Download Evaporation and transpiration Open water evaporation Combination

Evaporation and transpiration
Open water evaporation
ƒ Lake evaporation
ƒ Free water evaporation
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Combination approach
Transpiration
Penman equation (1948)
ƒ Loss of water from plants to the atmosphere
s(R n − G ) /(ρ w λ v ) + γ
E=
2
0.622ρ e sat (T ) − e
ρw P
ra
s+γ
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Transpiration
Transpiration
ƒ Plants lose water vapour through stomata
ƒ Transfer of water vapour
esat (Ts )
Leaf surface
e0
Stomata
Stomatal
cavity
Et
Guard cells
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Transpiration
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Transpiration
ƒ Resistance
ƒ Resistance – serial connection
rtot = Σ ri
r
a
r
ƒ Surface resistance – entire canopy
a
rS = rl/LAIactive ≅ rl/(0.5LAI)
• Total resistance
r
rtot = rS + ra
l
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Penman-Monteith equation
Soil evaporation
ƒ Similar to the Penman equation – with rS
s(R n − G ) +
LE =
ρ c p (e sat − e)
Water loss directly from the soil surface
ra
⎡ rS ⎤
s + γ ⎢1 + ⎥
⎣ ra ⎦
The actual evaporation is determined by
the flow of water to the evaporating surface
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Evapotranspiration = Evaporation + Transpiration
U,T
E
P
H
Rout
Rin
Ee
I
Etr
Etr+e
Rin
Limiting factors for
evaporation
D
Limiting factors for
Ee
Rout
G
transpiration
• Energy to vaporise
• Root water uptake ability
• Turbulence
• Maximum sap flow rate
• Surface soil
moisture
• Photosynthetic capacity
• Vegetation state (sick or well)
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U
T
P
I
D
E
G
H
Rin
Rout
Wind speed
Temperature
Precipitation
Interception
Drainage
Evapotranspiration
Soil heat flux
Sensible heat flux
Incoming radiation
Outgoing radiation
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Evapotranspiration
Interception and Interception loss
• Interception, I:
ƒ Potential evapotranspiration, PET:
“the process by which precipitation that falls on vegetative
surfaces and is subject to evaporation ”
“Potential evapotranspiration from a surface is the
maximum evapotranspiration rate when the surface is
well supplied with water”
ƒ Maximum interception storage:
Imax = LAI • Ic
[mm]
where Ic is the interception constant [mm3/mm2]
ƒ The potential evapotranspiration is a function of climatic and
meteorological conditions together with the surface
characteristics (affecting energy and vapour transfer) of the
surface in question
• Interception loss, IL:
“the amount of intercepted water that is evaporated”
IL calculated using the Penman evaporation
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Evapotranspiration
Actual evapotranspiration
ƒ Actual evapotranspiration, ET
ƒ ET ≤ PET
• Evapotranspiration at potential rate (PET) will only
take place if the soil is well supplied with water
ƒ Factors affecting actual evapotranspiration:
•
•
•
•
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Water availability
Soil type
Plant type
Nutrients, minerals, pesticides, illness, pests
• Actual evapotranspiration will be lower than potential
evapotranspiration when the soil dries out
• We could apply the Penman-Monteith equation and
predict ET(θ) as a function of rS(θ) – however, rS(θ) is
not well known
ƒ Modelling, ET:
• Instead a more empirical approach is often used
where ET = f(θ) PET
• Penman-Monteith, rS = f(θ)
• PET approach, ET = f(θ) PET
where θ is the water content of the soil
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Actual evapotranspiration
Root system model: sink term approach
ƒ ET as a function of θ
C
ET/PET
∂ψ ∂
∂ψ ∂K
= (K
)+
−S
∂t ∂z
∂z
∂z
50% of available water content
1.0
S depends on space, time and soil water content/pressure head
Water content θ
0
θwp
θfc
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Flow components in root zone
S-shaped function of van Genuchten (1985)
S (z, t ) = β(z) α(ψ ) Tp
Precipitation
Infiltration
α (ψ ) =
Soil evaporation
Evapotranspiration
Soil surface
Root zone
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Transpiration
1
ψ p
1+ (
)
ψ 50
Tp: potential transpiration
Percolation
α: dimensionless water stress
response function
Recharge
Water table
α
1.0
ψ = ψ50
0.5
0
1.0
Reduced pressure head ψ/ψ50
β: potential root water uptake
distribution function which integrates
to unity over the root zone depth
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