Agriculture Reference
In-Depth Information
16.4 INFLUENCE OF TARGET CHARACTERISTICS ON SPLASH
PARAMETERS
It is difficult to quantify the influence of target characteristics, such as target type
(artificial, leaf or fruit surface), superficial properties (wetness, rugosity, presence of
fungal lesions), internal and mechanical properties (turgidity, flexibility), on the
splash process. The scale of complexity increases from that of an artificial rigid
target to that of a plant surface and ultimately to that of a three-dimensional crop
canopy. Of particular interest is the leaf scale because of its importance in the
quantitative approach to the study of rain-splash.
16.4.1 Inert rigid target
An artificial rigid surface can be used as a reference target to scale mass and energy
transfer during splash under repeatable conditions, so that mass and energy transfer
from real leaves, which are neither inert nor rigid, can be studied by comparison
with transfer from the artificial leaves. Important characteristics of the surface are
the presence or absence of water and the thickness of the water film on a wet target.
The variability in the mass of water splashed from dry glass targets is much larger
than that from wet targets and decreases as incident drop size increases. The angle of
inclination of the target also greatly affects the splash process, especially with large
incident drops (>3 mm); the mass of water splashed by large drops (impacting onto a
glass target wetted with a water film 0.2 mm deep) decreased by about 50% as the
angle from horizontal increased from 0° to 15° (Huber
et al.
, 1997).
Fig. 16.4a illustrates the power law relationship between drop diameter and the
mass of water splashed per incident drop for drops falling from a height of 11 m
onto artificial rigid glass targets of various types (dry, wet with water films of
thickness 0.1 to 0.6 mm). Walklate
et al.
(1989) compared splash from various
target types and developed the model:
∆
z = z*
ln(
DV/C*
)
(16.3)
where
z
is the maximal splash height above the impact level, and
z*
and
C*
are
splash parameters; estimates of
z*
were directly affected by target surface properties.
For a rigid metallic target, the critical impact velocity above which splashing occurs
is a power law of drop diameter
D
(1.4 <
D
< 4.5 mm) (Stow and Hadfield, 1980),
which decreases (in a range less than 3-4 ms
-1
) with increasing surface roughness.
∆
16.4.2 Plant surfaces
Experiments using leaf or fruit surfaces have indicated that the variability in splash
variables is much greater for artificial targets. For example, the pattern of mass
transfer of splash droplets was different between healthy or diseased (white leaf
spot, caused by
Mycosphaerella capsellae
)
sub-horizontal oilseed rape leaves at
three stages of maturity and tobacco leaves, showing that leaf surface characteristics
