1. No category

Evaporation,
Transpiration,Sublimation
Processes by which water
changes phaseLiquid 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
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
What controls evaporation?
1.
2.
3.
4.
5.
Energy inputs
Temperature
Humidity
Wind
Water availability
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.
http://www.ctahr.hawaii.edu/faresa/courses/nrem600/10-02%20Lecture.ppt
• 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 plantsoil-water surfaces.
• Convection is the vertical
component of sensibleheat transfer.
http://www.ctahr.hawaii.edu/faresa/courses/nrem600/10-02%20Lecture.ppt
What controls evaporation?
1.
2.
3.
4.
5.
Energy inputs
Temperature
Vapor content
Wind
Water availability
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
What controls evaporation?
1.
2.
3.
4.
5.
Energy inputs
Temperature
Vapor content
Wind
Water availability
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,
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
=es – ea
kPa
VPD
www.fsl.orst.edu/~bond/fs561/lectures/humidity%20and%20transpiration.ppt
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)
• 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 (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) 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 evaporation?
1.
2.
3.
4.
5.
Energy inputs
Temperature
Vapor content
Wind
Water Availability
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
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 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
• 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 soil
3. Soil color – affects albedo and thus energy
inputs
4. Depth to water table
-
If shallow such as wetlands, almost unlimited
evaporation
5. Vegetation
- provides shade- limits insolation (energy and heat)
- reduces windspeed at ground level
- increase vapor pressure through transpiration
How do we measure/estimate
evaporation?
1. Direct measurement
– Pans
– Lake water balance
– Lysimeters
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
http://fr.cfans.umn.edu/courses/FR3114/FieldMeas%20-%20Transpir_10_03_06.pdf
http://fr.cfans.umn.edu/courses/FR3114/FieldMeas%20-%20Transpir_10_03_06.pdf
http://fr.cfans.umn.edu/courses/FR3114/FieldMeas%20-%20Transpir_10_03_06.pdf
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
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!)
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
• 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/nrem600/10-02%20Lecture.ppt
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/humidity%20and%20transpiration.ppt
Stomatal conductance balances the
atmospheric demand for evaporation with the
hydraulic capacity to supply water
DEMAND: VPD
Transpiration =
VPD * LAI * leaf conductance
VPD Vapor pressure deficit
LAI Leaf area index
SUPPLY
Flow of liquid water =
(Yleaf – Ysoil) * K
www.fsl.orst.edu/~bond/fs561/lectures/humidity%20and%20transpiration.ppt
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
Effects of Vegetative Cover
fine soils with ample
soil-moisture storage,
warm summers, cool
winters, and little
change in precipitation
throughout the year
PET
AET
Effects of soil type
and climate
P
PET
AET
P
coarse soils with
limited soil-moisture
storage,
warm, dry summers,
cool, moist winters.
Available Soil Water
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
• 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
• 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
• 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
• 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
• 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
Model
T
Penman
Priestly-Taylor
Jensen-Haise
Blainey-Criddle
Thornthwaite
x
x
x
x
x
RH or e Lat Elev Rad. Wind
x
x
x
x
x
x
x
x
(mm/yr)
JAWRA 2005
Evapotranspiration
• > 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
http://www.ctahr.hawaii.edu/faresa/courses/nrem600/10-02%20Lecture.ppt
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