Vegetation: Transport Processes Inside and Outside of Plants - Transport in the Atmosphere-Vegetation-Soil Continuum - page 245

will usually imply conditions where Q* < 0 (cloud-free conditions) and H and G are
not too strongly negative.
Two sources of water vapour are available. 6 First, water vapour can be extracted
from the atmosphere (dewfall): a downward turbulent moisture lux. For this it is nec-
essary that the stable stratiication does not suppress turbulence completely and hence
some wind is needed to maintain turbulence. On the other hand, if the wind is too
strong, H will become strongly negative and of the order of Q* so that the surface can-
not cool below the air temperature. Thus dewfall can occur only for a limited range
of wind speeds. The second source of water vapour can be the soil (dewrise). The soil
can both be a direct source of water vapour (in the case of unsaturated soils water
vapour diffusion will occur) or indirectly through evaporation from the soil surface.
In the latter case the resulting dew is energetically neutral as far as the control volume
that contains both the soil and the canopy is considered: irst energy is consumed to
evaporate water at the soil surface and subsequently latent energy is released on con-
densation. For a maize canopy, Jacobs et al. ( 1990 ) showed that the contribution of
dewrise is at least an order of magnitude smaller than that of dewfall. 7
Typical amounts of dewfall range from 0.05 to 0.5 mm per night (Xiao et al., 2009 ).
The amount of dew need not be uniformly distributed over the depth of the canopy.
The vertical distribution of dew found by Jacobs et al. ( 1990 ) in a maize crop can be
seen in Figure 6.25 . Despite the fact that the leaf area distribution changes signii-
cantly during the observation period, the peak of the dew formation always occurs at
z/h = 0.7. This is probably due to the fact that under the conditions studied dewfall
dominates and dew interception is concentrated in the top of the canopy. Furthermore,
the location of maximum cooling (see Figure 6.20 ) is a tradeoff between the location
of maximum foliage area (large cooling leaf area, deeper in the canopy) and maxi-
mum longwave cooling (per unit leaf area, top of the canopy).
Jacobs et al. ( 2006 ) estimate the contribution of dew to the water balance of a
Dutch grassland to be 37 mm per year, or nearly 5% of the precipitation. The num-
ber of nights in which some dew occurs is 250 per year or nearly 70% of the nights.
Whereas for temperate climates the contribution to the water balance is only limited,
in arid conditions the annual dewfall can be an important source of water with a lower
variability than rainfall (Zangvil, 1996 ).
Apart from the impact on the water balance, the wetness of leaves has other impli-
cations as well. First, fungal spores and other plant pathogens can develop in the layer
of liquid water. The length of the period of leaf wetness is a critical parameter in this
6 Another source of liquid water on leaves is guttation: water emerges from special pores due to the supply of water
through the vascular system. If the air is close to saturation the guttation droplets will not evaporate but accumulate
on the leaf. Because no phase change occurs, guttation cannot be accounted for by looking at latent heat luxes
(Hughes and Brimblecombe, 1994 ).
7 For semi-arid conditions, with irrigated soils, the reverse may hold: there the soil will be the primary source of water
vapour whereas the air is too dry to give a signiicant contribution (Weiss et al., 1989 ). Other atmospheric factors
affecting the relative importance of soil and atmosphere factors related to atmospheric transport (stability, wind speed).
Furthermore, the architecture of the canopy (height, bare soil fraction) will have an effect (Garrat and Segal, 1988 ).