Available Energy: Net Radiation and Soil Heat Flux - Transport in the Atmosphere-Vegetation-Soil Continuum - page 44

is not fully transparent for longwave radiation it will also have a non-zero emissivity.
This implies that the heated ilter will transfer energy to the sensor by radiation. If
the temperature of the ilter is measured (as is done in some instrument types) this
radiation input can be corrected for. But in addition, convection within the instrument
may transport energy. This may amount to 10-20 W m -2 in bright sunshine (Kohsiek
et al. 2007 ). Again, ventilation of the sensor will reduce these errors by reducing the
thermal contrast between different parts of the instrument.
The aforementioned error sources for pyranometers and pyrgeometers are equally
valid for net-radiometers. An additional problem with net-radiometers is that the
sensitivity of the sensor should be equal for longwave radiation and shortwave radi-
ation (i.e., 100 W m -2 of longwave radiation should produce the same voltage as 100
W m -2 of shortwave radiation). For many instruments these sensitivities are far from
equal, leading to errors during daytime in particular, when the relative importance
of longwave and shortwave radiation varies considerably (Halldin and Lindroth,
1992 ).
A inal remark concerns the scale and spatial homogeneity of the observed radi-
ation. Upward pointing sensors collect radiation from the entire hemisphere and the
measured radiation will not depend much on the exact location of the sensor in a ield.
The only important consideration is that no obstacles should be located in the ield of
view of the sensor (unless one is explicitly interested in the effect of those obstacles,
e.g. to study the radiation inside a canopy; see Chapter 6 ). On the other hand, a down-
ward pointing sensor receives radiation from a limited area only: the footprint from
which roughly 50% of the lux originates is a circle with radius equal to one time
the instrument height. For a 90% recovery, the circle has a radius of three times the
instrument height (Schmid, 1997 ). When measurements are made over a surface with
heterogeneous radiative characteristics, the exact location of the sensor will directly
inluence the observed net radiation.
2.3 Soil Heat Flux
Although invisible to the eye, the heat transport that occurs below the soil surface
can be both important and hard to determine. The heat transport can also be modi-
ied strongly by the presence of vegetation cover or snow, and when freezing of soil
moisture occurs. In this section we irst discuss the basics of heat transport in the
soil, with special emphasis on the speciic properties of soils (as opposed to simple
solids or luids). Second, we look at an idealized case where the temperature at the
soil surface varies as a sine (e.g., diurnal or yearly cycle). Then simpliied models are
treated for bare soil conditions and vegetated surfaces. Finally, snow cover and frost
penetration into the soil are dealt with. Note that in this section the vertical coordi-
nate z d is used, which is taken positive downward (i.e., it indicates depth rather than
height).