The Atmosphere-Vegetation-Soil System - Transport in the Atmosphere-Vegetation-Soil Continuum - page 4

input

output

∆ S

Figure 1.3 Concept of a control volume. Change in storage is due to an imbalance
between input and output. Note that inputs and outputs can occur at any face of the
control volume, not just the sides.

Figure 1.4 Control volume for water: liquid water (left) and water vapour (right).
The direction of the arrows holds for typical daytime conditions. Dark grey arrows
denote transport of liquid water, whereas light grey arrows are used for transport of
water vapour. Arrows between the boxes signify phase changes (e.g., C l→g is evapo-
ration): molecules of water do not leave the control volume but only move from one
phase to another. The dashed arrows with C l↔s and C g↔s have their other ends located
in the control volume for solid water, not drawn here. Other symbols are explained
in the text.

1.2.1 Water Balance
Figure 1.4 shows the water balance of the control volume. 1 Because under typical
terrestrial conditions water occurs in all three phases (gas, liquid and solid), a distinc-
tion has been made between water in the liquid phase and water in the gas phase (ice
and snow have been discarded for simplicity, but could be accounted for analogously;

1 Note that it is implicitly assumed that the control volume has a horizontal extent of one square meter. As a result
all luxes and storage terms, as used in Eqs. ( 1.2 ) and ( 1.3 ) should be interpreted as luxes per 1 m -2 of ground area.
This assumption is not a physical necessity, but simpliies the transition to the other chapters where usually luxes
are interpreted as lux densities (i.e., per unit area).