Agriculture Reference
In-Depth Information
16.5 RELEVANT CHARACTERISTICS OF RAINFALL-CANOPY
INTERACTIONS
To understand spore dispersal by rain-splash, a more specific description of the
properties of rainfall is required. From experimental studies with single incident
drops, it is known that drop size is an important factor affecting the splash process
on a target (Fitt
et al.
, 1989) because the terminal velocity and hence the kinetic
energy of a drop is a power function of diameter (Ulbrich, 1983:
V
(
D
) proportional
to
D
0.67
). Although large drops are a small proportion of the total number of drops in
rain, they contribute a large proportion of total rainfall volume and play a decisive
role in splash dispersal. Therefore, the size distribution of raindrops is important
because a given rainfall intensity and the drop size distribution can change
significantly during a rain event (Ulbrich, 1994). The type of rain (e.g. drizzle,
shower, widespread and stratiform, orographic, thunderstorm, etc.) is known to
affect the frequency distribution of drop sizes (Mason and Andrews, 1960; Ulbrich,
1983). A description of drop size distribution, with the assumption of spherical
raindrops at terminal velocity, is needed. Drip drops within a canopy travel at speeds
less than the terminal velocity because of the limited fall height and an analysis of
rainfall modification by the canopy is a prerequisite to studies on spore dispersal by
drip drops (also named secondary drops or gravity drops).
16.5.1 Characteristics of rainfall
Rainfall is composed of discrete drops scattered in a volume of air and several
physical and chemical processes contribute to the raindrop size distribution (DSD)
in the free atmosphere. Evaporation and drop break-up increase the number of small
drops, whereas condensation and coalescence increase the number of large drops.
These processes combine to produce a drop size range from 0.2 to 5.5 mm. The
temporal variability in DSD during a rain event can also been considered. For
instance, Smith and DeVaux (1992) showed that during a one-hour storm, the mean
drop diameter ranged from 0.5 to 2 mm and the standard deviation of diameter
varied from 1 to 6 times the mean diameter. Moreover, the number of drop impacts
s .
Classical characteristics of rain distribution were discussed by Madden (1992).
on the ground ranged between 50 and 5000 drops m
-
2
-
1
The size distribution was described initially by Marshall and Palmer (1948), using a
-
negative exponential function of drop diameter:
N
(
D
) =
N
exp(
Λ
D
), where
N
(
D
)
0
3
is the number of drops in a unit volume (m ) of air per length unit (cm) of drop
diameter
D
(cm);
N
0
and
are parameters dependent on the type of drop size
distribution and rain intensity, respectively. This function was used in some
epidemiological studies (Aylor and Sutton, 1992; Madden
et al.
, 1996). The
Marshall-Palmer equation was generalised as a gamma distribution by Ulbrich
(1983):
Λ
-
N
(
D
) =
N
0
D
µ
exp(
Λ
D
)
(16.4)
