36
3 A Conventional Experimental Technique …
kinetic energy to natural rainfall, and the droppers are susceptible to clogging especially if the supplied water is turbid. The thread-droppers and needle-droppers may
be used in a relatively small experimental area. Presently, they could be hardly found
although they had been used in the early studies.
In practice, a mesh screen is often suspended under the thread droppers to produce
droplets with small sizes close to those of natural rainfalls. Nevertheless, due to
surface tension, water may be accumulated in some areas of the screen and then
outsized drops might be formed. In addition, in the early phases, rainfalls from the
thread-droppers or needle-droppers are unstable; shortly after the power source is
turned off, some “redundant” raindrops may be produced with large sizes. Hence,
rainfalls in the two stages mentioned above should be excluded in a rainfall simulation
experiment in order to ensure the accuracy of the experimental results.
3.3 Rainfall Simulators with Spouts or Sprayers
Since the 1960s, rainfall simulators have been developing from non-pressurized
rainfall simulators into sprayer/spout rainfall simulators. The principle behind the
use of pressurized water is that the drops sprayed out from the nozzle have an initial
velocity which may be sufficient for the drops to reach their terminal velocity at a
considerably less falling height than drops falling from the skies. This reduction in
necessary falling height is a notable advantage for pressurized simulators over nonpressurized simulators which rely on gravity and free fall of drops to attain terminal
velocity. The pressurized water rainfall simulators could be divided into two main
groups: the spout rainfall simulators and sprayer rainfall simulators.
The spout rainfall simulators are widely used in experimental studies of soil
erosion. A relatively short distance is needed from the simulator to the model surface
because the water ejects from the nozzle with an initial speed that is large enough
to make water drops break and scatter. Hence, the characteristics of the simulated
rainfall are closer to those of the natural rainfall. In order to study the interflow in
soil, a scientist from the former Soviet Union designed a kind of simulator with
fine nozzle clusters. Nevertheless, the major drawback of this rainfall simulator is
that the area covered by this simulator is small, and the height of the simulator
is not sufficient to make raindrops attain their terminal velocities at which natural
raindrops with the same sizes impact the ground (Zhao and Li 1989). A small spouttype rainfall simulator was also developed by the Institute of Geographic Sciences and
Natural Resources Research, Chinese Academy of Sciences, which could be used in
infiltration experiments with areas of 1 m
2 . Some of the rainfall simulators designed
by the authors of the monograph are shown in Figs. 3.1, 3.2, 3.3 and 3.4. The SX2002
Spout-type Rainfall Simulator designed by Xu et al. (2006b) might be used in an
experimental plot measured 2.5 m × 3.5 m. A high initial spouting speed together
with a falling height of 4 m is enough to make the raindrops reach their terminal
velocities. The uniformity coefficients of the rainfall intensities for the simulators
exceeded 80%. Moreover, the rainfall intensity could be adjusted by controlling the
3 A Conventional Experimental Technique …
kinetic energy to natural rainfall, and the droppers are susceptible to clogging especially if the supplied water is turbid. The thread-droppers and needle-droppers may
be used in a relatively small experimental area. Presently, they could be hardly found
although they had been used in the early studies.
In practice, a mesh screen is often suspended under the thread droppers to produce
droplets with small sizes close to those of natural rainfalls. Nevertheless, due to
surface tension, water may be accumulated in some areas of the screen and then
outsized drops might be formed. In addition, in the early phases, rainfalls from the
thread-droppers or needle-droppers are unstable; shortly after the power source is
turned off, some “redundant” raindrops may be produced with large sizes. Hence,
rainfalls in the two stages mentioned above should be excluded in a rainfall simulation
experiment in order to ensure the accuracy of the experimental results.
3.3 Rainfall Simulators with Spouts or Sprayers
Since the 1960s, rainfall simulators have been developing from non-pressurized
rainfall simulators into sprayer/spout rainfall simulators. The principle behind the
use of pressurized water is that the drops sprayed out from the nozzle have an initial
velocity which may be sufficient for the drops to reach their terminal velocity at a
considerably less falling height than drops falling from the skies. This reduction in
necessary falling height is a notable advantage for pressurized simulators over nonpressurized simulators which rely on gravity and free fall of drops to attain terminal
velocity. The pressurized water rainfall simulators could be divided into two main
groups: the spout rainfall simulators and sprayer rainfall simulators.
The spout rainfall simulators are widely used in experimental studies of soil
erosion. A relatively short distance is needed from the simulator to the model surface
because the water ejects from the nozzle with an initial speed that is large enough
to make water drops break and scatter. Hence, the characteristics of the simulated
rainfall are closer to those of the natural rainfall. In order to study the interflow in
soil, a scientist from the former Soviet Union designed a kind of simulator with
fine nozzle clusters. Nevertheless, the major drawback of this rainfall simulator is
that the area covered by this simulator is small, and the height of the simulator
is not sufficient to make raindrops attain their terminal velocities at which natural
raindrops with the same sizes impact the ground (Zhao and Li 1989). A small spouttype rainfall simulator was also developed by the Institute of Geographic Sciences and
Natural Resources Research, Chinese Academy of Sciences, which could be used in
infiltration experiments with areas of 1 m
2 . Some of the rainfall simulators designed
by the authors of the monograph are shown in Figs. 3.1, 3.2, 3.3 and 3.4. The SX2002
Spout-type Rainfall Simulator designed by Xu et al. (2006b) might be used in an
experimental plot measured 2.5 m × 3.5 m. A high initial spouting speed together
with a falling height of 4 m is enough to make the raindrops reach their terminal
velocities. The uniformity coefficients of the rainfall intensities for the simulators
exceeded 80%. Moreover, the rainfall intensity could be adjusted by controlling the