THE STUDY OF DETACHMENT AND DEPOSITION ON A HILLSLOPE USING A MAGNETIC TRACER

Abstract

Soil erosion by water involves the processes of detachment, transport and deposition of soil materials by the erosive forces of raindrops and surface flow of water. The redistribution of sediment within a field-sized area is important in estimating the effect of erosion and deposition on productivity, in helping the conservation planner to target efforts to reduce erosion, and to evaluate erosion models. The objective of this study was to use a magnetic tracer, with size and density similar to soil aggregates, to study detachment and deposition on a hillslope. Two interconnected plots were established on a hillslope. Two rainfall intensities (35 and 70 mm h-1) combined with two different inflow rates (4 and 10 l min-1) were applied to the upper of the two plots. No rain or water was applied to the lower plot, which was used to study the deposition of eroded sediments from the upper plot. A 5% concentration of magnetic tracer was placed in the upper plot and mixed to depth of 3 cm. From this initial condition, areas of tracer detachment and deposition were identified using a magnetic sensor. Areas of detachment were associated with a decrease in magnetic signal, while areas of deposition were associated with an increase in the magnetic signal. In the lower plot, deposition of tracer correlated well with the magnetic susceptibility readings. Results indicated that the tracer was effective for identifying areas of net detachment and deposition, however, the tracer to soil ratio did not remain constant for all treatments. For this reason, a wider range of sizes and densities of the tracer should be tested if the method is to be useful to quantify erosion rates.