A Case Study of Terrestrial Laser Scanning in Erosion Research: Calculation of Roughness and Volume Balance at a Logged Forest Site

The microtopography of the soil surface (mm to dm range) is a crucial factor in understanding and monitoring soil erosion. It influences the dissipation of rain-drop energy, determines the hydraulics of surface runoff, and it is changed by transport and sedimentation of soil particles. We used a high-resolution terrestrial laser scanner to assess erosion risks due to mechanized logging with crawler harvesters on steep slopes. Four measuring fields of about 20 m2 were scanned before and after the logging operation and after one year of exposure to the rain. To avoid shading, all soil vegetation and coarse litter was removed. The scans yielded approximately 50,000 irregularly distributed three-dimensional coordinates per m2 and were interpolated to gridlike elevation models. The surface models and the balanced volumes revealed an overall compaction, track forming, and an increase of surface roughness by the logging operation. During one-year weather exposure a further weak volume loss occurred and sharp relief contrasts were partially smoothed. However, the surface models did not allow for the concluseion that soil erosion occurred. We attributed the volume loss during the weather exposure mainly to consolidation of loosened soil material. A calculated roughness index was highly variable in dependence of interpolation parameters. Therefore the interpretation of roughness indices requires the showing of the scale dependency, e.g. as fractal dimension. We conclude, that terrestrial laser scanning is very performant in assessing microtopography, but the problem of shading by soil vegetation complicate its general use in soil erosion monitoring.