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Modelling Tillage Erosion

A wide variety of materials and methods have been used to measure tillage erosion. Tillage translocation is normally measured with a tracer. A tracer is incorporated into the soil in plots. The distributions of tracer before and after tillage are used to calculate translocation, forward translocation from the distribution parallel to the direction of tillage and lateral translocation from the distribution perpendicular to the direction of tillage (see Figure 1).

Figure 1. Simplified illustration of soil translocation by tillage. = forward translocation, = lateral translocation, WT = unit width of tillage.

There are two methods of calculating translocation from tracer distributions. For the more common method, the quantity of soil translocated per unit width of tillage is calculated directly from the tracer distributions . For the less common method, a summation curve is generated from a tracer distribution by employing convolution, and translocation per unit width of tillage is calculated from this curve. Both methods provide accurate measures of translocation, but the later provides additional information regarding the distance over which translocated soil is dispersed. Translocation can be expressed as a volume, a mass or a depth-averaged length.

The calculation of tillage erosion from translocation measurements is simply the difference in translocation between two points divided by the separating distance. Tillage erosion is expressed similarly to wind and water erosion, i.e. a change in soil mass per unit area or a change in elevation.

Within a complex soil-landscape, numerous measurements of tillage translocation may be required. Plots should be placed so that the variability in translocation is characterized in detail. Where slope gradient has been assumed to be the dominant factor affecting soil movement, plots should be located over the full range of gradients. Greater attention should be paid to those areas where slope gradient is changing, i.e. convex and concave areas, since it is changes in slope gradient that result in soil loss and accumulation. A pair of plots positioned at the steepest point along a slope profile, one tilled upslope and the other tilled downslope, provides measures of the minimum and maximum translocation, net downslope translocation, and the total soil loss from the upper slope and the total soil accumulation on the lower slope. Although the minimum and maximum values of translocation occur at this point on the hillslope, if the gradient in this region of the slope is uniform, no soil loss or accumulation will occur at this point.

Tracers consist of bulk tracers and point tracers. Point tracers are individually labeled tracers of various shapes and materials, e.g. steel nuts, plastic spheres . Bulk tracers are used to label a volume of soil; they can be physical, e.g. gravel, or chemical, e.g. 134Cs, Cl. Point tracers also represent a volume of soil, but they have the distinct advantage that they can be used to characterize the complexity of soil movement in three dimensions; however, bulk tracers provide a more accurate measure of bulk soil movement in two dimensions.

After tillage, soil is sampled and analyzed to determine the distribution of tracer. Sampling usually involves destruction of the site, eliminating the possibility of successive translocation measurements from recurrent tillage operations. Non-destructive sampling methods do exist. Sampling must be carried out as soon as possible after tillage to isolate the movement of soil by tillage from that by other processes. Other measures can also be taken to isolate soil movement by tillage. The precise relocation of plots and the recovery of tracer are important considerations when sampling.

The design of plots is possibly the most important consideration when using tracers. Even under controlled operating conditions, the movement of soil during tillage is inherently variable due the nature of soil-tillage interactions. Soil strains and fails during tillage, and tools and implements flex and shift - the 'flow' of soil during tillage is not steady. For accurate measurement of soil movement by tillage, plots must be sufficiently long and wide to filter out this 'noise'. The length of plots used in tillage erosion studies has ranged from a few centimeters to a few meters. As stated above, a longer plot provides a better measure of the average movement of soil by tillage. The movement of soil varies across the width of tillage as a function of the tool type, arrangement and spacing. For accurate measurement of soil movement by tillage, the width of the plot must be a multiple of the unit width of tillage. For a single pass of a simple tillage implement, the unit width is the distance between two adjacent tools. However, for an implement with multiple tool types or for a sequence of different implements, establishing the unit width of tillage may not be possible. To reduce measurement errors associated with plot width, the plot must be as wide as possible. The paths of a tillage implement overlap in the field (as much as 10%), with the exception of some implements such as the moldboard plow, causing additional soil movement at the edges of each path. To account for this additional movement, plot widths must be equal to the width of implements; however, this is not practical in most cases. The depth of plots must exceed the maximum depth of the tillage operation(s) under examination. Tillage depth can vary greatly within a landscape, so caution must be used in estimating the maximum tillage depth - it is better to overestimate tillage depth than to underestimate.

Tillage translocation can also be measured using detailed elevation surveys of the soil surface. The redistribution of soil volume, along with soil bulk density, can be used to measure soil translocation. There are two good examples of this technique. In the study of tillage erosion by contour moldboard plowing, the profiles of furrow slices turned upslope are compared to those of furrow slices turned downslope - the difference is a measure of net downslope translocation. In the study of tillage erosion by hoeing on steep slopes (soil is only hoed downslope), the profiles of the furrow created at the top of the slope and the ridge created at the bottom are measures of downslope translocation.

 Tillage Translocatiom and Tillage Erosion is a subsite of the Soil Science Dept , University of Manitoba