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Evaporation, Transpiration,Sublimation. Processes by which water changes phase- Liquid or solid to gas vapor. Learning Objectives: Evapotranspiration (ET) Learn what conditions are necessary for evaporation to occur Learn what factors control evaporation rates Learn how to measure ET

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evaporation transpiration sublimation

Evaporation, Transpiration,Sublimation

Processes by which water changes phase-

Liquid or solid to gas vapor


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
  • Become aware of common equations used to estimate ET
  • Understand how ET varies in time and space
  • 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


what controls evaporation
What controls evaporation?
  • Energy inputs
  • Temperature
  • Humidity
  • Wind
  • Water availability
what controls evaporation5
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
  • Energy that is absorbed during phase changes of water is not available to increase the surface temperature.
energy budget
Net radiation: Rnet is determined by measuring incoming & outgoing short- & long-wave rad. over a surface.

Rnet can – or +

If Rnet > 0 then can be allocated at a surface as follows:

Rnet = (L)(E) + H + G + Ps

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.

Energy Budget


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.

Advection is movement of warm air to cooler plant-soil-water surfaces.

Convection is the vertical component of sensible-heat transfer.


what controls evaporation8
What controls evaporation?
  • Energy inputs
  • Temperature
  • Vapor content
  • Wind
  • Water availability
  • 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
what controls evaporation10
What controls evaporation?
  • Energy inputs
  • Temperature
  • Vapor content
  • Wind
  • Water availability
measuring the vapor content
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)

Humidity can be describe in many ways, for example,


Volumetric concentration cwv mol m-3

Vapor pressureea, also pH2OkPa

(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


vapor pressure e
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)
relative humidity rh
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)
dew point t d
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)
precipitable water vapor pwv
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
what controls evaporation17
What controls evaporation?
  • Energy inputs
  • Temperature
  • Vapor content
  • Wind
  • Water Availability
  • 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
additional factors affecting evaporation from free water surface
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
additional factors affecting evaporation from free water surface20
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
evaporation from soil
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
evaporation from soil22
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

how do we measure estimate evaporation
How do we measure/estimate evaporation?
  • Direct measurement
    • Pans
    • Lake water balance
    • Lysimeters
pan evaporation
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 > lake evap why?
  • Use a pan coefficient (usually 0.6-0.8)
  • Map of pan evap






soil lysimeter
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
  • ET = change in weight – precipitation
  • Either prevent seepage or collect and measure
  • 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!)
water availability
Water Availability
  • An open water surface provides a continuous water source
  • Transpiration can provide water up until a certain limit based upon the plant’s ability to pull water up through its roots and out its stomatae (rate of transpiration)
water movement in plants
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.



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


Stomatal conductance balances the atmospheric demand for evaporation with the hydraulic capacity to supply water


Transpiration =

VPD * LAI * leaf conductance

VPD Vapor pressure deficit

LAI Leaf area index


Flow of liquid water =

(Yleaf – Ysoil) * K


leaf conductance
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

fine soils with ample soil-moisture storage, warm summers, cool winters, and little change in precipitation throughout the year



Effects of soil type

and climate



coarse soils with

limited soil-moisture storage,

warm, dry summers, cool, moist winters.



pet potential evapotranspiration
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
actual evapotranspiration
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
pet equations
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
physically based theoretical methods e g penman monteith
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
physically based methods
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
aet equations
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
variables used in common et models
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



JAWRA 2005

> 70% annual precipitation in the US

In General: ET/P is

~ 1 for dry conditions

ET/P < 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 bin to store part of precipitation



human effects
Human effects
  • Change in vegetation affects ET
    • Agriculture, horticulture, urbanization, deforestation, etc.
  • Change in climate will affect ET
    • Think about the factors that affect ET
  • Reservoir storage affects ET
    • By 2000, Evap losses were greater than total domestic use in 1950 and is increasing