14.1. Fluxes¶
14.1.1. Surface energy balance¶
The surface energy balance (SEB) can be defined for an extensive simple surface as (units: \(\mathrm{W \ m^{-2}}\)):
where \(Q^*\) is the net all wave radiation, the turbulent sensible heat flux (\(Q_H\)) and the turbulent latent heat flux (\(Q_E\)) and the soil heat flux (\(Q_G\)) (units: \(\mathrm{W \ m^{-2}}\)).
Note other notation that is commonly used for the SEB are (in the same order as above):
Bowen ratio
The Bowen ratio (\(\beta\)) is the ratio of sensible to latent heat flux:
14.1.2. Radiation balance¶
The \(Q^*\) (or \(R_n\)) within the SEB consists of:
which includes the incoming (\(\downarrow\)) and outgoing (\(\uparrow\)) shortwave (K) and longwave radiation (L) fluxes.
SEB and Radiation Balance Measurements
Each of these fluxes can be directly measured or modelled using several methods (and data inputs).
Radiation fluxes
Various types of radiometers are used. For shortwave radiation, pyranometers are used, and for longwave radiation, pyrgeometers are used. The source area or field of view of a radiation sensor is fixed by geometery.
Soil Heat Fluxes
Soil heat flux plates are used with temperature sensors above to determine the heat gain/loss between the plate (e.g. at 0.05 m below the surface) and the surface. In more complex environments the storage heat flux (heating and cooling) of the whole volume needs to be accounted for. For example in a forest, the trees (trunk, branches, leaves, air) as well as the soil itself. So in most environments the soil heat flux is one part of the storage heat flux.
Turbulent heat Fluxes
The turbulent heat fluxes and momentum can be measured using Eddy Covariance techniques which requires a sonic anemometer and an infrared gas analyser. An alternative method is it use scintillometry.
As the source area of EC and scintillometer sensors varies with wind direction, fetch, stability etc, the surface characteristics may change with time if the fetch is not homogeneous.
SEB and Radiation Balance Modelling
Generally, to calculate a convective or conductive flux, data are needed to determine the size of the gradient (e.g. temperature difference) and the ability of the medium (e.g. air, soil) to transfer heat (or mass). The latter may use a resistance scheme, an eddy diffusivity, or other approach, which changes with the state of the medium (e.g. stability if air, moisture state if soil).
Tip
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