Agriculture Reference
In-Depth Information
The limited distances travelled by splash droplets and steep spore droplet
deposition gradients have been confirmed in different experiments (Fitt and
McCartney, 1986), with targets ranging from liquid spore suspensions (Fernandez-
Garcia and Fitt, 1993) to cow pats (Granvold, 1984). Negative exponential models
generally fitted these deposition gradients better than inverse power models. Half-
distances of pathogen dispersion in splash droplets produced by simulated drops are
in the range of 5 to 15 cm (Fitt
et al.
, 1989). A diffusion model closely related to the
exponential model and based on physical hypotheses also described deposition
gradients of spores well (Yang
et al.
, 1991a; Madden
et al.
, 1996).
In an extensive analysis of the drop impaction and splash droplet movement
from a strawberry fruit surface (Yang
et al.
, 1991b), mass and kinetic energy
reflection of impacting drops (0.5-4 mm) falling from release heights between 0.25
and 1.5 m were determined for different combinations of drop diameter and release
height. Frequency distributions and cumulative distributions of droplet diameter and
droplet distance of travel were positively skewed and fitted well by the Weibull
distribution. On a log-log scale, total kinetic energy and total mass of splash droplets
were linearly related to kinetic energy of drops impacting on the strawberry fruit
surface (Fig. 16.3a). On a log-linear scale, mass and kinetic energy reflective factors
were expressed as quadratic functions of droplet velocity and asymptotically limited
by a constant value (Fig. 16.3b). By comparison, the percentages of drop mass and
kinetic energy transferred to droplets increased with increasing drop velocity to
reach maxima of
c.
70% for mass and 10% for kinetic energy. Essentially, most of
the mass or kinetic energy of splash droplets is derived from high velocity, high
mass, high energy incident drops.
The few experimental data sets available show that external variables, such as
drop size and impact velocity, cannot always explain splash variability. At low
release heights, total kinetic energy or mass of splash droplets per incident drop can
be expressed as a power law of the kinetic energy (proportional to
D
3
V
2
) of the drop
falling on a surface (Yang
et al.
, 1991b). For a wide range of release heights (1, 2,
11 m), the maximum splash droplet height was predicted well using log(
DV
)
(Walklate
et al.
, 1989). Using a weighing technique to determine the mass balance
between incident drops (falling from 1.5 m or 11 m) and splash droplets, Huber
et al.
(1997) directly obtained the mass reflective factor (proportions) and total mass
during splash events for various target configurations. For given impacting surface
and target characteristics, Pietravalle
et al.
(2001) confirmed the potential use of the
kinetic energy of impacting raindrops for estimating maximum splash height and
total number of splashes on a vertical cylindrical splash meter (Shaw, 1987, 1991).
The impacting kinetic energy of individual drops or simulated rainfall was a good
indicator of rust spore dispersal from wheat leaves or seedlings, respectively
(Geagea
et al.,
1999, 2000).
