Agriculture Reference
In-Depth Information
16.6 CHARACTERIZING RAINFALL IN RELATION TO SPLASH-DISPERSED
PATHOGEN DISEASES
Spore dispersal by rainfall is not all by rain-splash; processes such as rain puff,
wash-off of spores by stem-flow and dripping water and wash out of air-borne
spores by falling raindrops can all disperse spores. These processes may operate in
isolation or in combination to increase disease spread if more susceptible hosts are
reached by infectious spores or to decrease disease spread if inoculum is lost. One
important topic which is rarely investigated is the influence of environmental
(particularly physical and chemical) conditions on inoculum during dissemination.
However, this influence is probably of much greater importance for air-borne spores
dispersed by wind than for splash-dispersed spores. Both spatial and temporal
variations in rainfall characteristics influence inoculum movement by rain-splash.
The temporal variation in drop size distribution is an external function influencing
the physical process of spore dispersal by rain-splash. DSD measurements require
expensive and sophisticated instruments (spectropluviometer or disdrometer);
however, these instruments are unique in providing the forcing function of rainfall.
In most cases, rainfall parameters are measured directly, for example by mechanical
integration of drop volume per unit time for different rain intensities. If the DSD is
measured, integral parameters of rainfall can be estimated by numerical integration
(section 16.5.1).
16.6.1 Rain drop size distribution (DSD) and integral parameters of rainfall
(a) Measurement of DSD
Instruments based on microwave back-scattering by rain have been developed
to estimate rainfall intensity over a large scale, nearly instantaneously, for
meteorological purposes (Jameson, 1991). To validate this methodology, other
instruments have been developed to measure the DSD, which is essential for
interpretation of the radar reflection factor. Based on optical and electromechanical
principles, these instruments are of two types: spectropluviometers provide the joint
statistical distribution of both drop size and drop speed and disdrometers provide
only the DSD. One spectropluviometer (Spectropluviomètre Précis Mécanique,
Bezons, France) operates by assessing the optical obstruction of a horizontal parallel
beam of light by falling raindrops that decrease the light intensity received by a
receptor when they pass through the parallelepipedic measurement zone. The
electrical signal produced by the photodiode receptor is transformed into positive
pulses whose height and duration fluctuations are used to determine both the size
and speed of drops. Drop size and velocity histograms can be measured at different
sampling intervals (from 15 to 120 s) with 16 drop diameter categories from 0 to 5
mm and 16 drop speed classes between 0 and 10 m/s. Even if this instrument shows
limited performance at measuring high rainfall rates (larger than 35 mm h
-1
), kinetic
energy and momentum can be estimated with an error less than 4% (Salles and
Poesen, 1999).
