30
3 A Conventional Experimental Technique …
uniformity. Due to a lack of understanding of the effects of raindrops on soil erosion,
the rainfall amount is generally used as the main parameter to simulate soil erosion
under natural rainfall. However, numerous studies have shown that simulated rainfalls
might result in obvious effects on soil erosion compared with natural rainfalls, even
though rainfall amounts are the same in the above-mentioned rainfall events (Zhou
et al. 1981).
Further studies have revealed that the effects of rainfall on soil loss varied with
the kinetic energy of the raindrop (Watung et al. 1996; Zhou et al. 1981). Since soil
erosion is triggered by the kinetic energy of falling raindrops, the energy is generally
calculated and discussed as the main parameter during the simulated rainfall process.
Wischmeier and Smith (1958) pointed out that the kinetic energy of the raindrop is
the most suitable parameter to simulate the role of rainfall during soil erosion. This
conclusion has been further confirmed by other researchers (e.g., Chen and Wang
1991; Park et al. 1983). In fact, the method of kinetic energy similarity is commonly
used to simulate the natural-rainfall event. However, observing the rainfall energy is
not as convenient as examining the rainfall intensity and drop size. Fortunately, there
is a close relationship between rainfall kinetic energy and rainfall intensity (Xu et al.
2006a). Hence, the congruence in kinetic energy could be achieved by adjusting the
rainfall intensity in rainfall simulations.
Many kinds of rainfall simulators have been developed and widely used in various
experiments. Bubzenzer (1979) identified 63 rainfall simulators which are successfully applied in studies of soil and water conservation. Shelton et al. (1985) classified
rainfall simulators into three forms, namely, hanging yarn, tubing tip, and nozzle,
and the latter, however, is used almost exclusively for field plots because it has the
capability to cover large areas. Wu and Xu (1995) further divided the rainfall simulators into four types. The first type is the thread dropper. Raindrops would drip
from a hanging yarn to the ground along cotton threads or wool fibers. A distinct
feature of this rainfall simulator is that the simulated raindrops are uniform and their
initial falling velocities are zero. The raindrop sizes are determined by the diameter
of the suspended thread instead of the water pressure of the water pipe. The second
type is the needle dropper. Raindrops would drip from the ends of the needles to the
ground. The simulated raindrops also fall to the ground with initial velocities of zero
and their characteristics are comparable to raindrops from the thread droppers. The
third type is the spout. The simulated raindrops are ejected at a certain initial velocity
from small holes drilled on some parallel slender tubes, and then they are dropped
to the ground. The raindrops have non-uniform diameters, and the rainfall intensity
could be adjusted by different aperture sizes or water pressure. The fourth type is the
nozzle/sprayer. Water drops are sprayed from the nozzle at a certain initial speed,
then they would be dispersed into the air in various sizes, and finally, they would fall
to the ground. The characteristics of this simulator are close to the features of the
spout. Presently, the technique to simulate rainfall has been greatly improved around
the world. Especially several rainfall simulation halls with automatic operation and
observation systems are boosting in virtue of the combination of rainfall simulators
and computers. Nevertheless, almost all the raindrop generators belong to the four
types mentioned above (Wu and Xu 1995).
3 A Conventional Experimental Technique …
uniformity. Due to a lack of understanding of the effects of raindrops on soil erosion,
the rainfall amount is generally used as the main parameter to simulate soil erosion
under natural rainfall. However, numerous studies have shown that simulated rainfalls
might result in obvious effects on soil erosion compared with natural rainfalls, even
though rainfall amounts are the same in the above-mentioned rainfall events (Zhou
et al. 1981).
Further studies have revealed that the effects of rainfall on soil loss varied with
the kinetic energy of the raindrop (Watung et al. 1996; Zhou et al. 1981). Since soil
erosion is triggered by the kinetic energy of falling raindrops, the energy is generally
calculated and discussed as the main parameter during the simulated rainfall process.
Wischmeier and Smith (1958) pointed out that the kinetic energy of the raindrop is
the most suitable parameter to simulate the role of rainfall during soil erosion. This
conclusion has been further confirmed by other researchers (e.g., Chen and Wang
1991; Park et al. 1983). In fact, the method of kinetic energy similarity is commonly
used to simulate the natural-rainfall event. However, observing the rainfall energy is
not as convenient as examining the rainfall intensity and drop size. Fortunately, there
is a close relationship between rainfall kinetic energy and rainfall intensity (Xu et al.
2006a). Hence, the congruence in kinetic energy could be achieved by adjusting the
rainfall intensity in rainfall simulations.
Many kinds of rainfall simulators have been developed and widely used in various
experiments. Bubzenzer (1979) identified 63 rainfall simulators which are successfully applied in studies of soil and water conservation. Shelton et al. (1985) classified
rainfall simulators into three forms, namely, hanging yarn, tubing tip, and nozzle,
and the latter, however, is used almost exclusively for field plots because it has the
capability to cover large areas. Wu and Xu (1995) further divided the rainfall simulators into four types. The first type is the thread dropper. Raindrops would drip
from a hanging yarn to the ground along cotton threads or wool fibers. A distinct
feature of this rainfall simulator is that the simulated raindrops are uniform and their
initial falling velocities are zero. The raindrop sizes are determined by the diameter
of the suspended thread instead of the water pressure of the water pipe. The second
type is the needle dropper. Raindrops would drip from the ends of the needles to the
ground. The simulated raindrops also fall to the ground with initial velocities of zero
and their characteristics are comparable to raindrops from the thread droppers. The
third type is the spout. The simulated raindrops are ejected at a certain initial velocity
from small holes drilled on some parallel slender tubes, and then they are dropped
to the ground. The raindrops have non-uniform diameters, and the rainfall intensity
could be adjusted by different aperture sizes or water pressure. The fourth type is the
nozzle/sprayer. Water drops are sprayed from the nozzle at a certain initial speed,
then they would be dispersed into the air in various sizes, and finally, they would fall
to the ground. The characteristics of this simulator are close to the features of the
spout. Presently, the technique to simulate rainfall has been greatly improved around
the world. Especially several rainfall simulation halls with automatic operation and
observation systems are boosting in virtue of the combination of rainfall simulators
and computers. Nevertheless, almost all the raindrop generators belong to the four
types mentioned above (Wu and Xu 1995).