The Atmosphere-Vegetation-Soil System - Transport in the Atmosphere-Vegetation-Soil Continuum - page 11

It is very tempting to extend Eq. ( 1.6 ) to cases other than molecular transport. In
the context of the atmosphere-vegetation-soil system one could think, for example,
of the transport of water through the soil and the turbulent transport of heat, water
vapour and momentum in the atmospheric surface layer. The shape of such a trans-
port description would be identical to that of Eq. ( 1.6 ). But the speciication of the
diffusion coeficient k is much less straightforward. Coming back to the examples:
for water transport in the soil, the diffusivity depends on the porosity of the soil, the
shape of the pores and on the water content. In the case of turbulent transport in the
atmosphere, the diffusivity depends on properties of the low, such as the intensity of
the turbulence, stratiication and the distance to the ground.

∂
∂

T
z

Question 1.3. Given Fourier's law for (vertical) heat transport: Hc ρ p where κ

is the thermal diffusivity for heat of air (about 2·10 -5 m 2 s -1 ), ρ is the density (about
1.2 kg m -3 ) and c p is the speciic heat at constant pressure (for dry air, 1004 J kg -1 K -1 ).
a. What are the units of the sensible heat lux H ? Check that these are consistent with
the units of the right-hand side.
b. What vertical temperature gradient is needed to generate a sensible heat lux of 100
W m -2 ?

=−

1.4 Setup of the Topic
The irst ive chapters are roughly divided along the lines of the part of the atmo-
sphere-soil-vegetation continuum they discuss. First the two compartments on either
side of the atmosphere-soil interface are discussed, followed by the plants that extend
in both the soil and the atmosphere:

Atmosphere

• Chapter 2 covers the interaction of radiation with the atmosphere and

Earth's surface, leading to the radiative input to the energy balance as net radiation.
• Chapter 3 deals with the turbulent transport of heat, water vapour and momentum in
the atmospheric surface layer. The inal aim of that chapter is the description of surface
luxes (in particular of sensible and latent heat) in terms of mean quantities such as verti-
cal gradients or vertical differences of mean temperature and wind speed.

The irst part of

Soil

• Chapter 2 deals with the transport of heat in the soil (it has been com-

bined with net radiation because net radiation minus soil heat lux provides the energy
available for sensible and latent heat lux).
• Chapter 4 presents the basic concepts of water low in the unsaturated part of the soil: the
vadose zone. To address the general low equation, measurements of soil water pressure
head, water content, hydraulic conductivity and root water uptake are treated. Speciic
attention is paid to iniltration, runoff and capillary rise.

The second part of