Combination Methods for Turbulent Fluxes - Transport in the Atmosphere-Vegetation-Soil Continuum - page 259

a) Evaporation increases with increasing energy input (radiation term).
b) Evaporation increases with decreasing aerodynamic resistance, that is, under conditions
of high wind speed, or strong convective turbulence (aerodynamic term).
c) Evaporation increases with increasing water vapour pressure deicit, thus with increasing
dryness of the air (aerodynamic term).
d) The relative importance of the radiation term and the aerodynamic term depends (at a
given values of Q*- G , VPD and r a ) on temperature: at higher temperatures, the radia-
tion term becomes increasingly important due to the increase of s with temperature. On
the other hand, at a given temperature, sunny conditions will favour the radiation term,
whereas windy conditions and/or dry air will favour the aerodynamic term.

The conclusions drawn under points (a) to (c) could be considered as common knowl-
edge: if one wants to dry the laundry outside, this works best when the sun shines (a),
when there is suficient wind (b) and when the air is dry (c).
Some remarks need to be made regarding the application of the Penman
equation:

• The Penman equation has a sound physical basis and as such it describes the evaporation

well (except for the small effect of the linearization in Eq. ( 7.11 )): if the input data have
been measured over the wet surface for which one wants to determine the evaporation,
the Penman equation should give a correct answer. But one should be cautious when
using the Penman equation to predict evaporation: if one would use observations at a
given location, not taken over the wet surface one is interested in (e.g., because the wet
surface is not yet there), the calculated evaporation will be in error, because the observed
Q* - G, T , VPD and r a will not be representative of the conditions over a wet surface (e.g.,
the observed temperature may be too high, the net radiation may be too low, due to a too
high surface temperature or a different albedo).
The aerodynamic resistance depends on wind speed • and stability (see Chapter 3 ). Hence,

to determine the evaporation from observed Q* - G , VPD, temperature and wind speed
(and assumed roughness lengths for momentum and heat) the stability needs to be deter-
mined as well. As this depends mainly on H (and to some extent on L v E ) an iterative pro-
cedure needs to be used to ind the correct combination of sensible heat lux, evaporation
and aerodynamic resistance.
Although water bodies (e.g., ditches, lakes, etc.) are wet surfaces as well, the application of •

the Penman equation to those surfaces needs to be done with care. First, usually no obser-
vations above the water body are available, but only observations over land surfaces near
the water body. Hence the irst point above is applicable (difference in surface temperature,
roughness lengths, albedo). Second, the storage of heat in the water takes over the role of
the soil heat lux in Q* - G . This storage may be due to exchange of energy at the water sur-
face, but also due to the penetration of solar radiation down to some depth in the water.

Question 7.4: Consider the Penman equation.
a) What happens with L v E if air is saturated?
b) What happens with L v E if there is no net energy input ( Q* - G = 0)?
c) What happens with r a if there is no net energy input?