The Atmosphere-Vegetation-Soil System - Transport in the Atmosphere-Vegetation-Soil Continuum - page 2

free troposphere

boundary layer

O(100 m)

surface layer

unsaturated zone

O(10 m)

saturated zone

substrate

Figure 1.1 Various layers in the soil-vegetation-atmosphere continuum. The rectan-
gular box indicates the vertical extent of the domain covered in this topic. The verti-
cal coordinate is roughly logarithmic.

and soil moisture has disappeared, whereas the top is located at the top of the sur-
face layer, which occupies roughly the lower 10% of the atmospheric boundary layer
(ABL, the layer where the diurnal cycle of surface heating affects the low). The
dynamics of the ABL itself are not part of this topic, but are dealt with elsewhere
(e.g., Stull, 1988 ; Vilà-Guerau de Arellano and van Heerwaarden, forthcoming).
The time scales considered range roughly from the diurnal cycle to the yearly
cycle, although for the atmosphere turbulent luctuations on time scales of less than a
second are dealt with as well. As an example of variations on time scales from days
to a year, Figure 1.2 shows daily rainfall luxes and simulated daily evapotranspira-
tion and drainage luxes of a grass on a sandy soil in a Dutch climate and with a deep
groundwater table. Although the rainfall luxes are well distributed over the year, the
evapotranspiration and drainage luxes have a clear yearly pattern. Potential evapo-
transpiration follows the pattern of solar radiation. In the summer season with a high
atmospheric demand, dry periods may cause a large difference between potential and
actual transpiration. Drainage occurs mainly in winter periods with a low atmospheric
demand and soil at ield capacity. Compared to rainfall luxes, drainage luxes are
much smaller and show a more gradual pattern.
Although the limitation of discussed time scales to a year is arbitrary, it suits the
discussion of most processes well. That is not to say that no interesting and relevant
processes occur at longer time scales, such as the interannual memory in vegetation
cover (e.g., Philippon et al., 2007 ).