**3. Rainfall simulator classifications**

There are three classification of rainfall simulator: drip, pressurised nozzle (PN) [3–5] and hybrid [6] rainfall simulator.

**Drip simulator**: also known as drop former (DF) [4] uses hanging yarn or hypodermic needles to produce drops of necessary size at zero velocity. Its impact velocity is attained by free fall which made others defined it as non-pressurised simulator [7]. The drilled holes and drop height determines the diameter of the raindrop and kinetic energy respectively [3] (see **Figure 1**).

It is capable of producing drops which ranges from 3 to 6 mm depending on the diameter holes [8]. Main advantage of the drip simulator is that it has the ability to produce relatively large drops at low application rate [5, 9]. It has the following disadvantages [10]:

i.It is impractical for field since it requires huge distance of at least 10 m height to attain terminal velocity.

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outdoor [7].

*A Revisit of Rainfall Simulator as a Potential Tool for Hydrological Research*

a limited plot depending on the size of the hanging yarn.

ii.Another constraint of this simulator is that simulation is only carried out on

iii.It does not produce distribution drops unless a variety of drop forming sized

**Pressurised Simulator (PN)**: produces drop distribution that includes both small and large range of drop sizes with nonzero initial velocity and an impact velocity similar to terminal velocity of raindrops. In order to obtain drops of suitable sizes while upholding high velocity, high discharge nozzles are required [7]. The application rates are reduced by means of an intermittent moving object which intercepts the rainfall. An example of this type of simulator was developed by [12]. The authors found out that it utilises the best nozzle known as "yet-for-rain simulation" (spraying system 80,100-veejet nozzle). But problem with the 80,100-veejet nozzles was that it did not simulate rainfall energy characteristic and is still better than other nozzles. This type of rainfall simulator provides about 80% of the required kinetic energy per volume of natural rain [5, 12, 13]. This nonzero pressurised nozzle has the following advantages over the hanging yarn simulator [10] as

i.They can be used in the field and their intensities can be varied more than

ii.Since the nozzle simulator has an initial velocity greater zero, it requires shorter height to reproduce the terminal velocity obtained from natural rain.

iii.According to Home, (2017), this simulator is often portable compared to

**Hybrid type simulator**: uses the principles of pressurised and drip former techniques of simulation incorporated together. It was first developed by [15] to reduce the kinetic energy impact of the rainfall, but the research indicated that the technique reduced the kinetic energy at the detriment of the rain uniformity [3]. Wildhaber et al. used a similar method by placing mesh 0.5 m of aperture 2 mm × 1.7 mm under a spraying nozzle. The obtained result was not far from the non-pressurised simulator type. Carvalho et al. also designed a pressurised nozzle simulator with mesh placed 2.35 m below the nozzle to change rainfall characteristics (see **Figure 3**), and varying the nozzles and mesh types. The results varied based on the aperture of the meshes employed. Conclusively, the hybrid simulator was noted as suitable tool to

According to its transportability, rainfall simulators are classified as indoor and

**Indoor rainfall simulator**: this rainfall simulator is used for modelling precipitation in a controlled environment. It is also known as Laboratory scale model. This simulator reduces lot of disadvantages incurred by the transportable type of rainfall simulator [7]. For example, Darboux et al. designed an indoor rainfall simulator system that simulated infiltration, run-off and erosion (see **Figure 4**), and the output was effective but the system was constrained with lack of non-recycle of the

**Outdoor rainfall simulator**: could be portable or large depending on the projected purpose. Many of these types of simulator have been used to relate soil surface characteristics and controlling to runoff, infiltration and erosion as influ-

*DOI: http://dx.doi.org/10.5772/intechopen.93532*

tubes are used.

presented by **Figure 2**:

drop former.

the drop forming type of simulator.

assessing erodibility of different types of soil.

water system as well as not portable.

enced by different parameters [18–22].

**Figure 1.** *Drop formers simulator [11].*

*Agrometeorology*

infiltration.

disadvantages [10]:

**2. Rainfall simulator**

**3. Rainfall simulator classifications**

[3–5] and hybrid [6] rainfall simulator.

to attain terminal velocity.

raindrop and kinetic energy respectively [3] (see **Figure 1**).

Rainfall simulators (RS) are device designed to model the characteristics of natural rainfall to the nearest possible. It can be used to determine inter-rill erosion rates and their dependent rainfall and soil parameters [1]. It has been a tool for agricultural research and has been used for different studies ranging from determination of soil characteristic, such as infiltration rate, surface runoff, storage or erosion process studies [2]. Yakubu and Yusop [3] pointed out two most important aspects to note while using rainfall simulator; the method used to simulating rainfall and runoff from plot. Consideration was not given to

There are three classification of rainfall simulator: drip, pressurised nozzle (PN)

It is capable of producing drops which ranges from 3 to 6 mm depending on the diameter holes [8]. Main advantage of the drip simulator is that it has the ability to produce relatively large drops at low application rate [5, 9]. It has the following

i.It is impractical for field since it requires huge distance of at least 10 m height

**Drip simulator**: also known as drop former (DF) [4] uses hanging yarn or hypodermic needles to produce drops of necessary size at zero velocity. Its impact velocity is attained by free fall which made others defined it as non-pressurised simulator [7]. The drilled holes and drop height determines the diameter of the

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**Figure 1.**

*Drop formers simulator [11].*


**Pressurised Simulator (PN)**: produces drop distribution that includes both small and large range of drop sizes with nonzero initial velocity and an impact velocity similar to terminal velocity of raindrops. In order to obtain drops of suitable sizes while upholding high velocity, high discharge nozzles are required [7]. The application rates are reduced by means of an intermittent moving object which intercepts the rainfall. An example of this type of simulator was developed by [12]. The authors found out that it utilises the best nozzle known as "yet-for-rain simulation" (spraying system 80,100-veejet nozzle). But problem with the 80,100-veejet nozzles was that it did not simulate rainfall energy characteristic and is still better than other nozzles. This type of rainfall simulator provides about 80% of the required kinetic energy per volume of natural rain [5, 12, 13]. This nonzero pressurised nozzle has the following advantages over the hanging yarn simulator [10] as presented by **Figure 2**:


**Hybrid type simulator**: uses the principles of pressurised and drip former techniques of simulation incorporated together. It was first developed by [15] to reduce the kinetic energy impact of the rainfall, but the research indicated that the technique reduced the kinetic energy at the detriment of the rain uniformity [3]. Wildhaber et al. used a similar method by placing mesh 0.5 m of aperture 2 mm × 1.7 mm under a spraying nozzle. The obtained result was not far from the non-pressurised simulator type. Carvalho et al. also designed a pressurised nozzle simulator with mesh placed 2.35 m below the nozzle to change rainfall characteristics (see **Figure 3**), and varying the nozzles and mesh types. The results varied based on the aperture of the meshes employed. Conclusively, the hybrid simulator was noted as suitable tool to assessing erodibility of different types of soil.

According to its transportability, rainfall simulators are classified as indoor and outdoor [7].

**Indoor rainfall simulator**: this rainfall simulator is used for modelling precipitation in a controlled environment. It is also known as Laboratory scale model. This simulator reduces lot of disadvantages incurred by the transportable type of rainfall simulator [7]. For example, Darboux et al. designed an indoor rainfall simulator system that simulated infiltration, run-off and erosion (see **Figure 4**), and the output was effective but the system was constrained with lack of non-recycle of the water system as well as not portable.

**Outdoor rainfall simulator**: could be portable or large depending on the projected purpose. Many of these types of simulator have been used to relate soil surface characteristics and controlling to runoff, infiltration and erosion as influenced by different parameters [18–22].

**Figure 2.** *Pressurised nozzle simulator [14].*

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**Figure 5.**

*Outdoor rainfall simulator [13].*

**Figure 4.**

*Rainfall simulation building [17].*

*A Revisit of Rainfall Simulator as a Potential Tool for Hydrological Research*

A research carried out at Duke University Durham, using large transportable

pressure washing nozzles which produced rainfall intensity of 62.43 mm/h and 32 mm h−1 with a corresponding uniformity coefficient (C.U) of 76.65 and 62% [13]. [23] developed a portable field simulator for use in hillside and obtained a consistent raindrop size of 2.58 mm with an intensities of 20 to 90 mm h−1 and C.U of 91.7% at an intensity of 60 mmh−1. In a similar event Abudi et al. [24] also designed and constructed a portable rainfall simulator for field investigation of runoff, the drop size obtained was 1.5 mm with a ground hitting velocity near that of natural rainfall and energy flux 76% of the natural rainfall. All the simulators offered good

(**Figure 5**). The system was tested with common

*DOI: http://dx.doi.org/10.5772/intechopen.93532*

rainfall simulator of area 15.12 m2

**Figure 3.** *Hybrid simulator [16].*

#### *A Revisit of Rainfall Simulator as a Potential Tool for Hydrological Research DOI: http://dx.doi.org/10.5772/intechopen.93532*

A research carried out at Duke University Durham, using large transportable rainfall simulator of area 15.12 m2 (**Figure 5**). The system was tested with common pressure washing nozzles which produced rainfall intensity of 62.43 mm/h and 32 mm h−1 with a corresponding uniformity coefficient (C.U) of 76.65 and 62% [13]. [23] developed a portable field simulator for use in hillside and obtained a consistent raindrop size of 2.58 mm with an intensities of 20 to 90 mm h−1 and C.U of 91.7% at an intensity of 60 mmh−1. In a similar event Abudi et al. [24] also designed and constructed a portable rainfall simulator for field investigation of runoff, the drop size obtained was 1.5 mm with a ground hitting velocity near that of natural rainfall and energy flux 76% of the natural rainfall. All the simulators offered good

**Figure 4.** *Rainfall simulation building [17].*

*Agrometeorology*

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**Figure 3.**

*Hybrid simulator [16].*

**Figure 2.**

*Pressurised nozzle simulator [14].*

**Figure 5.** *Outdoor rainfall simulator [13].*

#### *Agrometeorology*

performance. The merit of these simulators is that it can be used to study field parameters required for hydrologic modelling on any surface including the ones covered with vegetation. But they were limited by problem of natural rainfall which resulted to dismantling the setup when experiment schedule was not over and water was not recycled.

From the numerous studies carried out on the simulation of rainfall both in field and laboratory experiment, two merits of rainfall simulator in a research carried out in 2010 using laboratory simulator [25] were pointed out as:

