Surface Energy Fluxes - Terrestrial Hydrometeorology - page 47

Latent and sensible heat flux : natural evaporation rate (typically a few mm
per day) is converted to latent heat flux by multiplying by the latent heat of
vaporization which has units of W m −2 (1 mm d −1 is equivalent to 28.6 W m −2 ).
The flow of heat leaving the surface and directly warming the air (in the same
units) is called the sensible heat flux .

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Energy balance : the energy budget of a volume with unit horizontal area that
intersects a horizontal, uniform ideal terrestrial surface that comprises soil
with overlying vegetation permeated by air is (

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l E + H ) = ( R n − G + A t − S t − P )

where:
l

E is the latent heat flux (see above);

H is the sensible heat flux (see above);

R n is the net radiation (the net flux of radiant energy at all wavelengths);

G is the soil heat flux (the flow of heat into or out of the soil by conduction);

A t is advected energy (the energy advected horizontally in the air by wind);

S t is the storage (the change in energy stored in vegetation, air and soil);

P is the biochemical storage (energy stored by photosynthesis/respiration).

Sign convention : radiation fluxes are positive toward the surface; other verti-
cal energy fluxes are positive away from the surface; storage terms are positive
when energy is absorbed, and advection positive when energy is brought in.

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Difference values of fluxes : examples given are:
— Dry Soil versus Wet Soil : During the day there is no latent heat flux for dry
soil and net radiation is greater for wet soil because less solar energy is
reflected; wet soil has higher thermal conductivity so soil heat fluxes are
greater.
— Moist Crop versus Moist Forest : Forests reflect less solar radiation so day-
time net radiation is higher, but also transpire less so latent heat is less
dominant than for crops; soil heat fluxes are small under dense
vegetation.

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Evaporative fraction and Bowen ratio : evaporative fraction is the ratio of the
latent heat flux to the sum of latent heat plus sensible heat (called the avail-
able energy ); Bowen ratio is the ratio of the sensible heat to the latent heat
flux.

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Open water evaporation : is related to wind speed and the difference between
the vapor pressure of the air and the (saturated) vapor pressure at the water
surface by empirical formulae that change with the evaporating area.

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References

Arya, S.P. (1988) Introduction to Micrometeorology. Academic Press, San Diego.
Long, I.F., Monteith, J.L., Penman, H.L. and Szeicz, G. (1964) The plant and its environ-
ment. Meteorologische Rundschau , 17 , 97-102.
McNaughton K.G. & Black, T.A. (1973) A study of evapotranspiration from a Douglas fir
forest using the energy balance approach. Water Resources Research. 9 (6), 1579-90,
doi:10.1029/WR009i006p01579.