The Atmosphere-Vegetation-Soil System - Transport in the Atmosphere-Vegetation-Soil Continuum - page 8

1.00

0.80

LAI

0.60

0.40

transpiration

0.20

evaporation

0.00

emergence

harvest

Time

Figure 1.6 The partitioning of evapotranspiration into soil evaporation and transpira-
tion over the growing period of an annual ield crop.

with radiation, whereas in more arid regions radiation is not limiting, and evapo-
transpiration correlates with rainfall (Teuling et al., 2009 ). Although these correla-
tions sketch the main picture on a seasonal or longer time scale, many regions may
show different behaviour on shorter time scales. For example, mid-latitude regions
that show a strong correlation between radiation and evapotranspiration may show a
stronger dependence on soil moisture and rainfall after a prolonged drought (Teuling
et al., 2010 and Chapter 8 ). On the other hand, semi-arid regions in which the sea-
sonal pattern of evapotranspiration follows the seasonal pattern of rainfall may show
a clear correlation of evapotranspiration with radiation within the rainy season (e.g.,
Schüttemeyer et al., 2007 ).

Question 1.2: Evapotranspiration is a combination of various luxes: evaporation from
intercepted water, soil evaporation and transpiration.
a. What is roughly the impact of each of the terms on the soil moisture content at var-
ious depths?
b. At which location is the energy, needed for the phase change from liquid to water
vapour, absorbed for each of the luxes?

1.2.4 Simpliied Balances
In many applications the control volumes presented earlier are vertically compressed
to become a control surface (see Figure 1.7 ). Because a surface has no volume, stor-
age terms will disappear. Besides, horizontal advection will vanish as well. Whereas
in Eqs. ( 1.2 ) and ( 1.3 ) all luxes occurred at the boundaries of the control volume
(i.e., at a certain height above the ground or at a certain depth below the surface) now