Components of Runoff Plot | Tools | Soil Science

The following points highlight the three main components of runoff plot. The components are: 1. Collector 2. Divisor 3. Coshocton Wheel Sampler.

Boundaries are required for demarcating the area of runoff plot; and also for making a clear-cut jurisdiction for catching the rainfall and collecting rainfall based observations from the given area within plot boundary. The plot boundary can be constructed by using the materials such as soil, brick, concrete, timber planks, metal strips, asbestos-sheet, plastic etc., depending on their availability and cost. The boundaries are permanently constructed so that runoff plot may remain for longer duration. However, sometimes, temporary boundaries are also constructed to permit farm implements in the plot.

The following points should be considered for construction of boundaries in runoff plots:

i. The leakage in/or out of the plot across the boundaries is a common problem with runoff plots; it causes erroneous measurements. In order to avoid this type of happening, there should be the provision of diversion drain at the u/s of plot to divert the water flow toward plot’s upper area.

ii. If there have been constructed several plots in series, then between two plots there should not be the common boundary; otherwise, leakage from one-plot may affect the result of other plots. To avoid this problem, there should be provided a buffer strip between the two plots.

iii. Construction of boundaries should be done with suitable foundation. The depth of foundation should be sufficient to check the leakage from below. The boundary height above ground surface should also be sufficient to prevent overtopping.

iv. At the outside of boundary, there should be earth heaping to check the effect of water ponding or water flowing inside through the boundary.

Component # 1. Collector:

This is a flow (rainwater + eroded soil particles) receiving pot, placed at the d/s end of runoff plot. Each runoff plots are equipped with an individual collector. All collectors finally lead to a common collecting unit (tank). However, in small runoff plots, there is no need of collector; the overland flow/runoff is directly lead into the collecting tank.

In larger plots it is essential to provide a trough or channel for directing the flow into collecting tank. The installation of collector should be done with great care; otherwise, there is possibility of occurrence of several errors in measurement.

Few important points about it, are outlined as under:

i. The edge of collecting trough should be in the ground level, otherwise, there is possibility of develop­ment of errors in measurement. For example – if the edge of collecting trough is higher than the level of normal soil surface, then the end sill will trap the collected sediments; and thus the observation will get affected. On the other hand, if the edge of collector is lower than the soil surface, then there will be excessive soil erosion at this point; and also there is possibility of formation of rills in the plot area.

ii. If the soil of runoff plot is highly eroded (sandy), then the eroded soil mass likely to get deposit at the sill of the trough, as result there develops a new soil profile, which elevation is higher than the general soil surface. To avoid this happening the plots should be constructed with the sill in such a way that it can be adjusted according to erosion rate.

iii. If there is leakage underneath the collecting trough, then it should be checked by compacting the soil below the collector. However, it can also be checked by providing cutoff wall; or by inserting impermeable membrane. Or, if the collecting trough is constructed with the help of brick or concrete, then by providing sufficient foundation depth the leakage problem can be checked to a significant level.

Component # 2. Divisor:

In small runoff plots, the total amount of runoff likely to be generated is normally very less, which can be collected in a single collection tank. But in larger runoff plots the total amount of runoff is very high; or when a very large amount of runoff is generated due to an unexpected heavy rain event, then a single collection tank of small capacity is not capable to collect huge runoff.

In this situation, a kind of device is used to divide the total runoff flow into fractions to collect accurately. The device used for dividing the runoff into fractions is called multi-slot divisor. In USA the widely used this device is the GEIB divisor.

It consists of a number of slots in rectangular shape. It is installed at the measuring point of runoff plot. The runoff is allowed to pass through the central slot of divisor, which is collected into the storage tank. The water passing through other slots are to the waste.

As precautionary measure, it is always suggested to verify the quantity of water sample stored, whether it is exactly in the proportion as supposed to be, or not. If it is not, then a suitable arrangement is required to follow to get the correct amount.

In this method, there is possibility of occurrence of several errors from different sources; few of them are outlined as under:

i. Variations in flow depth.

ii. Blockage at the outlet.

iii. Interference in sampling mechanism.

iv. Interference in flow path towards divisor.

v. Variation in approaching velocity towards slot.

vi. Construction materials used.

vii. Often, the divisors are likely to get interfere with the flow conditions. This causes into deposition of sediments within the system. If it is there, then deposited sediments should be removed time to time.

Component # 3. Coshocton Wheel Sampler:

There have been developed several types of divisors for collecting the sediment samples for rainfall based erosion studies. Their suitability depends on several factors like sediment concentration, flow rate and its duration, catchment size, particle size distribution, availability of debris contents in the sediment load etc.

In the situation when there is very high amount of debris contents in the total sediment load, then general multi-slot divisors are not being suitable, because of high risk of their choking due to deposition of debris; and cause mechanical failure.

For such condition the divisor with moving component is found very suitable. The Coshocton Wheel sampler is one of such type of devises used. This sample is installed in combination of flume (H-flume).

The runoff discharge from flume gets fall en the sampler wheel, as result the wheel starts rotating on vertical axis. The rotation of wheel affects the sediment sample size. The size of sediment sample is not constant for all the flow rates, because of variations in rotor speed.

It is considered as one of the disadvantages of this device. For removing this demerit a motor is equipped in the system to provide a constant speed of rotation. This device is suitable only for experimental stations, where constant supervision is required, particularly when extreme events are taking place at very odd times such as at night etc.