Geoscience Reference
In-Depth Information
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The Atmosphere-Vegetation-Soil System
1.1 Introduction
Whereas roughly 70% of Earth's surface is covered by oceans, the remaining 30% of
land has a profound inluence on processes in the atmosphere (e.g., differential heat-
ing, drag, evaporation and resulting cloud formation, composition of the atmosphere).
This impact is due to the large variability in the properties (e.g., albedo, roughness,
soil type, land cover type, vegetation cover) and states (e.g., soil moisture availabil-
ity, snow cover) of the land surface. The processes occurring at the land surface are
often grouped under the terms biogeophysical and biogeochemical processes (Levis,
2010 ): they inluence the state and composition of the atmosphere both through phys-
ical and chemical processes, and biological processes play an important role in both.
Although the interface between Earth and atmosphere is located at the surface,
subsurface processes in the soil are of major importance because part of the energy
and water exchanged at the surface is extracted from or stored in the soil. Plants play
an important role in extracting water from deeper soil layers and providing it to the
atmosphere. In return, processes in the soil and plants (e.g., transport of water, sol-
utes, and energy) are strongly inluenced by atmospheric processes (e.g., evaporation
and precipitation).
The interface between Earth and atmosphere is part of the continuum that ranges
from the substrate underlying soils to the top of the atmosphere. The overarching sub-
ject of this topic is the transport of energy, matter (water, solutes, CO 2 ), and momen-
tum in the atmosphere-vegetation-soil continuum. In some cases this transport occurs
within one of the compartments (e.g., redistribution of solutes in the soil); in other
cases exchange over the interface between different compartments takes place (e.g.,
transpiration of water by plants).
In the context of this topic we limit the extent of our subject both in the vertical
direction (see Figure 1.1 ) and in the time scales considered. The lower boundary of
the domain is located at that level in the soil where the yearly variation in temperature
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