Rn
α
Is
εL
σ
T
H
λE
λ
E
G
β
D
ew
ea
ƒ(u)
Eeq
ε
D(Pa)
ρ
gb
ζ
Gv
Tk
J
W
Definition
Values and Units
Net radiation
Reflectivity of a surface (albedo)
Intercepted incident Shortwave radation
Emissivity compared to a perfect black body
Stefan-Boltzmann constant
Temperature in Kelvin
Sensible heat flux (heating the air)
Latent heat flux (evaporation)
Heat of vaporization of water
Evaporation rate
Storage of heat by soil and stems (trunks)
Bowen Ratio, ratio of sensible to latent heat losses or H/ λE
Water pressure deficit
Saturated vapor pressure of water at the surface temp
Vapor pressure of the air
Function of air circulation associated with wind speed
Evaporation rate in equilibrium with specific surface
Change of latent heat relative to change of sensible heat
Air saturation deficit
Density of Air
boundary layer conductance for water vapor
= ρ (ε +1) Gv Tk (This is a combination term)
Gas constant for water vapor
Air temperature in Kelvin
Joules
Watts
J m-2s-1 or W m-2
0.9 to 0.98 for soils/veg
5.67x108 W m-1K-4
K
W m-2
W m-2
2454 kJ kg-1 at 20°C
kg m-2s-1
m s-1
=0.7185e0.0544T
kg m-3
m s-1
0.462 m3 kPa kg-1 K-1
m-2kgs-2
m-2kgs-3
Equations
Rn=(1- α) Is + εL σ T4 (surface) - σ T4 (sky)
Net Radiation = short wave radiation + long wave radiation
2.1
2.2
Rn - G = H + λE
Net Radiation – Heat Storage = Sensible Heat Flux + Latent Heat Flux
Net Radiation – heat storage = Heating of the ATM without evaporation and the evaporation of
water
λE = Rn /(1 + β)
Prediction for latent heat loss (assuming heat storage is minimal)
2.3
2.4
E = (ew – ea) f(u)
Evaporation rate = evaporation close to surface * air circulation
ew > e a
Water evaporates from the surface into the air
ew = e a
Local equilibrium so no evaporation net change in water
Water condenses from the air to the surface
ew < e a
E = Eeq + D gb / ζ
Calculating evaporation without knowing surface temperature
2.5
Eeq = (ε / ρ λ (ε + 1) Rn
2.6
ζ= ρ (ε +1) Gv Tk
So:
E = (ε / ρ λ (ε + 1) Rn) + D gb /(ρ (ε +1) Gv Tk)