evaporation

Question 1

Daltons Aerodynamic influences

Answer 1

- Temperature
Air and evaporating surface
- Water vapour capacity of air
Saturation deficit
- Wind speed
- Atmospheric pressure
High, anticyclone
Low, damp unsettled weather
- Evaporating surface characteristics
Irregular: causes turbulence, enhances evaporation
- Size of surface

Question 2

Energy/Thermodynamic influences

Answer 2

Rn = net radiation (W m-2 day-1)
H  = energy heating air (W m-2 day-1)
G  = energy heating water (W m-2 day-1)
E = evaporation (m day-1)
λ = latent heat of vaporisation (J kg-1)
ρ = density of water (kg m3)

Question 3

Penman/s equation

Answer 3

E     = evaporation (m day-1)
Δ   = gradient of vapour saturation curve (kPa K-1)
K   = net short-wave radiation input (MJ m-2 day-1)
L   = net long-wave radiation input (MJ m-2 day-1)
γ   =  pyschrometric constant (kPa K-1)
KE = coefficient relating to efficiency of vertical transport
ρw = density of water (kg m-3)
λv  = latent heat of vaporisation (MJ kg-1)
νa  = wind speed usually at 1 or 2 m (m s-1) 
ea* = saturation vapour pressure at the air temperature (kPa)
Wa = relative humidity (Ø)

Question 4

Penman-Monteith

Answer 4

ET  = evapotranspiration (mm day-1)
Δ   = gradient of vapour saturation curve (kPa K-1)
K   = net short-wave radiation input (MJ m-2 day-1)
L   = net long-wave radiation input (MJ m-2 day-1)
γ   =  pyschrometric constant (kPa K-1)
ρw = density of water (kg m-3)
λv  = latent heat of vaporisation (MJ kg-1)
ca  = specific heat capacity of dry air  (MJ kg-1 K-1)
ea* = saturation vapour pressure at the air temperature (kPa)
Wa = relative humidity (Ø)
Cat = atmospheric conductance (m s-1), leaf surface to atmosphere transfer
Ccan = canopy or stomatal conductance (m s-1), stomatal cavity to leaf surface