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Saturday, April 2, 2011

EROSION : VOLUMETRIC MEASUREMENTS

Estimates of soil loss based on three-dimensional measurements of volume can be used in different ways. For erosion from rills or roads, the length of the eroding section and changes in cross-sectional area are measured. For gully erosion, usually information is needed not only on the volume lost, but on how much the gully is increasing, so changes in length as the gully cuts back also have to be measured. The other volumetric approach is to measure or estimate the volume deposited as an outwash fan, or in a catchpit or reservoir.
  • Rills and roads
Measuring the cross-section of all the rills in a sample area or along a sample transect is quick and easy, so the method is suitable for measuring change over short time periods, such as the change caused by a single heavy storm. The cross-section may be re-estimated from measurements of average width and depth if the shape is fairly uniform, or by summing the area of segments if the cross-section of the rill is irregular. (The arithmetic of cross-sections is discussed in the section Measuring streamflow.) The accuracy of estimates of total soil loss based only on measurements of rill erosion will depend on how much inter-rill erosion by splash and sheetwash is also occurring. Where inter-rill erosion is low, the underestimation from rill erosion alone may be from 10-30% (Zacher 1982).

A simple method for an immediate estimate of soil loss with minimum calculation dates back to 1937, when it was pioneered by A.N. Alutin of the United States Soil Conservation Service. A fixed-length transect is set out across the slope, and the cross-section area of each rill along the line is calculated from average width and average depth and summed. In the original units the transect was 13.7 feet, and the total cross-section of rills in square inches is numerically equal to the total soil loss in tons/acre (Hill and Kaiser 1965). The metric equivalent is a unit transect of 15 m, when the rill area in cm² is numerically equal to ten times the soil loss in tonnes/ha. Usually the results from a number of transects would be averaged. This assumes a soil bulk density of 1.5, and that the transects measured are typical of the area being studied.

Estimates of erosion from rill measurements have been compared with estimates of the volume deposited in outwash fans in England by Evans and Boardman (1994), who found that agreement was better when the measurements were made by experienced field workers. They suggest that estimates by measuring rills can be expected to be between twice and one half of the true value. The opportunity for a detailed assessment of the method occurred in 1985 when water from a burst water main cut a large gully through sandy soil drilled to a winter cereal. Nearly all the eroded soil was redeposited in the field (Plate 2) and estimated to be 304 m3. The soil eroded from the gully was measured as 320 m3, the discrepancy probably the result of fine particles being carried away in the runoff.

A further simplification of estimating soil loss from rills was tried out by Watson and Evans (1991) who compared direct measurements of rills in the field with estimates made from a study of colour slides taken in the field. They concluded that "It is possible for an experienced observer to make reasonably accurate decisions about volumes of soil eroded by looking at photographs of fields taken on the ground". On eight of the eleven fields measured in the study, the ratio between estimates from field measurements and estimates from the photographs was between 0.81 and 1.11, with extreme values of 0.67 and 2.12. The discrepancies were thought to arise mainly from the difficulty of estimating the length of rills on the photographs because of foreshortening. They conclude that there is room for improvement in the technique, but that it does offer a quick and simple method of estimating field soil loss where rill erosion is the dominant process.

Clearly figures in tonnes/ha from these methods must not be treated as if they were reliable accurate measurements, but they may be useful in providing a quick simple comparison of the effect of alternative cropping or cultivation practices.
  • Gullies and streambanks
When the progress of gully erosion is being studied, measurements are needed both of the horizontal spread of the gully and vertical changes within the gully.

To measure the surface area, and changes from cutting back or bank collapse, a rectangular grid of erosion pins is set out at an appropriate grid interval of perhaps 2 or 5 m as in Figure 10. From measurements along the grid lines from the nearest pin to the gully edge, the surface area can be plotted on squared paper. The grid lines also serve as the transects for cross-sections across the gully. A string is stretched at ground level along a grid line with markers at fixed intervals of, say, 1 m. At each marker the depth is measured from the gully floor using a survey staff or a ranging rod, and the section can be plotted. The volume of soil lost from the gully is calculated as in Figure 11, and subsequent measurements will quantify the changes.

The bed of a gully may at any one point have cycles of cutting down at some times and deposition at others, for example when a large bank collapse puts a large quantity of soil into the flow. The use of erosion pins with washers may provide information on such changes of level, as described in the section Measuring change of surface level.

Another method of assessing the cutting back of streambeds or gully sides is to drive in horizontal small-diameter metal rods. An increase in the length of rod exposed shows how much the bank has retreated, and the measurement can be simplified by spray painting collars round the exposed rods. However this technique should not be used if placing the rods will affect the soil's resistance to erosion. In gravel soils, driving the rods can loosen, and increase their erodibility, or in alluvial soils with low tensile strength, the rods can act as a tension reinforcement and reduce slumping, toppling, or cantilever failure (Thorne 1981).

Changes in a gully may be interpreted from the use of sequences of photographs. The position of the camera and the direction of the photograph must be carefully recorded. It is surprising how seldom 'before and after' photographs of gullies are lined up accurately. For studies of the long-term development of gullies, aerial photography can be a useful tool. An interesting example from Zimbabwe allowed the correlation of the changes in a gully with the changes in land use and vegetation in its catchment over a period of forty years (Keech 1992).
  • Catchpits
Surveys of sediment in reservoirs can be used to make quantitative estimates of erosion as discussed in chapter 5, but simple catchpits may be used to demonstrate comparisons (Plate 3). It is not possible to get a reliable estimate of the total soil movement unless the receiving reservoir is large enough to contain the whole flow and sediment load, but smaller pits which only catch an unknown proportion of the sediment can still be used to obtain comparative information. This was done successfully in the FAO project in Java previously referred to in connection with erosion pins (FAO 1976a) where small catchpits were dug on two small parallel catchments, one of which was terraced and the other not. Previously skeptical farmers were convinced of the effectiveness of terracing when they saw that there was much less soil in the catchpit below the terraced plot than the untreated plot.

Another example is the Japanese study previously referred to (Takei, Kobaski and Fukushima 1981). Two small plots were set up side by side with a simple catchpit below, with one plot left bare and the other reforested. Again a clear difference between the amount of sediment accumulated demonstrated the effect of reforestation, although the actual amounts of soil caught in the pits could not lead to quantitative estimates of the amounts of erosion.

In Colombia, on-farm demonstration trials initiated by the International Centre for Tropical Agriculture (CIAT) used plastic-lined channels to compare the soil movement under different cassava-based cropping systems (Howeler 1987) (Figure 12). More recently a farmer-operated trial in Thailand with catchpits lined with polythene sheets demonstrated a huge difference is soil loss from a plot under hill rice with no conservation measures, and a plot with grass strips and strip cropping (Sombatpanit et al. 1992) (Plate 3).

A simple method for measuring relative soil movement at different points in the catchment uses 'mesh bags'. A 30 cm by 30 cm square of 5 mm mesh nylon fabric is fastened on 3 sides over the same size of 2 mm mesh. The bags are pinned to the soil surface with the open edge uphill in a line across the contour to measure horizontal variation, or up-and-down slope to measure variation down the catena. Some of the soil moved by surface flow is trapped in the mesh bag and may be dried and weighed at intervals. The method is an inexpensive and simple way of studying relative soil movement at different points in the field (Hsieh 1992).

As an alternative to excavating catchpits, gully checkdams can be used to give an approximation of the effect of different treatments in their catchments.
  • Landslips and landslides
Geomorphological processes tend to require careful and long-term study rather than reconnaissance estimates and so are out of the remit of this Bulletin. Readers wishing to consider this topic are directed to the section Further reading relating to this chapter.
Source : FAO.org
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