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In this article we will discuss about:- 1. Meaning of Runoff Plot 2. Types of Runoff Plots 3. Site Selection 4. Construction 5. Design and Installation 6. Care and Maintenance 7. Use 8. Problems.
Contents:
- Meaning of Runoff Plot
- Types of Runoff Plots
- Site Selection for Runoff Plot
- Construction of Runoff Plot
- Design and Installation of Runoff Plot
- Care and Maintenance of Runoff Plot
- Use of Runoff Plot and Runoff Plot Data
- Problems with Runoff Plot
1.
Meaning of Runoff Plot:
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It is a structure used to measure the overland flow and soil loss thereby under controlled condition. Runoff plots are constructed directly in the field. For erosion/soil loss studies these are cropped with the crops taken into consideration for study. Also, the soil type and slope of the plot can be changed as per requirement.
The size of plot should ideally be as large as the estimated size for the project. Smaller runoff plots should be avoided, because the results obtained from such plots are rather misleading. Runoff plots are extensively used for erosion or soil loss studies. Plot design, instrumentation and data collection procedures vary greatly from place to place.
2. Types of Runoff Plots:
In general, the runoff plots are of following types:
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i. Runoff plot with natural rainfall
ii. Runoff plot with artificial rainfall facility; and
iii. Bounded or un-bounded runoff plots.
i. Runoff Plot with Natural Rainfall:
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The runoff plots with natural rainfall are simple in construction. These are constructed as per design norm and in proper size, in the open field. The collection of data is carried out depending on the occurrence of rainfall.
The soil and other parameters like slope, vegetations etc., are changed in the runoff plot, as per need. The generation of data with such type of runoff plots is dependent on the rainfall occurrence, is considered as one of the main demerits of this type of runoff plot.
ii. Runoff Plot with Artificial Rainfall Facility:
The runoff plots with artificial rainfall facility is very common for research studies. These plots are equipped with rainfall simulator. The rainfall simulator facilitates the artificial rainfall on the plot area.
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In rainfall simulator there is provision to simulate the rainfall of different intensities. As result, the impact of rainfall of various intensities on runoff and soil erosion or soil loss can be easily evaluated on desired soil types and soil steepness.
Also, with this type of runoff plots there is no waiting of rainfall occurrence. For runoff plot based studies there have been developed different types of rainfall simulators. For researchers, there are various options about selection of most suitable rainfall simulators. However, the simulator imposes limitations about involvement of heavy expenditure of money for using on large size runoff plots. The simple and cheaper simulators are restricted to small plots of few square meters area.
From runoff plots the measurement of runoff and soil loss can be easily done. Nowadays, the devices have been designed for continuous measurement of soil loss rate. In runoff plots they can be easily equipped for the purpose. Normally, they are the flumes which measure the runoff and soil movement both. But they involve heavy costs and sophisticated instrumentation.
iii. Bounded or Un-Bounded Runoff Plot:
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The bounded runoff plots are constructed with a well-defined boundary. In other words, they have a specific area. This type of runoff plot facilitates the measurement of runoff and soil loss, both from a fixed area. The measured value of runoff or soil loss data can be extended for larger area, by multiplying the unit value of measured parameter with the area. These runoff plots can also be equipped with rainfall simulator, and cannot be, also.
The unbounded runoff plots have no well-defined boundary, i.e., their area is not fixed- Such types of runoff plots are usually referred to as Gerlach Troughs after inventor’s name. These runoff plots consist of collecting unit called gutter, which is let into the soil surface and connected to a small container at d/s side for collecting the sample. The gutters are constructed with the designed specification.
Runoff Plot Size:
The size of runoff plots is decided on the basis of purpose of their use. The size may vary from 1 sq m to 1 ha.
The size of runoff plots is divided into following three categories:
1. Micro Plots:
Size of micro runoff plots varies from 1-2 sqm. These runoff plots are suitable for very simple studies; and conducting the research trials for making comparison between two treatments. In other words, such plots are suitable for conducting small-scale research works, e.g., to demonstrate the effect of vegetation on runoff, soil erosion etc.
The accuracy in measured data is not so good with such runoff plots. These plots can also be used for study of slope steepness/length effects on soil erosion and splash erosion. These are cheaper and simple in construction; and could be used for field level investigation.
2. Small Scale Plots:
The small-scale runoff plots are usually in the size of about 100 sqm area. This type runoff plots are very common in USA for soil loss studies. These are most commonly used for runoff-soil erosion/loss based trials on cropping practices, cover effects, rotations, and those practices which can be applied to small plots as it would be on field scale, and where the effect is expected to be unaffected by plot size. Its original size is 6-feet as the width and 72.6 feet length covering 0.01 acre area.
In this type of runoff plot, there is significant bordering effect on runoff and soil loss. However, as per research findings the bordering effect could be minimized by keeping the plot width 5m and length 20 m. These runoff plots are not found appropriate to evaluate the effects of channel terraces, bench terraces, or any other mechanical measures of soil erosion/soil loss control.
3. Large Scale Plots:
Size of these runoff plots is about 1 ha. These are suitable for evaluating the effect of any type of treatments that cannot be applied accurately in small or micro runoff plots. Since size is very large, therefore, the treatments such terracing, bunding etc. can be accurately applied to study their effect on runoff and soil loss. Also, the tillage operations using heavy machineries such as tractors etc. can be performed.
In other words, it is fully possible to use all cultivation practices and farm operations in such type of runoff plots. However, the major source of error in such runoff plots is leakage of water across plot boundaries. To eliminate this problem, these plots are constructed with a low height concrete boundary.
Collecting Unit:
It is an important component of runoff plot. Its function is to collect the data on runoff and soil loss generated from the plot area due to rainfall (natural or artificial). The size of collecting unit depends on the plot size. For factor of safety, the plot size should also be decided by considering the extreme events of rainfall (in case of natural rainfall). For a plot of 2 m x 2 m size with 100 mm rainfall (80% runoff) needs collecting tank of 320 lit capacity.
Similarly, for the plot of 5 m x 20 m size, the required size of collecting tank is about 8000 lit. If large size of tank is not manageable then to fulfill the requisite capacity of tank more than one tank should be constructed in series. For the plots of 1 ha or more in size, a special instrumentation is required for collecting the runoff and continuous sampling. The following two factors must be taken into consideration while designing the size and capacity of collecting unit.
They are:
i. Must be capable to handle the maximum probable flow rate; and
ii. Must be capable to store the maximum probable runoff quantity.
Length – Width Ratio of Runoff Plot:
There is no standard for the length: width ratio of runoff plot. The shorter plots are normally undesirable because they are likely to get interfere with the rill formation during overland flow. However, the width of plot should be such that there should not be any obstruction in soil manipulation. Also, for nullifying the bordering effect and also for easy access of recording, there should be buffer strips at the sides and top of the plot, both.
3. Site Selection for Runoff Plot:
The construction site of runoff plots plays very important role regarding collection of needful data and data authenticity. Therefore, a proper care should always be taken about selection of construction site of runoff plots in the field.
The followings are few important points to consider, while selecting the site for construction of runoff plots:
i. The site should not be selected, especially where susceptibility of rill formation.
ii. The soil of construction site should not have cracking problem.
iii. At construction site there should not exist gullies crossing the plot.
iv. There should also not be the depressions.
v. The soil gradient along the plot should be regular, and free from local depressions.
4. Construction of Runoff Plot:
Following steps are followed for constructing the runoff plots:
i. Select the suitable site, adopting proper guidelines etc.
ii. Demarcate the dimension of runoff plot to be constructed on the ground surface.
iii. Erect the boundary using suitable materials. The stone, bricks etc., can be suitably used for this purpose. However, for belter result the boundary should be constructed with some depth of foundation. Also, for better result place metal sheet or wooden planks around the plot to stop the water incoming into the plot from outside; and vice-versa. The sheet must be driven into the soil for at least 15 cm depth.
iv. At the lower end of plot provide a gutter to collect the runoff. The gutter should have the gradient about 1% towards collection tank.
v. Backfill and compact the soil around the gutter.
vi. To allow a smooth flow of water from the plot into the gutter, construct an apron between the gutter and the lower end of plot.
vii. Construct a collecting tank of suitable size at the lower end of gutter. The tank may be constructed using stone masonry, bricks, concrete blocks, or sometimes in the form of buried barrel. The tank should be covered to protect evaporation loss and direct rainfall.
viii. Install rain gauge for measuring the rainfall data.
The view of standard runoff plot is shown in Fig. 9.1.
5. Design and Installation of Runoff Plot:
The basic purpose of runoff plots is to facilitate the rainfall based soil erosion/soils studies either it is in the field or in laboratory under different soil and crop conditions. In which, the selection of crops and soils for plot studies is not difficult. But for proper design and construction of plots, the parameters such as selection of slopes, sites, sizes, replications and appropriate installation procedure are very important to take decision in very careful manner.
The slope of runoff plot depends on the terrain of area; the main slope on which the test crops are grown, and availability of the sites for such studies. In steep land condition, it is not essential to limit the slope 9% or to follow the standards of Universal Soil Loss Equation. For example in Jamaica and Thailand the runoff plots have been constructed on 30% slope, which is representative of hill cultivation conditions of their localities. In general, the site selection for construction of runoff plot should be in such a way that the collection of observations, supervision, cares and maintenance can be conveniently carried out.
The size and replications on plot length, width and total area is usually constrained by the site. The selection of proper size of runoff plot however should not only be chosen based on the economical maintenance point of view, but also for getting good results. For example – if adopting USLE standards then on dissected hilly areas, it may be difficult to find a constant slope length of 22.1m.
Mutchler et al. (1963) have reported a minimum length of 9-10 m as sufficient. The appropriate width of plot depends on the crop. Very narrow plots have bordering effect. The size of plot should also be in manageable range, i.e., must suit the instrument capacities and manpower employed for measuring and maintaining the runoff plot.
Instrumentation:
The devices such as H-flumes, water level recorders and sediment samplers are few important instruments required for runoff plot system. Since, these instruments are very sophisticated type, therefore they require a routine care and maintenance. For this purpose, a well-equipped care and maintenance cell should be there in nearby place. Otherwise, if these instruments become out of order due to some unforeseen reasons, then they need to be sent elsewhere for their repairs. In mean-time, there is possibility of getting loss of data recording.
Sometimes, due to heavy sediment loads in the runoff from the plots, there is malfunctioning of the instruments. For dealing such problems in proper way, it is always advised to open a repairing centre in nearby place. If such facilities are there, then use of automatic devisees should be recommended; otherwise, strictly no. In this condition, manual collection of data with simple devices and tanks is the best.
In action, the flumes and slots are basically the runoff divisors. They connect two tanks and allow only a part of runoff to go into the second tank. Design of tanks is done on the basis of maximum daily rainfall depth for a given return period.
Normally, 10-years return period is sufficient for this purpose. This is an important criterion for tank design, because it removes the possibility of generation of unexpected very high runoff volume due to an extreme rain event in the area; as result the designed tank does not gets overflow during such rainfall events; and the measurements are being more accurate.
In brief, the following instruments are mainly required to accomplish the tasks related to runoff plot studies:
i. Rainfall simulator
ii. Multi slot divisors; and
iii. Flumes
Treatments:
In erosion/soil loss studies, under different cropping systems, land slopes, soil types etc. the application of different treatments is very important to have a better result. In principle, the treatments provide different sets of data from the same field of land under variety of parameters.
The number of treatments are decided on the basis of required study. Basically, for different treatments based studies there should be individual runoff plots for each of them. However, if plot size is very large then more than one treatments could be adjusted in one plots. Also, for making the comparison of different treatment’s result with the natural condition based result, there should be a control runoff plot.
6. Care and Maintenance of Runoff Plot:
A proper care and maintenance of runoff plots is very important to keep the plot in proper order for longer duration. Also, a well maintained runoff plot provides more accurate result as compared to the not maintained runoff plots. Regarding runoff plots, the maintenance is required in different aspects.
Few important amongst them are outlined as under:
i. On continuous use of runoff plots the volume of soil filled in the plot gets reduce, which causes bad effects on observation.
ii. The slope of topsoil surface in plot also gets disturbed because of repeated flow of water. This needs formation of requisite slope after each rain event.
iii. In runoff plots due to occurrence of soil erosion/soil loss the topsoil surface become stony or covered with roots of plants, also make erroneous result. This also needs immediate removal from the plot.
iv. If there have been developed several rills in the plot, then they should also be leveled.
v. In off-season when collection of observation is not there, then there is possibility of development of holes by rodents in the plot area. This causes destruction of runoff plot. Such type of defects must be removed, immediately.
vi. Also, the boundaries of runoff plot get destroyed, if proper care is not taken. If there is any defect in the boundaries of plot, they should also be immediately removed.
Observation’s Recording:
The recording of observations depends on various points, such as:
i. Objective of the experiment.
ii. Recording interval, i.e., daily, weekly, monthly or annual.
iii. Purpose of data, and
iv. Storm-wise or not.
After deciding the type of data required, the next step is to depute a person to take the observations. The accuracy in collection of observations depends very much on the observer. If the deputed person is skilled, and he knows the importance of data, then he would like to collect the data, accurately.
Otherwise, if the deputed person is not skilled, and he does not know the importance of data, then he will take the observations in very usual way. In this condition, the collected data might be erroneous. Especially, for data recording the deputed person must have a good training. He must have knowledge about procedure to take the measurements; keeping of the record; and also about operation of automatic instruments.
Apart from above, the followings are also few important points to follow for accurate data recording:
i. The recording site should be at a good access point.
ii. At the site, there should be a person throughout observation to make sure that the instrument is working properly.
iii. The measurement and collection of data should be done after every runoff-producing rain event. In other words, it should be done almost every day during the rainy season, if rain happens.
To accomplish the task of collecting observations, normally 2-3 hours are required. If the measurements are carried out after several storms, then not only the result is going to get affect, but the collecting tanks will also get overflow, as result there is inaccuracy in measurement.
Under runoff plot studies, the following works are also required to carry out:
i. Measuring and recording of rainfall and runoff
ii. Sampling of sediment
iii. Weighing of sample, and
iv. Clearing of collecting tanks.
Data Analysis:
Normally, the estimation of runoff, sediment load, nutrient loss and crop production are the main parameters for analysis, based on the runoff plot data. Under runoff estimation the total amount of runoff and its percentage in relation to rainfall are evaluated. In manual recording, the runoff timing and peak runoff cannot be measured.
In sediment analysis the sediment load (soil loss); particle size distribution of sediment fractions, suspended load etc. are determined. The nutrient loss is determined for different conservation measures (treatments) used under study. The treatments may be on the soil type, crops, rainfall intensity etc.
Estimation of Soil Loss:
For estimating the soil loss from the runoff plot it is very important to collect the water sample from the collecting tank, accurately. Normally, after lapse of few time the soil particles of runoff water collected in the tank get settle down over the tank bottom; and distribution of soil particles in the runoff water become non-uniform.
In this condition, if water sample is taken from the top of water depth, then the estimated soil loss would be very less than the actual value; while if the sample is taken from the bottom then the estimated soil loss would be very high. To avoid this variation, it is very important to stir the runoff water thoroughly in the collecting tank, so that the soil particles may get mix in the water, completely; and then sampling should be done.
After taking the water sample of known volume, the sample is filtered, dried and weighed. Filtering can be done by using blotting paper of suitable density. The drying of filtered sediment should be done in oven. If oven facility is not available, then it can also be done in sunlight, but it should always be taken care that there should not be addition of soil particles by blowing winds. The dried soil mass is weighted by using accurate weighing machine. The determined soil mass of a given volume of runoff is converted into kg/unit area.
7. Use of Runoff Plot and Runoff Plot Data:
Use of Runoff Plot:
Various uses of runoff plots are outlined as below:
i. Demonstration of Rainfall-Runoff Based Phenomenon:
Runoff plot is the best tool for making demonstration of runoff and soil loss events at field level. For example, if a person wants to show the happening of soil loss generation in the laboratory, then by operating the rainfall simulator he can show the mechanism of runoff phenomena, and thereby the happening of soil loss, too.
Not only this, at any time during the year, either that is rainy season, winter season or summer season, but the runoff or soil erosion/soil loss processes can be easily demonstrated. Also, by using runoff plot the effect of vegetation, soil type, soil slope, mulches etc., on runoff and soil erosion/soil loss can be demonstrated to the concerns.
ii. For Comparative Studies:
The comparative studies on various aspects such as effect of soil types, soil slope, vegetations etc., on runoff and soil erosion or soil loss can be suitably studied with the help of runoff plot, provided that the plot is equipped with rainfall simulator. For such type of studies the runoff plot provides a kind of independency to the researchers. Throughout year, one can conduct the research works. Due to this reason, the runoff plots are considered as tool for study of hydrological based soil erosion phenomenon.
iii. Generation of Data for Model Validation:
Models are either the empirical or conceptual, they all need validation for testing their validity or accuracy. For validation, the observed data are required, which are not easily available for each and every place (watershed). In this condition, the runoff plot is only the tool by using which the requisite data for the concern model can be generated either in the laboratory or in the field with least effort. Also, one can maintain the desired level of accuracy in data generation. The USLE, which is an empirical soil loss-computing model was developed on the basis runoff plot based data, for about 10000 plots.
Use of Runoff Plot Data:
Using runoff plot a variety of data related to the runoff, soil erosion, soil loss, nutrient loss etc., could be obtained under different treatments like soil types, soil slopes, vegetations, mulches etc. Accordingly, the generated data can be used for that specific purpose for which they have been collected.
In broad sense the runoff plot data can be utilized for following specific purposes:
i. For Verification of Soil Loss Equations:
There are host of soil loss predicting equations/models have been developed, worldwide. Their applicability for any other places is evaluated based on their testing for the specific conditions. For this purpose the data generation of requisite parameters becomes essential.
For generating data set in short span of time, the runoff plot is very useful tool. The USLE, MUSLE (Modified USLE) and the Simplified Process (SP) model can be tested for the specific condition using runoff plot data. The runoff plot data have been used for verifying various models in Jamaica; and the findings revealed that the USLE and MUSLE generally over-estimate the soil loss as compared to the actual soil loss.
On the other hand, the SP model, though somewhat overestimating the soil loss, but much close to the actual value. The higher values of factors LS and C of the USLE and MUSLE probably the main cause of overestimation of soil loss on steep slopes. The SP model performed extremely well in predicting runoff.
In another study using runoff plot data in Jamaica to verify MUSLE and the Morgan and Finney model reported that the MUSLE grossly over-predicts the soil loss in comparison to the measured values, because of high LS values. The Morgan and Finney model produced close value to the actual one.
ii. For Conservation Planning:
The runoff plot acts as a tool for approximating the soil loss from a given condition. In this way, a person just by determining the value of soil loss of his field, he may go for making the plan for growing the crops, by which there would be less soil loss.
On this issue, the plot data, together with the results from a demonstration area can provide a basis for planning of soil conservation and watershed projects in terms of estimating erosion rates, effectiveness of conservation measures and crop production potentials on treated lands.
8. Problems with Runoff Plot:
Few important problems associated to the runoff plot are outlined as under:
i. The runoff plots are expensive because of involvement of heavy expenditure of money in their construction, maintenance and operation.
ii. In order to collect accurate data on any parameter skilled staffs are required, so that it could be recorded, properly.
iii. The construction of runoff plot should be at easy access location, so that a proper care and maintenance could be done, timely; and also the recorded data by the deputed staff could be checked time to time by the scientist.
iv. The facilities like operating staff, laboratory facilities etc. are essentially required for handling the samples, and performing other miscellaneous works associated to the runoff plot based runoff-soil loss studies.
v. The application of agronomical treatments in runoff plots for study purposes is very difficult.
vi. The collection of soil loss data from runoff plot is problematic. There is possibility of huge scopes for occurrence of faults and errors.
vii. Like in open fields, the tillage operations using implements is not possible to perform in the runoff plot, as result the soil environment which exists in natural field does not exists in the runoff plot. This causes erroneous observations as compared to the collected observations in actual field condition.
viii. The application of treatments related to soil conservation such as bench terraces or channel terraces, is not possible on small size runoff plot, as result the evaluation of soil loss under such treatments is not possible by runoff plot. It is a type of constraint with runoff plot.
ix. The hydrological effects of channel terraces cannot be produced on runoff plots because the mechanism of runoff is completely different.
x. Similarly, the study on effect of contour farming etc., on soil loss is also not possible with the runoff plot.
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