Agriculture Reference
In-Depth Information
Although the physics determining trajectories of splash droplets in the air is
understood, little is known about the processes controlling variability in the initial
conditions of droplet trajectories. Thus, it is very difficult to proceed further in
studying the mechanics of splash on a small scale, because modelling the splash
process is physically complex and computer-intensive and the upscaling of
physical mechanisms from single splash events to canopy level is complicated.
Therefore, most efforts to predict the characteristics of a series of splash events for
use in plant disease epidemiology and ecology, for given rain/target conditions,
are made without analyses of each individual event. The splash process is
considered as a black box, ignoring the impact stage, and the outcome is described
macroscopically using mean or cumulative variables. For example, water splash
by single incident drops can be characterised by considering the overall efficiency
of the splash in transferring mass or energy from the incident drops to splash
droplets.
Alongside these macroscale studies a new physical approach was developed by
Saint-Jean
et al
. (2004) to model water transfer by rain-splash in a 3-D canopy using
Monte-Carlo integration. In the framework of this mechanistic and probabilistic
modelling work, each individual splash event at the plant-atmosphere interface and
the interception of splash droplets by plant organs is simulated. The first results
show the potential of such a model as a research tool for studying effects of
obstruction patterns on splash droplet trajectories. By assuming that in-flight
evaporation of splash droplets can be ignored (which makes sense under high
relative humidity of the air during rainfall) and drag force can be ignored, a more
operational 2-D parabolic trajectory model was developed to predict rain splash
height (Pietravalle
et al
., 2001).
16.3.3 Water and energy transfer during splash events
The splash mechanism can be studied experimentally and numerically by
investigating the physical properties (surface tension, viscosity) of the water drops
and the target. In epidemiology, splash events can be studied as exchange systems,
using knowledge of empirical relationships between impacting drop characteristics
(e.g. size and velocity) and splash parameters (e.g. number and mass of droplets,
flight distance, kinetic energy). The concept of distance travelled by a splash droplet
during a single splash event is different from that of distance travelled by a spore.
A spore may move by a series of successive 'jumps' in several splashes but droplet
movement and consequently the distance travelled depend only on the initial
conditions of droplet ejection, atmospheric turbulence characteristics and obstruction
patterns.
In still air with no obstructions, deposition gradients from a point source have
been studied extensively (Fitt
et al.
, 1989). Most average or median distances
travelled by splash droplets from point sources are
15 cm and such droplets rarely
travel beyond 1 m (Fitt and McCartney, 1986). Because the spore-carrying droplets
of greatest importance are the large ones travelling close to the source (Fig. 16.1),
splash dispersal is a short-range phenomenon.
≤
