14
Rn S G LE = O
(2-14)
where:
Rn -
net radiation incident upon soil surface [W*m"2]
S
soil heat flux [W*m'2]
G
sensible energy flux into atmosphere [W*m'2]
LE =
flux of latent energy from soil [W*nT2]
An expression for the rate of evaporation of water from the soil can be
determined by substitution of equation (2-13) into (2-14) and
rearranging terms. This method is referred to as the Bowen ratio
equation and yields valid results for a wide range of conditions.
Fritschen states that it is imperative that efforts be undertaken to
assure that the assumption of no horizontal divergence of heat or
moisture be met in order for the Bowen ratio equation to yield
satisfactory estimates of the evaporation. Soil heat flux must be
measured as well, since omitting soil heat flux from the analysis could
lead to large errors.
In using the Bowen ratio, the soil surface temperature must be known as
in some of the other methods.
Penman (1948) used a combination of the energy and mass balance
methods with the objective of eliminating the need for surface
temperature. The underlying assumptions are the temperature of
evaporating surface is the same as the ambient air and the vapor
pressure is the saturated vapor pressure evaluated at the surface