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January
18, 2007 A single drop is
harmless, but
when billions of raindrops fall from a cloudburst onto bare soil they
strike
like billions of tiny hammers, dislodging tons of soil per acre which
is
carried away by surface runoff. This process, called
splash
erosion, is of critical importance to agriculture. It is the initial
stage of
water erosion, which causes an estimated $27 billion in on-site
economic losses
in the Despite its
importance, there is
a lack of understanding of the fundamental processes involved in this
natural
phenomenon that makes it difficult for researchers to assess the
reliability of
the experimental data that exists. This lack is addressed by the first
study to
use a high-speed camera to analyze the interaction between individual
rain
drops and soil particles, published on January 16 in the Journal
of Geophysical Research. In the new study
researchers from
Vanderbilt and When the model is
used with new
sources of information like Doppler radar, which can provide data on
average
raindrop size and velocity in actual rainstorms, it could provide more
reliable
estimates of the amount of splash erosion taking place in different
environments. "The more we
understand the
basic physics of the splash erosion process, the better we can become
at
controlling it in the farmer's field," says David Furbish, professor of
earth and environmental sciences at Vanderbilt, who directed the study.
His
collaborators were Mark Schmeeckle, assistant professor of geography at
The experiments
consisted of
mounting a 20-foot PVC pipe vertically and attaching a syringe at the
top of
the pipe. The distance was great enough so that the raindrops nearly
reached
terminal velocity, the fastest speed that they can travel through still
air.
The pipe prevented errant air currents from deflecting the drops. A
sand
target, which was two centimeters deep and 2.5 centimeters in diameter,
was set
flush to a surrounding surface that is covered with sticky paper. The
syringe
produced individual water drops of different sizes. When a drop hit the
target,
a high-speed camera, operating at 500 frames per second, recorded the
dynamic
interactions between the water and the sand. The grains ejected by each
impact
stuck to the surrounding paper where they hit, allowing the researchers
to plot
their positions precisely. The researchers
investigated the
impacts of raindrops of three sizes: two, three and four millimeters in
diameter. Natural raindrops come in sizes up to 6 millimeters. The
water drops
were dropped onto three grades of quartz sand: fine, medium and coarse
with
grain sizes of 0.18, 0.35 and 0.84 millimeters, respectively. The
experiment
also investigated the effect of slope by setting the target at six
inclinations
(0, 10, 15, 20, 25 and 30 degrees). The high-speed
camera revealed
that when small drops fell onto coarse sand, they hit without a splash
and
disappeared with scarcely a trace. But when a large drop falls onto
fine sand,
it flattens out and pushes a ridge of grains ahead of it. At about the
same
time that it blasts the sand grains into the air, the drop begins to
contract,
pulled back by its own surface tension, leaving behind a small impact
crater. The difference in
the impact of
different size raindrops didn't come as a surprise. The energy in
raindrops
increases dramatically as they get bigger. For one thing, they weigh
more. The mass
of a five millimeter raindrop is 125 times greater than that of a one
millimeter drop. In addition, larger drops travel faster. The terminal
velocity
of a five millimeter drop is twice that of a one millimeter drop. As a
result,
a five millimeter drop has 250 times the destructive energy of the one
millimeter drop. When they began
tilting the
target to see what happens on sloping surfaces, however, they did
discover
something they didn't expect. For more than 50 years, scientists have
known
that soil particles detached by rain splashes move down slope farther
than they
move sideways or upslope. The generally accepted explanation has been
that this
is caused by the fact that particles ejected down slope travel farther
before
coming to rest than those ejected in other directions. The experiment
confirmed
this effect but found that there was a second, unexpected contribution:
More
grains are ejected in the down-slope direction than in other
directions, and
they are ejected at higher velocities. This is particularly important
because
splash erosion does the most damage on sloping surfaces. Impacts on
inclined
surfaces also replicate those of wind-driven drops. "By providing a
clearer
explanation for this phenomenon, our study will inform further studies
aimed at
relating transport rate to sediment size and drop intensity," says
Furbish. Scientific efforts
to understand
and predict soil erosion date back to the "dust bowl" days of the
Great Depression in the 1930's. The U.S. Department of Agriculture
began an effort
that resulted in the Universal Soil Loss Equation. This is a purely
empirical
formula designed to relate agricultural practices to soil erosion. "The Universal Soil
Loss
Equation was created by using a large sledge hammer, namely going out,
measuring soil properties, estimating rain properties, looking and
categorizing
land use practices, putting in erosion pans and gutters to catch
sediment in
order to measure the amount of erosion, plotting the data, analyzing it
statically to come up with a formula that could be handed back to the
farmer," says Furbish. "Here's the problem: This formula was created
from a finite data set pertaining to a finite range of slopes, a finite
range
of soil properties, a finite range of rain properties, and so on. It
has since
been applied universally, which means that under many circumstances it
has
performed poorly because it is being applied to conditions beyond those
for
which it was developed. "The aim of research
like
ours is to produce a theoretical foundation for describing these
processes that
can be used to model landscape evolution as well as improve upon the
Universal
Soil Loss Equation," Furbish says.
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