4. Evapotranspiration Flashcards
Evaporation
Direct transfer of water from open water bodies or soil and vegetation surfaces to the atmosphere
Criteria that must be met in order for evaporation to occur (3)
- A flow of energy to the evaporating surfaces
- A flow of liquid water to these surfaces
- A flow of vapor away from these surfaces
How does evaporation occur? (4 elements)
- Water molecules move permanently
- Some break away from water surface into the atmosphere, while some cross back to the water surface (dynamic process)
- As long as more water molecules leave the water surface than return, net evaporation occurs
- If net evaporation continues, eventually an equilibrium will occur: no more net evaporation; the air reached its saturation vapor pressure
Two basic controls on evaporation
1) Evaporation requires energy (provided by water temperature, which in turn is driven by the E budget)
2) Evaporation is driven (or limited) by the vapor pressure deficit (i.e. the ability of air to hold more water)
Control #1: ENERGY
- name?
- Latent heat of vaporization is the energy that is required for a water molecule to escape the water surface.
- At 100°C, it is equal to 2270 kJ/kg
Control #1: ENERGY
- where does it come from
- mostly from solar radiation (insolation
- other terms of the energy balance (e.g. sensible or ground heat flux)
Control #1: ENERGY
- what can be used to approximate this energy
Air temperature is a good proxy for average energy availabilty
Control # 2: VAPOR PRESSURE DEFICIT (VPD)
- def
Vapor pressure deficit is the difference between the amount of moisture in the air and how much moisture air can hold when saturated.
- VPD = es -ea
where - es = saturation vapor pressure at air temperature
- ea = actual vapor pressure at air temperature
Relative humidity formula
RH = ea / es x 100%
where
- es = saturation vapor pressure at air temperature
- ea = actual vapor pressure at air temperature
Saturation vapor pressure (es)
- Saturation vapor pressure is the maximum amount of moisture that air can hold at a given temperature
- It is a property of air
- It is only a function of air temperature
→ es = 0.6105 x e^( 17.27 x T / (237.3 + T) )
Actual vapor pressure (ea)
- Actual amount of moisture that air holds at air temperature
- formula: ea = es * RH / 100
- So we either need to know the air temperature (and thus es) and the relative humidity OR the dew point temperature to derive ea
** reminder: dew point temperature = the temperature at which air starts to condensate (i.e., is saturate) (i.e. temperature when RH = 100%)
Factors that affect evaporation (6)
- Solar radiation (evap ↑ when solar rad ↑)
- Vapor pressure difference between a water surface and the overlaying air (evap ↑ when P diff ↑)
- Temperature (evap ↑ when T ↑)
- Wind (evap ↑ when wind ↑)
- Atmospheric pressure (evap ↑ when P ↑)
- Quality of water (evap ↑ when water quality ↑)
Three methods to estimate evaporation
- water budget
- energy budget
- evaporation pans
Water budget method
ΔS / Δt = (P+Q+Qr+Qs) - (Q0+Qd+E)
E = (P+Q+Qr+Qs) - ΔS / Δt - Q0 -Qd
basically: Evap = Inputs - Storage - other outputs
Advantage of the water budget method (1)
Simple
Disadvantages of the water budget method (2)
- Difficult to estimate subsurface seepage loss (Qd) and subsurface runoff (Qs)
- Unreliable; accuracy increases when Δt increases
Energy budget method
Es = (Ea +Rt) - (Rr + Ee + Hn + R1)
Ee = (Ea +Rt) - Es - Rr - Hn - R1
Evap = Energy inputs - Energy stored - Other energy outputs
Advantage of the energy budget method
most accurate method since evaporation depends on the energy state of the system
Disadvantages of the energy budget method (3)
- Difficult to evaluate all the terms
- Energy balance equation must be simplified
- Empirical formulas are used (although radiation measurements are preferred)
Radiation budget (total radiation and net)
- R total = total solar radiation inputs on a horizontal plane at the Earth’s surface
- R net = R total *(1 - albedo)
Energy budget equation simplified
R net = λE + H + G
E = (H + G)/λ
where
E = evaporation
λ = latent heat of vaporization for water
H = sensible heat transfer to heat
G = heat conducted to the ground
Evaporation pans method equation
Ep = P - (H2 - H1)
where
- Ep = pan evaporation
- P = precipitation
- H1 = height of water at t=0
- H2 = height of water after a certain amount of time
Why do pans measure more evaporation than there actually is in natural bodies? (3 reasons)
- Pans have less heat storage capacity (due to their smaller volume)
- Heat transfer
- Wind effects
What is done to account for the fact that pans compute more evaporation than there is in actuality?
A coefficient is applied to the Epan measurement :
Etrue = Cp x Epan
Cp usually is between 0.7 and 0.95; it varies from month to month but is fairly consistent from year to year
Advantages to the pan method (2)
- easy method
- inexpensive
Disadvantages of the pan method (3)
- Although we use a pan, the main focus is NOT what evaporates from the pan;
- What we want to know is the regional evaporation from land surface or the evaporation from a lake
- Overestimates evaporation
Transpiration (def)
Indirect transfer of water from the root-stomatal system to the atmosphere
Criteria that must be met in order for transpiration to occur (3)
- A flow of energy to the transpiring surfaces
- A flow of liquid water to these surfaces
- A flow of vapor away from these surfaces
How does water move in plants? (what drives the movement)
- Energy differentials drive the water movement from the soil into the roots, up the stalk, into the leaves and out into the atmosphere
- Water always moves from a less negative moisture tension in the soil to a more negative tension in the atmosphere
BETTER EXPL:
The driving force of transpiration is the difference in water vapor concentration (i.e. vapor pressure difference), between the internal spaces in the lead and the atmosphere around the leaf.
How does water get to the leaf?
- water is pulled not pumped
- water within the whole plant forms a continuous network of liquid columns from the film of water around soil particles to absorbing surfaces of roots to the evaporating surface leaves
- it is hydraulically connected
What are stomatae?
- Stomatae are vert small pores in plant leaves (or stems), with a slit of variable width
- This slit is what allows the movement of gases in and out of the intercellular space
What do stomatae do? (3)
- Allow for plants to acquire CO2 from the air (essential for photosynthesis
- Allow plants to “expel” water (transpiration)
- Open and close diurnally and in response to soil water tension
Stomatal conductance
Stomatal conductance, estimates the rate of gas exchange and transpiration through the leaf stomata
How does stomatal conductance vary with temperature?
As temperature increases, stomatal conductance increases
How does stomatal conductance vary with photosynthetically active radiation (PAR)?
As PAR increases, stomatal conductance increases
How does stomatal conductance vary with vapor pressure deficit (VPD)?
AS VPD increases, stomatal conductance decreases
How does stomatal conductance vary with pressure?
As pressure increases, stomatal conductance decreases
Do crops (herbaceous and cereal) have maximum stomatal conductance or minimum stomatal conductance?
Maximum
How does stomatal conductance influence transpiration?
As stomatal conductance increases, transpiration increases.
Evapotranspiration (def)
Evapotranspiration summarizes all processes that return water to the atmosphere in vapor form, so it includes:
- Evaporation: direct transfer of water from open water bodies, soil surfaces or vegetated surfaces
- Transpiration: indirect transfer of water from the root-stomatal system
FOREST: percent of evaporation vs interception vs transpiration (place in order largest % to smallest %)
transpiration > interception > evaporation
MEADOW: percent of evaporation vs interception vs transpiration (place in order largest % to smallest %)
transpiration > interception = evaporation
AGRICULTURAL LAND: percent of evaporation vs interception vs transpiration (place in order largest % to smallest %)
evaporation > transpiration > interception
BARE SOIL: percent of evaporation vs interception vs transpiration (place in order largest % to smallest %)
evaporation»_space; transpiration & interception
→ evaporation accounts for 100%
How to measure evapotranspiration? (5)
- Lysimeter measurements
- Flux tower micrometeorological measurements
- Evapotranspiration equations
- Study of groundwater fluctuations
- Water balance priniple
LYSIMETRY: how does it work?
- Crop of interest is grown under natural conditions in an isolated tank in a large field of the same crop
- It takes direct measurement of ET
LYSIMETRY: advantages (4)
- the terms that are difficult to measure using the water balance method are carefully controlled and measured
- precision is most accurate (0.05 mm/hour of resolution)
- can be used to determine weather effects on ET
- can be used to evaluate estimating methods of ET
LYSEMTRY: disadvantages (5)
- difficult and expensive to construct
- requires careful operation and maintenance
- primarily research application
- soil inside and outside the tank must be similar
- vegetation inside and outside the tank must perfectly match (height, leaf area, density, vigor)
FLUX TOWER: how it works
- Measurement of vertical transfer of water vapor driven by convective motion
- Gas fluxes are measured directly, by sensing properties of eddies as they pass through a measurement level
- Evaporation can be calculated by using measurements made at different elevations above the land surface, along the eddy flux tower
FLUX TOWER: Eddy covariance principles
- directly measures how much CO2 or H2O vapor blows in or out of a site in wind gusts
- allows to link changes in CO2 or H2O concentration in the air above a canopy with the upward or downward movement of that air
FLUX TOWER: what instruments are needed? (4)
- pyranometer
- net radiometer
- quantum sensor
- 3-D sonic anemometer
FLUX TOWER: advantages (3)
- measurements are continuous and in high temporal resolution
- fluxes are determined without disturbing the surface being monitored
- great tool to look at ecosystem physiology
FLUX TOWER: disadvantages (4)
- expensive
- requires the air to be turbulent for eddies to be measured
- requires flat terrain and homogenous underlying vegetation
- when data acquisition fails, gap-filling the dataset is difficult
Potential evapotranspiration (PET or ET0)
- ET that would occur if there was an adequate soil-moisture supply at all time (i.e. no water limitation)
- PET is determined by local weather conditions (VPD, wind) and energy status
Actual evapotranspiration (ET or AET)
- Actual evaporation rate from any surface under prevailing conditions of moisture availability and radiative input
- AET is determined by local weather conditions, energy status and water availability
Which is smaller? AET or PET?
AET
Is it possible for AET = PET?
Yes, at open water surfaces or over saturated bare soils
Reference crop evapotranspiration (ETrc)
ET that would take place under strictly prescribed biologic and surface moisture conditions :
- well watered grass, 0.12m high
- canopy resistance (rs) of 70s/m
- albedo of 0.23
- actively growing, completely shading the ground
EVAPOTRANSPIRATION EQUATIONS: name the 6 equations
- Bradley-Criddle method
- Thornthwaite model
- Penman model
- Penman-Monteith model
- Hargreaves method
- Hamon equation
Which of the equations have low data requirements?
- Bradley-Criddle method
- Thornthwaite model
- Hargreaves method
- Hamon equation
Which of the equations have high data requirements?
- Penman model
- Penman-Monteith model
Which of the equations only require air temperature?
Which of the equations only require air temperature data and daylight hours data?
- Bradley-Criddle method
- Thornthwaite model
Which of the equations only requires air temperature data and radiation data?
Hargreaves method
Which of the equations only requires air temperature data, daylight hours data, and saturated vapor pressure?
Hamon equation
What is ET/P in dry conditions?
ET/P ≈ 1
→ in dry climates, the effect of vegetative cover on ET is limited
What is ET/P in humid conditions?
ET/P < 1
→ in humid climates, the vegetative cover affects the magnitude of ET and thus of Q
