In this article we will discuss about how to measure splash erosion.
Introduction to Splash Erosion:
The splash erosion is the initial phase of sediment production from the hill slopes. It has very fundamental importance in the soil erosion studies. Regarding evaluation of splash erosion, very few studies have been made so far under field condition. However, in contrast to field measurement, a lot of laboratory studies have been made on splash erosion evaluation hut laboratory measurement is not so authentic as the field measurements.
There are several reasons of it, such as the exact situation like soil condition, vegetation and others cannot be maintained in the laboratory. Also, the rainfall, which causes splash erosion, cannot be simulated in the laboratory, exactly as happen in natural condition. The laboratory measurements provide a kind of approximation, and an idea to the researchers for studies.
The most commonly developed devises for measuring the splash erosion are listed as under:
i. Splash boards.
ii. Small funnels or bottles.
iii. Painted stones.
iv. Radioactive tracers.
v. Splash cup.
The splash-board method is used for Held measurement of splash erosion. The board is kept on the soil surface; and when rainfall takes place, the soil particles get splash due raindrop impact which get deposited on the board surface. After end of rainfall the board is taken back.
The board along with deposited soil mass is kept for drying al suitable place (ovane or in shade). After getting dry, the deposited soil particles are carefully collected from the board; and weighted. The obtained soil weight is counted as the soil loss caused by splash erosion.
In small funnel or bottle method, the bottle is inserted in the soil in such a way that its mouth is slightly above the general ground level so that the soil may not enter directly due to surface flow.
On occurrence of rainfall, the soil particles get splash, and deposit in the funnel. At the end of rainfall event the bottle is removed from the soil; and deposited soil mass is dried and weighted, carefully. The obtained soil weight is considered as the soil loss due to splash erosion.
The splash cup method was developed by Morgan (1978), has been used at different places throughout the world, and reported to be an accurate method/device for measuring the splash erosion in field condition.
The design requirement depends on the objective, such as cither to be used for determining the splash detachment or to obtain sufficient data for development of splash process model.
However, the basic design requirements of any device for measuring the splash erosion in field condition, should be as below:
i. The device should provide the data on total weight of splashed soil particles by raindrops (splash detachment).
ii. The device should be adequately isolated from the effects of sediment movement by overland flow/runoff.
iii. It should not be affected by the relative change in the height of device with respect to the soil surface as result of ground lowering, compaction, frost or swelling and shrinking of the soil etc.
iv. It should also be acceptable, environmentally.
This device was developed by Morgan (1978). It consists of an inner hollow cylinder, which is 110 mm in length (height) and 100 mm in diameter. For measurement of splash erosion the device is inserted into the soil just by pushing into the ground. At the surrounding of cup, a circular catching tray of 300 mm diameter, with 100 mm high boundary wall is also placed.
Its inner cylinder catches the splashed soil particles from the ground surface. This device is capable to catch the splashed particles for the distance less than the radius of the catch tray, and also to the particles splashed from greater distances with the angles of ejection up to 20°.
This device also checks the splash- in of 90% of the soil particles detached by the raindrop impact outside the catching tray. At the end of rainfall, the deposited soil mass is collected separately from the upslope and down slope compartments of the catching tray, dried and weighed carefully. The weights of soil obtained from upslope and down slope are added together. The obtained weight of soil mass so, is the splash detachment. And, the difference of down slope soil weight and the upslope soil weight is the net down slope splash transport.
The measurement of sheet erosion, i.e., inter rill erosion and rill erosion is carried out by using following methods:
1. Paints and dyes method, and
2. Burried nails/stakes method.
1. Paints and Dyes Method:
In this method, the soil particles are colored by using paints/dye and coloured soil particles are spread over the runoff plot/natural field plot. During rainfall (or artificial rainfall), these soil particles get eroded, which are collected into collection unit. Later on, by taking sample the soil loss/erosion rate is determined.
This technique of inter rill and rill erosion measurement has been used by many researchers. This method can also be used for evaluating the relative importance of different mechanisms involved in soil movement, especially over a short distance, such between rills etc.
In Puerto rico, Lewis (1976) measured the soil creep. He found that in soil movement the antecedent moisture, soil texture, temperature, through flow and density of plant roots are very effective. He used the same technique in western Nigeria, and found that a majority of rain events caused the movement of soil particles by splashing action, only few rain events with the intensity greater than 2.5 cm/h caused substantial soil movement.
The maximum transportation distance of splashed soil particles was about 4 m, down slope side. Mtakwa et. al. (1987) also used this technique, he found completely unsuitable. He reported that the painted soil particles get cemented together and form aggregates which movement gets significantly affected as compared to the uncolored soil mass.
2. Burried Nails/Stakes Method:
In this method the nails or stakes (wooden) are inserted into field (runoff plot) at various points. On rainfall occurrence, and thereby the movement of soil mass (sheet erosion/rill erosion) is observed in the form of exposed height of nails/stakes from ground level. The measurements are taken for different time intervals.
This method provides an approximation in erosion measurement. It has few limitations, e.g., sometimes the burried nails/stakes obstruct the sheet flow and thus affect the natural movement of soil mass. For making clear visibility about depth of soil removed, the nails/stakes are coloured, usually by bright red colour.
This method of interrill/rill erosion measurement has been used by many researches. Millington (1981) used this technique in Sierra. He reported that the measured soil loss by this technique is very high as compared to the plot measurement (runoff plot). In south western Nigeria, Mlakwa ct. al (1987) compared the soil loss measured by the nail method with the runoff plot based soil measured values and from the rill volume method.
He reported that the rill volume method is least satisfactory, underestimated the soil erosion rate. The nail method was satisfactory for predicting the rate of soil erosion from small plots, but for bigger areas (Watershed) was not satisfactory. He also observed that on small size plots (20 sqm) at least 20 nails are required to measure the erosion rate.
Lal (1990) listed following limitations of this technique:
i. Observation in sheet flow and erosion rate, thereby.
ii. Obstruction in cultivation.
iii. Creation of hindrance in cultural operations like weeding etc.
iv. Removal of nails/stakes due to intense rainfall.
v. Not so authentic measurement.
The amount of soil removed (eroded) under rill erosion can also be predicted by computing the volume of rills formed after an intense storm, in the field. In this method, the field plot is kept open to rainfall event. After end of rain storm, the volume of formed rills are determined. This method is found suitable relatively for small plots (10 to 20 m2).
Many researchers have used this technique for determining the rill erosion. Gerasimenko (1980) evaluated the accuracy of rill volume method, in USSR. He pointed that the accuracy depends very much on the micro channels (rills) typo and agricultural practices followed. In western Nigeria Franzen (1886) and Mtakwa et.al. (1987) used for measuring the rate of rill erosion.
Mtakwa et.al (1987) found that this method underestimate the erosion rate, because of the reason that –
i. First the sheet erosion/splash erosion is not measured to this method, because sheet flow (overland flow) does not develop rills until it attains the state of critical flow.
ii. The slope Length should be long. The effect of rill cannot be measured on the plot length less than 10 m.
This technique estimates the erosion better from long slopes.
Portable Rill Meter:
This device is used for measuring the soil loss occurring from rill section. Rill meter can be easily used in relatively inaccessible locations such as high elevation, surface mined spoil banks and re-vegetated sites in remote areas. It can also be used in rangelands and in croplands to quantify the rill and inter-rill erosions.
In field, it can be used to measure the size (cross sectional area) of rills at the end of erosion season; to quantify the effect of slope length, slope steepness, and climatic variations on rill based soil loss. On runoff plots, the rill meter can be used in conjunction with the runoff collection unit to monitor rill development throughout the erosion season; and to derive the relationship between rill and inter-rill soil losses.
The design and construction of rill meter is described as under:
The design and construction of rill meter includes following specifications:
i. The width of runoff plot for its use should be 1.83 m or in the multiple or in fraction of 1.83 m. The maximum rill depth in the field should be about 380 mm, and the desired resolution of complete system for measurement is arbitrarily at 1 mm depth.
ii. Slope up to 50%.
iii. Rapid field operation of 5-min or less per reading is desirable.
iv. The meter needs to be rugged in order to withstand against transport.
The rill meter developed by Curtis & Cole (1972) and Foster & Meyer (1972) is further modified based on the suggestions from several other researchers.
The modified specifications are outlined as below:
(a) Working width of rill meter is 1.829 m
(b) Diameter 3.2 mm
(c) 610mm long stainless steel pins on 12.7mm centers as depth indicator
(d) Maximum depth of rill is 410 mm.
The recording device consists of 35 mm single lens reflex camera with 28 mm wide-angle lens. A grid background is there in the system that can read to an accuracy of 2.5 mm or could be used as a reference scale if other means are used to digitize the data. The reading can be taken at every 2-min intervals under good soil condition without vegetation. The weight of rill meter is about 33 kg without camera. This device can be easily transported across steep fields.
The design is further improved by using stiff Aluminum alloy welding rods for the depth indicator pins; the light Aluminum tubing is replaced with Aluminum angle for folding the arms; lighter connecting parts, and many of the connections are welded rather bolted. The new rill meter is stronger than the original, and is much easier to carry, weight is only 25 kg without camera.
The measurement site should be with fully developed rill system of proper size that could be rapidly measured. Normally, the width of rill should be from 1.83 to 12.80 m. The rill meter is installed at one end of transect located farthest down slope, and leveled across up and down slope. The pins are gently lowered to the soil surface and checked; identification card is placed; camera setting is checked, and thereafter photograph is taken.
After this the pins are lifted, so that when the device is shifted along transect, the holes left by the back legs could facilitate positioning of the meter. After measuring transects, the slope length and steepness between each transect are determined, and soil samples are also collected.
If the rill meter is used in conjunction with runoff plots, then the legs can be inverted for attachment of collars; and the device is set on alignment pins. The rill meter is left installed for entire season or permanently in the plot to collect the readings from the same location in the plot.
The data on slope length and steepness are collected on colour slides, which are analyzed by reducing and measuring with the help of polar planimeter. Also, a special light table is used with a mirror to project the image from the colour slide, using ‘racing paper. The distance between lens-to-paper is adjusted to produce the image which is one-half of the full-length scale. The pin-top locations are then traced manually; and the ground surface is predicted before erosion.
The positions of camera and the lens configuration are adjusted in such a way that the existing ground surface is clearly visible in the colour slides, to assess the position of original ground surface. Finally, the cross sectional area of the rills is measured with the help of planimeter. The extent of rill erosion is predicted on the basis of horizontal distance between the measured transects, and the surface bulk density with reference to a given measuring section.
The measurement using polar planimeter provides accurate result but tedious and time consuming. Due to this reason, the collected data are analyzed by using manual electronic digitizer. For this purpose, the digitizer is connected to a programmeable desktop calculator to integrate the eroded area by the trapezoidal rule. The calculator program includes the correction for camera. It is necessary to sample only the prints and to calculate three average correction factors, which are applied to the outer, intermediate and central zones of the enlargements.
The colour slide and tracing paper method has the advantage that the actual ground surface can be seen, and at the same time the pre-erosion ground surface can also be estimated at the pin tops. This is especially helpful if the soil surface is cloddy or irregular.