Combination Methods for Turbulent Fluxes - Transport in the Atmosphere-Vegetation-Soil Continuum - page 267

a) Compute the latent heat lux (use c p = 1013 J kg -1 K -1 and ρ = 1.22 kg m -3 ).
b) Compute the sensible heat lux.
c) Compute the surface temperature.
7.2.4 Analysis of Evapotranspiration from Different Surface Types
Using the Penman-Monteith equation, some hydrologically very relevant differences
in evapotranspiration from different surfaces can be explained. We consider grass
and a forest. The important differences are the differences in roughness (and hence
aerodynamic resistance) and the difference in canopy resistance. We do not consider
the difference in albedo (affecting net radiation) and differences in soil heat lux. In
Table 7.1 the relevant resistances for both surfaces are given, for the situation that
the canopy is dry as well as when it is wet (as a result of, e.g., rain or dew fall). Also
given is the evapotranspiration for all cases (computed using the Penman-Monteith
equation and decomposed in the radiation term and the aerodynamic term).
First we focus on the situation that both grass and forest are dry. In that case grass
produces signiicantly more transpiration than forest. This is due mainly to the radiation
term: the difference between surface temperature and air temperature is much larger for
grass than for forest because forest is more closely coupled to the air temperature. This
is a result of the low aerodynamic resistance for forest. The result of the higher surface
temperature of grass is that the saturated vapour pressure in the stomata will be higher,
leading to a larger humidity contrast between surface and atmosphere. The difference in
the aerodynamic term between the two surfaces is only marginal because for forest the
lower in r a in the numerator is nearly compensated by the lower of r a in the denominator.
The r a has a dominant effect on the value of the denominator because (1/ r a ) it is multi-
plied with a large r c . One could wonder how forest is able to get rid of the energy input,
if both the latent heat lux and the surface temperature are low. But the smaller vertical
temperature contrast (approximately half of that for grass) is more than compensated by
the smaller aerodynamic resistance (one quarter of that for grass).
For a wet canopy the situation is reversed. Water is readily available at the surface
of the canopy and hence there is no stomatal control for the evaporation ( r c = 0). In
this case the radiation term is identical for both surfaces and hence the only difference
in evaporation can come from the aerodynamic term. The fact that the aerodynamic
resistance of forest is only 25% of that of grass directly translates in an aerodynamic
term for forest that is four times the value for grass. The total evaporation for grass
is slightly larger than the available energy, but for forest the evaporation amounts
to 2.24 times the available energy. Thus a signiicant amount of energy needs to be
extracted from the air (negative sensible heat lux).
To summarize: owing to the low aerodynamic resistance of forests, the variations
in evapotranspiration due to variations in the canopy resistance are magniied as com-
pared to surfaces with higher aerodynamic resistances. The water loss depends not