Rate at which ET would occur in a situation of
  unlimited water supply, uniform vegetation cover,
  no wind or heat storage effects
• First used for climate classification criteria
• Usually assume short grass as the uniform
  vegetation
• Compute as function of climate factors

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Actual Evapotranspiration

• Amount actually lost from the surface given the
  prevailing atmospheric and ground conditions
• Provides information of soil moisture conditions
  and the local water balance
• Measured by a lysimeter (difficult to maintain,
  not many in existence) that weighs the grass,
  soil, and water above

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PET equations

• Penman- Monteith (based on radiation balance)
• Jensen-Haise (developed for dry, intermountain
  west)
• Priestly-Taylor (based on radiation balance)
• Thornthwaite (based on temperature)
• Hamon, Malstrom (based on T and saturated vapor
  pressure)
• See table 4.3 p 95 in text

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Physically-based theoretical methods- e.g. Penman
Monteith

• Energy budget
• Mass balance on energy inputs and outputs
• Incoming solar radiation reflected solar
  radiation (albedo) net longwave radiation net
  energy advected to vegetation ET energy (latent
  heat) sensible heat transfer from veg to air
  changes in energy storage in heating soil and veg
• Can measure all but latent heat which equals ET

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Physically-based methods

• Turbulent mass transfer
• Function of wind speed and vapor pressure deficit
• Evap k uz ( ew ez)
• K is a constant, U is wind velocity, e is vapor
  pressure, z is some reference height, w is level
  at water surface
• Can only measure precisely over short distances
• Useful only for experimental situations

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AET equations

• Blainey-Criddle
• Good for crops and ag situations
• f tp/100
• f is consumptive factor, t is mean monthly air
  temperature in Fahrenheit (tmax tmin/2)
• p is mean monthly percentage of annual daytime
  hours
• Compute f for each month of interest
• U K S fi
• Where U is total consumptive use in inches per
  season
• K is crop coefficient, sum over the number of
  months of growth

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Variables used in common ET models

• Model T RH or e Lat Elev Rad.
  Wind
• Penman x x
  x x x
• Priestly-Taylor x x
• Jensen-Haise x
  x x
• Blainey-Criddle x x
• Thornthwaite x

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fine soils with ample soil-moisture storage,
warm summers, cool winters, and little change in
precipitation throughout the year
coarse soils with limited soil-moisture storage,
warm, dry summers, and cool, moist winters.
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Available Soil Water
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Evapotranspiration

• gt 70 annual precipitation in the US
• In General ET/P is
• 1 for dry conditions
• ET/P lt 1 for humid climates ET is governed by
  available energy rather than availability of water

• ET affects water yield by affecting antecedent
  water status of a watershed ? high ET result in
  large storage to store part of precipitation

http//www.ctahr.hawaii.edu/faresa/courses/nrem600
/10-0220Lecture.ppt