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geog 322 > 4. Evapotranspiration > Flashcards

4. Evapotranspiration Flashcards

1
Q

Evaporation

A

Direct transfer of water from open water bodies or soil and vegetation surfaces to the atmosphere

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2
Q

Criteria that must be met in order for evaporation to occur (3)

A
  1. A flow of energy to the evaporating surfaces
  2. A flow of liquid water to these surfaces
  3. A flow of vapor away from these surfaces
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3
Q

How does evaporation occur? (4 elements)

A
  • 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
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4
Q

Two basic controls on evaporation

A

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)

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5
Q

Control #1: ENERGY
- name?

A
  • 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
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6
Q

Control #1: ENERGY
- where does it come from

A
  • mostly from solar radiation (insolation
  • other terms of the energy balance (e.g. sensible or ground heat flux)
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7
Q

Control #1: ENERGY
- what can be used to approximate this energy

A

Air temperature is a good proxy for average energy availabilty

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8
Q

Control # 2: VAPOR PRESSURE DEFICIT (VPD)
- def

A

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
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9
Q

Relative humidity formula

A

RH = ea / es x 100%

where
- es = saturation vapor pressure at air temperature
- ea = actual vapor pressure at air temperature

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10
Q

Saturation vapor pressure (es)

A
  • 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) )
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11
Q

Actual vapor pressure (ea)

A
  • 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%)

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12
Q

Factors that affect evaporation (6)

A
  1. Solar radiation (evap ↑ when solar rad ↑)
  2. Vapor pressure difference between a water surface and the overlaying air (evap ↑ when P diff ↑)
  3. Temperature (evap ↑ when T ↑)
  4. Wind (evap ↑ when wind ↑)
  5. Atmospheric pressure (evap ↑ when P ↑)
  6. Quality of water (evap ↑ when water quality ↑)
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13
Q

Three methods to estimate evaporation

A
  1. water budget
  2. energy budget
  3. evaporation pans
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14
Q

Water budget method

A

ΔS / Δt = (P+Q+Qr+Qs) - (Q0+Qd+E)
E = (P+Q+Qr+Qs) - ΔS / Δt - Q0 -Qd

basically: Evap = Inputs - Storage - other outputs

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15
Q

Advantage of the water budget method (1)

A

Simple

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16
Q

Disadvantages of the water budget method (2)

A
  • Difficult to estimate subsurface seepage loss (Qd) and subsurface runoff (Qs)
  • Unreliable; accuracy increases when Δt increases
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17
Q

Energy budget method

A

Es = (Ea +Rt) - (Rr + Ee + Hn + R1)
Ee = (Ea +Rt) - Es - Rr - Hn - R1

Evap = Energy inputs - Energy stored - Other energy outputs

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18
Q

Advantage of the energy budget method

A

most accurate method since evaporation depends on the energy state of the system

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19
Q

Disadvantages of the energy budget method (3)

A
  • Difficult to evaluate all the terms
  • Energy balance equation must be simplified
  • Empirical formulas are used (although radiation measurements are preferred)
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20
Q

Radiation budget (total radiation and net)

A
  • R total = total solar radiation inputs on a horizontal plane at the Earth’s surface
  • R net = R total *(1 - albedo)
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21
Q

Energy budget equation simplified

A

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

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22
Q

Evaporation pans method equation

A

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

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23
Q

Why do pans measure more evaporation than there actually is in natural bodies? (3 reasons)

A
  • Pans have less heat storage capacity (due to their smaller volume)
  • Heat transfer
  • Wind effects
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24
Q

What is done to account for the fact that pans compute more evaporation than there is in actuality?

A

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

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25
Q

Advantages to the pan method (2)

A
  • easy method
  • inexpensive
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26
Q

Disadvantages of the pan method (3)

A
  • 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
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27
Q

Transpiration (def)

A

Indirect transfer of water from the root-stomatal system to the atmosphere

28
Q

Criteria that must be met in order for transpiration to occur (3)

A
  1. A flow of energy to the transpiring surfaces
  2. A flow of liquid water to these surfaces
  3. A flow of vapor away from these surfaces
29
Q

How does water move in plants? (what drives the movement)

A
  • 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.

30
Q

How does water get to the leaf?

A
  • 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
31
Q

What are stomatae?

A
  • 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
32
Q

What do stomatae do? (3)

A
  • 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
33
Q

Stomatal conductance

A

Stomatal conductance, estimates the rate of gas exchange and transpiration through the leaf stomata

34
Q

How does stomatal conductance vary with temperature?

A

As temperature increases, stomatal conductance increases

35
Q

How does stomatal conductance vary with photosynthetically active radiation (PAR)?

A

As PAR increases, stomatal conductance increases

36
Q

How does stomatal conductance vary with vapor pressure deficit (VPD)?

A

AS VPD increases, stomatal conductance decreases

37
Q

How does stomatal conductance vary with pressure?

A

As pressure increases, stomatal conductance decreases

38
Q

Do crops (herbaceous and cereal) have maximum stomatal conductance or minimum stomatal conductance?

A

Maximum

39
Q

How does stomatal conductance influence transpiration?

A

As stomatal conductance increases, transpiration increases.

40
Q

Evapotranspiration (def)

A

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
41
Q

FOREST: percent of evaporation vs interception vs transpiration (place in order largest % to smallest %)

A

transpiration > interception > evaporation

42
Q

MEADOW: percent of evaporation vs interception vs transpiration (place in order largest % to smallest %)

A

transpiration > interception = evaporation

43
Q

AGRICULTURAL LAND: percent of evaporation vs interception vs transpiration (place in order largest % to smallest %)

A

evaporation > transpiration > interception

44
Q

BARE SOIL: percent of evaporation vs interception vs transpiration (place in order largest % to smallest %)

A

evaporation&raquo_space; transpiration & interception
→ evaporation accounts for 100%

45
Q

How to measure evapotranspiration? (5)

A
  1. Lysimeter measurements
  2. Flux tower micrometeorological measurements
  3. Evapotranspiration equations
  4. Study of groundwater fluctuations
  5. Water balance priniple
46
Q

LYSIMETRY: how does it work?

A
  • 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
47
Q

LYSIMETRY: advantages (4)

A
  • 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
48
Q

LYSEMTRY: disadvantages (5)

A
  • 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)
49
Q

FLUX TOWER: how it works

A
  • 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
50
Q

FLUX TOWER: Eddy covariance principles

A
  • 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
51
Q

FLUX TOWER: what instruments are needed? (4)

A
  • pyranometer
  • net radiometer
  • quantum sensor
  • 3-D sonic anemometer
52
Q

FLUX TOWER: advantages (3)

A
  • measurements are continuous and in high temporal resolution
  • fluxes are determined without disturbing the surface being monitored
  • great tool to look at ecosystem physiology
53
Q

FLUX TOWER: disadvantages (4)

A
  • 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
54
Q

Potential evapotranspiration (PET or ET0)

A
  • 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
55
Q

Actual evapotranspiration (ET or AET)

A
  • 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
56
Q

Which is smaller? AET or PET?

A

AET

57
Q

Is it possible for AET = PET?

A

Yes, at open water surfaces or over saturated bare soils

58
Q

Reference crop evapotranspiration (ETrc)

A

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

59
Q

EVAPOTRANSPIRATION EQUATIONS: name the 6 equations

A
  1. Bradley-Criddle method
  2. Thornthwaite model
  3. Penman model
  4. Penman-Monteith model
  5. Hargreaves method
  6. Hamon equation
60
Q

Which of the equations have low data requirements?

A
  • Bradley-Criddle method
  • Thornthwaite model
  • Hargreaves method
  • Hamon equation
61
Q

Which of the equations have high data requirements?

A
  • Penman model
  • Penman-Monteith model
62
Q

Which of the equations only require air temperature?

A
63
Q

Which of the equations only require air temperature data and daylight hours data?

A
  • Bradley-Criddle method
  • Thornthwaite model
64
Q

Which of the equations only requires air temperature data and radiation data?

A

Hargreaves method

65
Q

Which of the equations only requires air temperature data, daylight hours data, and saturated vapor pressure?

A

Hamon equation

66
Q

What is ET/P in dry conditions?

A

ET/P ≈ 1
→ in dry climates, the effect of vegetative cover on ET is limited

67
Q

What is ET/P in humid conditions?

A

ET/P < 1
→ in humid climates, the vegetative cover affects the magnitude of ET and thus of Q

geog 322 (10 decks)

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