3.2 Rainfall Simulators with Thread Droppers or Needle Droppers
31
3.2 Rainfall Simulators with Thread Droppers or Needle
Droppers
Various kinds of rainfall simulators may be designed to meet the demands of soil
conservation experiments according to the mechanisms of the raindrop generator. As
mentioned above, the spot and sprayer are frequently used all over the world. Table 3.1
shows the types and parameters of the rainfall simulators developed in recent years.
It could be concluded that the hand-controlled basic simulators will continue to play
an important role in the soil erosion experiments in virtue of convenient manipulation
and low cost.
As mentioned before, the initial velocities of the raindrops generated from the
thread droppers and needle droppers are zero. The raindrop falls on the ground along
cotton or wool fibers from the water-supply pipe. One of the major drawbacks of these
simulators is that the particle sizes of the simulated raindrops are larger than those of
natural rainfalls. Moreover, different from the natural rainfall events, the distribution
of drop sizes is almost uniform. Besides the cotton threads and wool fibers, the
rainfall droppers may also be produced from fine glasses or brasses. Bowyer-Bower
and Burt (1989) made a rainfall simulator with the Tygon tubing in a length of 15 mm,
an internal diameter of 0.7 mm, and an external diameter of 2–3 mm. The former
determined the rates of water drop formation while the latter controlled the sizes of
the raindrops. The drop sizes were 2–3 mm or so.
The sizes of the raindrops formed by the needle-style simulators are close to
those created by the thread droppers. However, relatively small particles would be
generated if some measures were adopted, e.g., a wire mesh suspended beneath
the drop-formers (Clarke and Walsh 2007). Gunn and Kinzer (1949) reported that
by blowing a co-axial air stream directed vertically downwards over a single sized
hypodermic needle and varying the rate of water flow, they could produce raindrops
within an astonishing range of 0.25–10,000 mg.
By the end of the twentieth century, a large-scale thread-dropper rainfall simulator
was built in the Institute of Geographic Science and Nature Resources Research,
CAS. The simulator was equipped with a computer system to control the rainfall
intensity, and an electric pendulum to crush the raindrops. In addition, a V-shaped
groove was installed under each branch of pipe to discharge the overflow, which is
very likely to generate large water drops. The rainfall intensity and raindrop size
distribution of this kind of rainfall simulator could be adjusted by replacing the
needles with different diameters. A high uniformity may also be achieved with the
thread-dropper rainfall simulator.
The thread droppers and needle droppers have advantages of low-intensity threshold, high-intensity uniformity, and easy performance. However, in general, a relatively great height is required for the simulators mentioned above, since the sizes
of the raindrops are large and their initial dropping velocities are close to zero. In
fact, the raindrops would be spouted with a certain initial velocity if high-pressure
water was used (Battany and Grismer 2000). In addition, the thread-droppers and
needle-droppers are often inferior to the spouts and sprayers in achieving a similar
31
3.2 Rainfall Simulators with Thread Droppers or Needle
Droppers
Various kinds of rainfall simulators may be designed to meet the demands of soil
conservation experiments according to the mechanisms of the raindrop generator. As
mentioned above, the spot and sprayer are frequently used all over the world. Table 3.1
shows the types and parameters of the rainfall simulators developed in recent years.
It could be concluded that the hand-controlled basic simulators will continue to play
an important role in the soil erosion experiments in virtue of convenient manipulation
and low cost.
As mentioned before, the initial velocities of the raindrops generated from the
thread droppers and needle droppers are zero. The raindrop falls on the ground along
cotton or wool fibers from the water-supply pipe. One of the major drawbacks of these
simulators is that the particle sizes of the simulated raindrops are larger than those of
natural rainfalls. Moreover, different from the natural rainfall events, the distribution
of drop sizes is almost uniform. Besides the cotton threads and wool fibers, the
rainfall droppers may also be produced from fine glasses or brasses. Bowyer-Bower
and Burt (1989) made a rainfall simulator with the Tygon tubing in a length of 15 mm,
an internal diameter of 0.7 mm, and an external diameter of 2–3 mm. The former
determined the rates of water drop formation while the latter controlled the sizes of
the raindrops. The drop sizes were 2–3 mm or so.
The sizes of the raindrops formed by the needle-style simulators are close to
those created by the thread droppers. However, relatively small particles would be
generated if some measures were adopted, e.g., a wire mesh suspended beneath
the drop-formers (Clarke and Walsh 2007). Gunn and Kinzer (1949) reported that
by blowing a co-axial air stream directed vertically downwards over a single sized
hypodermic needle and varying the rate of water flow, they could produce raindrops
within an astonishing range of 0.25–10,000 mg.
By the end of the twentieth century, a large-scale thread-dropper rainfall simulator
was built in the Institute of Geographic Science and Nature Resources Research,
CAS. The simulator was equipped with a computer system to control the rainfall
intensity, and an electric pendulum to crush the raindrops. In addition, a V-shaped
groove was installed under each branch of pipe to discharge the overflow, which is
very likely to generate large water drops. The rainfall intensity and raindrop size
distribution of this kind of rainfall simulator could be adjusted by replacing the
needles with different diameters. A high uniformity may also be achieved with the
thread-dropper rainfall simulator.
The thread droppers and needle droppers have advantages of low-intensity threshold, high-intensity uniformity, and easy performance. However, in general, a relatively great height is required for the simulators mentioned above, since the sizes
of the raindrops are large and their initial dropping velocities are close to zero. In
fact, the raindrops would be spouted with a certain initial velocity if high-pressure
water was used (Battany and Grismer 2000). In addition, the thread-droppers and
needle-droppers are often inferior to the spouts and sprayers in achieving a similar