ARS | Publication request: THE STUDY OF SOIL DETACHMENT AND DEPOSITION ON A HILLSLOPE USING A MAGNETIC TRACER

Submitted to: Catena
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 27, 2001
Publication Date: January 1, 2002
Citation: Ventura, E., Nearing, M.A., Amore, E., Norton, L.D. The study of soil detachment and deposition on a hillslope using a magnetic tracer. 2002. Catena 48(3):149-161.

Interpretive Summary: In order to conserve soil from erosion it is helpful to know where soil erodes and how the sediment that is produced moves across the landscape. It sounds like a very simple matter, but measuring the movement of sediment across the landscape or even down a hillslope is not an easy task. It is difficult to know how far the material from a given portion of the land moves during a storm, or where it is deposited. Scientists have developed models to predict such movements, but they really have little data to verify the models. The purpose of this study was to develop a technique to measure the movement of sediment across landscapes. We use magnetized particles, which can be seeded in the upper layer of the soil. As erosion occurs, the magnetized particles are redistributed. A magnetometer is used then to map the magnetic content of the soil both in the area where the magnetic beads are placed and downslope. The impact of this study is that we now have a new method to identify how sediment is redistributed across landscapes so that we can more efficiently target conservation practices and combat erosion.

Technical Abstract: 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 combined with two different inflow rates 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 five percent concentration of magnetic tracer was placed in the upper plot and mixed to depth of 3 cm. Areas of detachment and deposition were identified using a magnetometer. 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 soil correlated well with the magnetic susceptibility readings, indicating that a quantitative estimate of deposition can be made using this non-intrusive technique. Results indicated that the tracer was effective for identifying areas of net detachment and deposition, however, the tracer to soil ratio of the sediment was not constant for all treatments. A wider range of sizes and densities of the tracer should be tested if the method is to be useful to quantify erosion rates.