Integrated Applications - Transport in the Atmosphere-Vegetation-Soil Continuum - page 296

Next it is assumed that the relative humidity above both surfaces is 70%. With the
derived air temperature this gives the vapour pressure and with that the total resis-
tance (aerodynamic + canopy resistance) can be derived from

c

ee
rr

− () T p

LE =− ρ γ

a sat s

(8.4)

v

+

a c

(which was used before in Chapter 7 in the derivation of the Penman-Monteith
equation).

8.4.2 Energy Balance during Normal Summers
Figure 8.7a shows the components of net radiation and the energy balance for the
composite grass and forest station. The downwelling radiation luxes are nearly equal
for both sites, which simpliies the analysis as both sites are exposed to the same radi-
ative input. However, the grass and forest do differ in albedo (0.10 vs. 0.18) and sur-
face temperature (21.6 °C vs. 18.8 °C, assuming a surface emissivity of 0.96), leading
to a difference in net radiation of more than 10%. The partitioning of the available
energy over sensible and latent heat lux is consistent with the indings in Chapter 7 :
grass allocates a larger proportion of the energy to evapotranspiration than the forest
(evaporative fraction: 53% vs. 38%; see Table 8.6 ). The relatively low evapotranspi-
ration for the forest is due to the strong coupling of the surface temperature to the air
temperature (resulting from the low aerodynamic resistance). As a result, the gradient
of water vapour between the stomata (saturated air at leaf temperature) and the air is
lower than for grass. This lower gradient is only partially compensated by the lower
aerodynamic resistance.
The air temperatures above grass and forest (derived using Eq. ( 8.3 )) appear to be
close to each other and the surface-to-air temperature difference is smaller for the for-
est due to the small aerodynamic resistance (strong coupling to the air). Furthermore,
the computed air temperature is roughly consistent with the climatological median
maximum temperature of the stations, 20.6 °C (note that the maximum temperature
occurs later than the time period used here: 9-13 UTC, which could explain the dis-
crepancy of about 2.5 °C).
Using Eq. ( 8.4 ) the total resistance ( r a + r c ) can be determined, which leads ‒ with
the assumed aerodynamic resistance values ‒ to canopy resistances of 76 and 83 s m -1
for grass and forest, respectively. Given the crudeness of the analysis and the assump-
tions made, these values are close to the expected values (see Chapter 7 ).
The analysis shows that under normal summer conditions grass and forest supply
approximately the same amounts of water vapour to the atmosphere, but forests sup-
ply more sensible heat than grass.