SVERIGES LANTBRUKSUNIVERSITET - Epsilon Open Archive - SLU

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Routine determination of AET, in a long term, is feasible only
with reliable and weather proof monitoring equipment that require
minimum of attendance and maintenance at the site. Automated weather
stations equipped with appropriate sensors could be used for this
purpose. These autostations should also be located on a relatively
homogeneous terrain, for example, on an relatively extensive
agriculturaI field. One approach to estimate AET is to apply the
measurement of the skin surface temperature of the evaporating surface
in addition to the standard meteorological data.
2. Theory
2.1. The aerodynamic surface temperature method
To obtain a working formula for AET we first consider the heat
balance of the soil surface. The surface either absorbs or emits a net
amount (R n ) of electromagnetic energy. When absorbed this energy is
converted into heat. The absorbed heat is further conducted into the
soil as soil heat flux (G) and transported into the air as sensible
(H) and latent (LE) heat. A small amount of heat can temporarily be
stored in the vegetation layer, but for most agriculturaI crops this
component can be neglected. For practical purposes the heat balance
can then be written as
G + H + LE (1)
The efficiency of the transport of sensible and latent heat may be
characterised with a single parameter, namely the turbulence transfer
coefficient (h), which is used to connect the flux of an entity to the
vertical concentration gradient within a vertical distance z of that
entity. For the sensible heat flux we have
where pCp is the thermal heat capacity of air, To is the skin surface
temperature and Tz is temperature at height z (usually the screen
height 2m) above the ground.
The transfer coefficient hHz for heat between the effective canopy
height and height z is mainly dependent on the stability of air,
height of the roughness elements and wind speed. Empirical algorithms
(2)