Agriculture Reference
In-Depth Information
The influence of leaf flexibility on splash was examined by comparing Brussels
sprout leaves that were either rigid (taped onto a rigid holder) or normally flexible
(Stedman, 1979). There was more splash from rigid leaves because flexible leaves
absorb a proportion of the drop momentum so that it is not all available for transfer
into splash droplet production, and maximum splash height values for oilseed rape
and wheat leaves were much less than those obtained with artificial targets
(horizontal surface with thin or accumulated water film) (Walklate et al. , 1989).
Except in a few cases (Fitt et al. , 1992), experimental results on drop splash
efficiency have been obtained with plant targets whose flexibility was not taken into
account.
16.4.3 Crop targets
Splash dispersal is greatly modified, and generally decreased, in a crop canopy by
comparison with splash from an artificial rigid target or an individual plant target.
Surface topography (including ground cover) and plant obstruction both have a large
influence on splash (Stedman, 1979, 1980; Yang et al. , 1991b; Soleimani et al. ,
1996; Huber et al. , 1997).
(a) Ground cover and surface topography
A controlled experiment with uniform-sized drops falling near terminal velocity on
barley straw, placed on a horizontal plane surface or on an open nylon grid showed
the effects of ground cover on maximum upward splash of water droplets (Walklate
et al. , 1989). Dispersal is greatly affected by surface roughness, quantified as the
standard deviation of elevation; the flux density of conidia (assessed as a number of
colonies growing on a selective medium) was greatest with a polyethylene ground
cover and smallest with straw ground cover (Yang et al. , 1990).
The effects of ground cover/topography on water splash were studied extensively
by Yang and Madden (1993) using simulated rainfall impacting on three types of
ground cover (soil surface, straw, plastic mulch) or strawberry plants. Flux densities
of droplet number and mass over horizontal distance were described well by
exponential models, and ground cover affected the steepness of gradients in both
numbers and mass of splash droplets; the steepest gradient was observed with the
straw ground cover and the shallowest with plastic mulch. The mass reflective factor
over a time period of uniform rainfall, defined here as the estimated total mass of
splashed water divided by the total mass of impacting raindrops on the intercepting
area, was directly influenced by the type of ground cover.
(b) Spatial organisation and temporal changes in plant canopies
The effects of the strawberry plant canopy on splash of water were assessed in the
experiments of Yang and Madden (1993). For three values of leaf area index (LAI =
1.4, 1.9, 2.2) and a plant free control (LAI = 0), droplet size distributions were
characterised by Weibull functions. The gradients of flux densities of droplet
Search WWH ::




Custom Search