In this article we will discuss about:- 1. Meaning of Rainfall Stimulators 2. Field Applications of Rainfall Simulators 3. Rainfall Simulator Characteristics 4. Components 5. Important Design Considerations 6. Operation 7. Maintenance 8. Advantages 9. Disadvantages.
- Meaning of Rainfall Stimulators
- Field Applications of Rainfall Simulators
- Rainfall Simulator Characteristics
- Components of Rainfall Simulator
- Important Design Considerations of Rainfall Simulators
- Operation of Rainfall Stimulators
- Maintenance of Rainfall Stimulators
- Advantages of Rainfall Stimulators
- Disadvantages of Rainfall Stimulators
1. Meaning of Rainfall Stimulators:
Rainfall simulation is the technique of applying water to the test plot in a manner to some aspects of natural rainfall. The device used for simulating the rainfall is called rainfall simulator. Nowadays, the rainfall simulators are considered as a best tool for simulating the rainfall for conducting research on evaluation of runoff and soil erosion/soil loss under different kinds of treatments through runoff plot. The rainfall simulator has been used from very long back for runoff, erosion and infiltration studies.
Neff (1977) has reported its common features, i.e., portability, availability of water when and where needed and easiness in conducting research on small size plots (test plots/runoff plots). In brief, with rainfall simulator the runoff and soil loss generated by a standardized rainfall on a plot with standard surface area can also be measured. The duration, intensity and kinetic energy of rain event can be simulated in such a way that a high sensitivity of test results for difference in soil properties can be obtained.
2. Field Applications of Rainfall Simulators:
The rainfall simulator is a device for creating artificial rainfall on a small area. In rainfall simulator, there is provision to change the rainfall characteristics as per requirement. On using runoff plot the experiments on rainfall-runoff and soil erosion/loss can be suitably done under different sets of treatments related to soil and vegetations.
In general, the rainfall simulators can be used for following purposes:
i. Studies on evaluation of relative protection developed by different crop cover densities.
ii. Experiment on evaluation of relative protection developed at different stages during crop period.
iii. Studies on prediction of relative erodibility of different soils.
iv. Evaluation of infiltration characteristics under different sets of treatments.
v. Evaluation of runoff and soil erosion under different types of treatments related to soil and vegetations.
The characteristics of rainfall simulator are related to the simulated rainfall. With rainfall simulator, it is always desirable to simulate the rainfall of same physical characteristics as of natural rainfall, but in practice it is never obtained. However, because of simple construction and affordable cost, it is commonly used for research works.
The main desirable characteristics of rainfall simulators are mentioned as under:
i. Drop size
ii. Drop size distribution
iii. Fall velocity of raindrop, and
iv. Intensity of simulated rainfall.
i. Drop Size:
As for as drop size is concerned, in natural rains it varies maximum up to 6 or 7 mm in diameter. If the size of any raindrop gets exceed the maximum limit of drop size, then they become unstable and most commonly they get break into smaller drops.
The median drop diameter by volume varies between 2 and 3 mm, depending on the intensity of rainfall, is shown in Fig. 9.3. In simulated rainfall all above characteristics are desirable. With rainfall simulator, the rainfall of any desired intensity can be formed; but it needs special design and construction.
ii. Drop Size Distribution:
The drop size distribution varies in each rain events. However, for research point of view a uniform distribution is always desirable. Uniformity in raindrop size distribution represents to some extent uniformity in rainfall characteristics.
Normally, the cyclonic rains in temperate regions are composed of small to average size drops. And high-intensity tropical thunderstorms are with larger drops. In simulated rainfalls the uniformity in drop size distribution is always desirable.
iii. Fall Velocity:
In natural rainfall the raindrops strike the ground surface with terminal velocity. Terminal velocity of raindrop refers to that velocity which is after balancing the gravitational force and resistance of drop falling through the air. Terminal velocity depends on the drop size. Its value can be maximum up to 9 m/s for the largest drops (Fig. 9.4).
In case of rainfall simulator the formation of rainfall with falling raindrop at terminal velocity is not very common; but by improving the design of simulator it can be achieved to some extent. It can also be done by adjusting the height of rainfall simulator.
iv. Rainfall Intensity:
It is the rate of fall of raindrops on the ground surface during rainfall. It is expressed as mm/h or m/h. Rainfall intensity is also one of the very important characteristics of rainfall desirable to get develop from rainfall simulator. In case of rainfall simulator it can be achieved by adjusting the pressure and selecting the spray nozzles.
Rainfall intensity signifies the following points:
a. Rainfall Intensiveness:
If intensity is very high, i.e., more than 7.5 mm/h then rainstorm is called intense rainfall. Such rain events take place for shorter duration and also for lesser aerial extent. Kinetic energy of these rainfalls is very high, as result they are very causative to produce soil erosion. In design of rainfall simulator it is considered as an important parameter. In rainfall simulators the rainfall intensity can be varied by varying the pressure.
b. Kinetic Energy of Rainstorm:
It is directly related to the rainfall intensity. The relationship between drop size (median volume diameter) and rainfall intensity is presented in Fig. 9.3. This property of rainfall is considered to be very important regarding soil erosion/soil loss point of view. The KE of drop determines its effectiveness on detachment of soil particles by varying the degree of impacting force on the ground surface.
A rainfall with high intensity creates greater impacting force, as result there is greater soil detachment from the soil mass; and vice-versa. This parameter is desirable from simulator in soil erosion studies. Fig. 9.4 presents the variation in kinetic energy of rainfall with intensity. The upper limit of rainfall intensity is about 75 mm/h.
The intensity of rainfall is not related to mean annual rainfall. The intensity of arid or semi-arid rainfalls can reach as high as in humid tropics, but with lesser frequency. On rainfall intensity and KE several researchers have developed their individual relationships for different locations, shown in Fig. 9.5.
4. Components of Rainfall Simulator:
A rainfall simulator essentially consists of following components:
i. Sprinkler nozzle.
ii. Adjustable support for the sprinkler.
iii. Ground frame.
The sprinkler nozzle of rainfall simulator is of special type. It is used to perform rainfall over the ground surface. In nozzle a built-in-pressure regulator is equipped for formation of standard rainfall. The adjustable support is used for providing support to the sprinkler system. The ground frame is placed on the soil surface; its function is to prevent the lateral movement of water from the runoff plot to the surrounding soil.
The components of rainfall simulators are described as under:
i. Sprinkler Nozzles:
In rainfall simulator system, the sprinkler nozzles are connected with a calibrated cylindrical reservoir (A), which capacity is about 2300 liters. In reservoir a scale is provided to read the water level. In simulator’s nozzles about 49 capillaries are provided for releasing the water or to form rainfall. The lower ends of the capillaries are fitted with a small tube, which function is to control the drop size and drop frequency.
The rate of water release from the system depends on the pressure head, the length and inner diameter of capillaries. The pressure head on capillaries can be varied with the help of aeration tube, by moving up/down. The aeration tube also controls the intensity of rainfall as per requirement.
ii. Adjustable Support:
The adjustable support (B) is provided for positioning and leveling the sprinkler nozzles. For this purpose 2 levels and 4 knobs are equipped in the system. The adjustable legs are provided in the ground frame. The legs are normally made of stainless steel.
iii. Ground Frame:
It is generally made of Aluminum sheet, which is fixed on the soil surface on 4 large legs. The function of frame is to prevent the lateral movement of water from the test plot to the surrounding soil. A gutter of suitable size is also provided at the d/s side of the plot for collection of runoff and sediment sample in collection box.
The following accessories are essentially required for rainfall simulators:
1. Soil wetting jar.
2. Water storage tank.
3. Sample collection box.
4. Sample bucket.
5. Transport case.
The soil-wetting jar is a plastic box; is fitted with perforated lid. Its function is to wet the plot area before sampling. The water storage tank is for storing the water; it is connected to a valve for allowing the water to the sprinkler nozzle. The sample collection box is for collecting the samples; it is made of plastics.
The sample bucket is also made of plastics; its function is to store and transport the sample materials. The transport case is made of aluminum; its function is to store the rainfall simulator and accessories; and facilitate in transpiration from one place to another. Transport case is equipped with two grips, a hinge and two locks, on which the brass padlock is fitted.
The rainfall simulator needs calibration before its use for collecting the data. Calibration provides guidelines about corrections required in collected data, depending on the variations in simulated rainfall pattern or other features in comparison to the desired rainfall.
The calibration is carried out under following steps:
i. Install the ground frame with the help of 4 legs/nails.
ii. Install the adjustable support on the ground frame; and level the support using 2 levels and 4 knobs.
iii. Close the aeration pipe with the plug.
iv. Equip the sprinkler nozzle upside on the support.
v. Install the water storage tank on the top of transport case, above the sprinkler nozzle.
vi. Connect the tube to the water storage tank. Also, connect the tube to the sprinkler system for allowing water, and finally to operate the rainfall simulator.
vii. Fill the water in storage tank to supply to the sprinklers.
viii. Fill the water to sprinkler system to escape the air.
ix. Disconnect the sprinkler, if it has been filled with the water and all the air has been escaped through the capillaries.
x. Keep the sprinkler to its sprinkling position.
xi. Adjust the rainfall intensity with the help of aeration lube.
xii. Record the water level in the storage tank.
xiii. Remove the plug from the aeration pipe; and starts the simulation and record the lime.
xiv. After 3 minutes, stop the simulation by placing the plug on aeration pipe.
xv. Record the requisite observations at the given time intervals.
xvi. Repeat the above steps; and re-adjust the aeration pipe.
5. Important Design Considerations of Rainfall Simulators:
There are host of practical considerations regarding design and construction of rainfall simulators; few important amongst them are outlined as under:
1. Design Factors:
The power source, water supply and accessibility are the important factors for design of rainfall simulators. In most of the simulators, the motors and pumps are used, which need to be operated with the help of power source for supplying the water to simulation unit. But when water source is located at higher elevation than the elevation of simulator, then motors/pumps are not required for the system, because water automatically gets flow to the simulator with pressure under gravity effect.
If electricity is available in the area, then electrical motors should be given priority, because they are cheaper as compared to the diesel pump sets. Sometimes, where electric power source is not there, and availability of diesel is also not very common, then batteries are also used for operating the system, but it is very expensive.
2. Water Supply:
The requisite capacity of water supply source and supply rate depend very much on the size of rainfall simulator. The smaller simulators equipped with dropper type nozzles require less supply of water, because they sprinkle the water over test plot with little wastage of water, outside the test plot.
If rainfall simulator is equipped with the sprayer for making rainfall then large water supply is required, because such rainfall simulators are usually run with higher intensities; and the area coverage is also being more. In this condition the capacity of storage tank/water source should be more. The correct capacity of water supply source can be determined based on the amount of water required, and mode of spray.
It is very important requisite for design and construction of rainfall simulator. As for as possible the test plot should be located at the site close to the road, because such locations make the system installation, operation and care & maintenance, easy. Also, if there happens any faults in the system then it can be easily transported for repair, if there is a link road. In brief, the accessible sites are always desirable for installation of rainfall simulator.
It refers to the construction material used for fabricating the rainfall simulator, must be of good quality. Normally, it is observed that when system is operated after erection, it does not work properly, which may be because of improper design and construction.
Also, after lapse of lime, few components become out of order; the motor gets burn; water supply line is clogged; pump becomes jam etc. In this condition, if the repair facility is not available in nearby market, then the system becomes failure to fulfill the objective of construction. Overall, for reliability point of view the simulator should always be as simple as possible, and easy to repair.
6. Operation of Rainfall Stimulators:
The rainfall simulators are constructed over test plots, called runoff plots. The size of simulator is generally tallied with the size of plot. The runoff plot and rainfall simulator together form a complete set for soil loss studies. In which, the runoff plot is constructed first and then the simulator over the same. The newly constructed simulator is tested for checking the nozzles discharge; the operating pressure; the leakage in pipeline; coverage area of simulated rainfall etc.
Also, the drop size distribution, rainfall intensity etc., are determined before using the simulator for real experimental work. If the test data favours the appropriateness of constructed rainfall simulator, then simulator is allowed for using specific research experiments.
The operation of rainfall simulator depends on the experiment requirement. For example – the water supply rate, pressure etc. are maintained for generating specific rainfall intensity, raindrop size distribution etc. as required for the experiment, particular.
The system should be avoided to operate, especially when wind velocity is very high. In high wind velocity condition the falling raindrops get distort, as result the drops are likely to fall out of the test plot; and also the rainfall intensity and drop size distribution get affected. A high wind velocity also causes evaporation effect on water dropping, which is the loss of water due to evaporation.
7. Maintenance of Rainfall Stimulators:
It is carried out in following respects:
i. Clean the sprinkler nozzles thoroughly, time to time.
ii. Clean the capillaries, thoroughly.
iii. Before storage or transportation of rainfall simulator, clean all the parts
iv. If there is any leakage in system, remove immediately.
v. If there is formation of rust in the pipeline, remove immediately.
vi. In test plot if there is hole, fill them with soils and remove.
8. Advantages of Rainfall Stimulators:
The main advantages of rainfall simulators are outlined as under:
i. Rainfall simulator based research works are cost effective. It is because of the reason that the degree of control over rainfall characteristics can be easily exercised, which results into reduction in cost per unit data collection as compared to the unit cost of long-term experiment based on natural rainfall. The long-term experiments not only require huge investment on instrumentation, but also require sufficient money on maintenance for keeping the runoff plot and instruments in proper order, especially when system is not in use.
ii. It enables the collection of data very quickly without waiting for natural rainfall.
iii. It provides a good control on rainfall characteristics, which enables to generate the data as per requirement.
iv. For an investigator it facilitate to provide the requisite characteristics of rainfall, i.e., a desired rainfall intensity, drop size distribution and other features for a given treatment. On the other hand, with natural rainfall it is never possible.
v. It provides a means for generating the data in very short time span.
vi. Rainfall simulator can be easily constructed over runoff plot.
vii. It results into application of desired treatments related to soil, vegetation or others, easily for study points of view.
viii. Rainfall simulator provides in-situ as well as laboratory based data generation.
ix. Simple and easy to install.
9. Disadvantages of Rainfall Stimulators:
These are mentioned below:
i. Its construction on large size plots, say for 100 m2, is very expensive.
ii. The generated data from the plot area ranging from few sqm to 100 sqm or little more, cannot be extrapolated to larger area.
iii. The data obtained from rainfall simulator does not represent the data from natural rainfall. It is always artificial. The rainfall simulators do not produce the drop size distribution as natural rainfall, causing into inappropriateness in collected data.
iv. Also, there is quite difference in rainfall intensity produced by the rainfall simulator and of the natural rainfall.
v. Rainfall simulators don’t create the drop size producing terminal velocity as normally found in case of natural rainfall. In this way, the generation of soil erosion data at terminal velocity is not possible with the rainfall simulator.
vi. The simulated rainfall can be affected by wind; however, it can be nullified by erecting windbreak across the blowing wind.
vii. For few studies the simulators are not suitable; they are given as under:
a. Crops grown on contour.
b. Comparison of treatments.
c. Studies on physical processes requiring variations in rainfall characteristics such as intensity, duration, frequency etc.