Agriculture Reference
In-Depth Information
(diameter 5 cm) of chromatography paper in the centre of the annular reservoir. The
impact of raindrops on the liquid surface produces splash droplets, some of which
are intercepted by the vertical cylinder; the logarithm of the amount of dye coverage
(above a threshold height of 5-10 cm) was a linear function of height. Before
regression, a correction factor was applied to dye coverage data to account for
the decrease in the apparent splash receptor area with distance above the target.
The response of this instrument to rainfall is dependent on its design characteristics.
The type of target greatly affects the amount of dye splashed; for a liquid target, the
depth of the water film affects the amount of water splashed (Huber
et al.
, 1997).
Further work is needed to improve target surface and reservoir characteristics to
optimize the response of the instrument to rainfall. As suggested by Shaw (1991), a
constant level reservoir for the splash meter is useful to avoid the need to refill the
reservoir when intense evaporation occurs during days before a rain event (Huber,
unpublished data); with a V-shaped aluminium rain-shield at the top of the vertical
cylinder, a non-fluorescent dye can be used to permit a visual assessment of the area
covered by splash without the need for the sophisticated equipment to quantify
coverage by a fluorescent dye. Effects of the angle of rainfall require investigation
because the impact force of drops decreases as the angle of rain versus the surface
normal increases. In the presence of gusty winds, variability in rain angles produces
fluctuations in the quantity and trajectory of splash droplets that may cause
significant variation in DSD over a period of a few hours or even of a few minutes.
Consequently, both wind and DSD characteristics can limit the accuracy of the
splash meter. However, this simple instrument should be used more extensively.
An alternative is the direct measurement of kinetic energy with an electronic
sensor. For studying soil erosion, methods based on rainfall intensity/kinetic energy
relationships to relate soil transport to rainfall characteristics proved to be inade-
quate (Kinell, 1981). As suggested by Walklate
et al.
(1989), kinetic energy
sensors based on the response of a piezoelectric crystal transducer (producing a
single analogue output after signal transformation and integration) appear to be very
useful (Madden
et al
., 1998). Recent work showed the potential use of such rainfall
kinetic energy sensors to assess maximal splash height (Lovell
et al.
, 2002), or
rainfall power over variable time periods (Madden
et al.,
1998). A potential problem
with sensors based on piezoelectric crystals is the low signal-to-noise ratio for
measurements of kinetic energy from impacting small to moderate sized raindrops.
The measurement of both rainfall intensity and kinetic energy at the same time is
relevant to the characterisation of potential rain-splash, because they vary
considerably depending on the time interval of measurements (Smith and deVaux,
1992). Methodologies developed by soil erosion scientists might be of interest for
studying spore dispersal by rain-splash or wash-off processes (van Dijk
et al.
, 2003).
(b) Estimates based on DSD measurements or indirect estimates
Two integral parameters (e.g. intensity and kinetic energy at ground level, or back-
scattering coefficient and liquid water content) measured at adequate time intervals
are sufficient to determine DSD parameters (section 16.5.1). Once the size
