Measurement of erosion Splash erosion

Techniques in farmers fields Figure 1. Measurement of erosion and deposition using erosion pins: (a) installation; (b) re-measurement (a)

Note on the use of erosion pins for the assessment of erosion rates in an existing soil and water conservation programme employing physical structures

Introduction Where bunding or trenches have been used in a soil and water conservation programme, they can be used to assess the erosion rates from the fields in which they have been constructed. The purpose of such an assessment may be: to reassess the priority areas for the SWC programme on the basis of the erosion rates from different soil types; to reassess the priority areas for the SWC programme on the basis of the erosion rates from different soil types; to predict how long a terrace between will take to level out which would have implications for the future modification of the cropping systems; to predict how long a terrace between will take to level out which would have implications for the future modification of the cropping systems;

to assess the amount of maintenance that would be required to keep terrace bunds at the correct height; to assess the amount of maintenance that would be required to keep terrace bunds at the correct height; to re-evaluate whether bunding is even required on the particular soil/slope/crop combination on which the observations have been made. to re-evaluate whether bunding is even required on the particular soil/slope/crop combination on which the observations have been made. Assumptions It is assumed in the observations that: either most of the sediment washed downslope is deposited behind the bund (or in the trench) or that the gains from the sedimented area are as great as the losses; either most of the sediment washed downslope is deposited behind the bund (or in the trench) or that the gains from the sedimented area are as great as the losses;

the bulk density of the sediment is constant and the same as the original soil; a standard of 1.5 g/cm 3 seems reasonable; the bulk density of the sediment is constant and the same as the original soil; a standard of 1.5 g/cm 3 seems reasonable; the deposit has a level surface; the deposit has a level surface; the deposit does not reduce the inter-bund spacing by a significant amount (though it would not be difficult to allow for this); the deposit does not reduce the inter-bund spacing by a significant amount (though it would not be difficult to allow for this);

Methodology There are a number of points that should be borne in mind if the method is to be of value. (a) The use of erosion pins is easiest where the "fanya juu" type of bunds are used, i.e. the soil is thrown up hill. If the soil is thrown downhill or partly up and partly down, it is more difficult to calculate the volume of sediment deposited against the bund. If fanya juu bunds are not used and the soil is thrown downhill, the shape of the hole from which the soil is taken should be known and the equation below modifed using the geometry of the actual hole.

(b) The pins should be inserted either between two contour bunds (or trenches) or between a contour bund and the top of a hill. Bunds which are not on the contour will cause side-flow along the bund and the assumptions will not be valid. (c) There should be no inflow from the next terrace uphill or out flow into the next terrace downhill. That is there should be no spillways or broken bunds in the measured area. (d) Because of the risk of leakage, it would be better to work with earth bunds rather than stone bunds (e) Areas where there are deep rills between the bunds (or trenches) should be avoided.

Calculations Calculations If fanya juu bunds are used the area, A, deposited behind the bund is calculated as follows: If fanya juu bunds are used the area, A, deposited behind the bund is calculated as follows:

If the pin is inserted a distance f uphill of the bottom of the bund where the depth of deposit is e, the area is given by: In the case of roughly parallel contour bunds, the sediment rate is calculated from: D = A/L D = A/L where L is the inter-bund spacing and D is the deposit per unit area.

If the bund used is the first one and is a distance L from the crest of the hill, then a better equation, assuming the contribution area to be a segment of a circle, would be: D = 2A/L D = 2A/L In the case of contour trenches, the erosion rate is given by D = e x width of trench L L where e is again the depth of sediment in the trench

Measurement using vegetation

Runoff plots

Note that caution should be exercised in extending runoff rates to catchment level

Gerlach troughs