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Abstract "Heft 41"

Freiburger Bodenkundliche Abhandlungen

Schriftenreihe des

Institut für Bodenkunde und Waldernährungslehre
der Albert-Ludwigs-Universität Freiburg i.Br.
Schriftleitung: P. Trüby

Heft 41

Thomas Schmid

Einfluss des Mikroreliefs auf die Erodierbarkeit des Bodens
und Veränderung des erosionswirksamen Niederschlagsspektrums durch die Krone
am Beispiel einer Raupenharvesterbefahrung

Freiburg im Breisgau 2003

ISSN 0344-2691


The aim of this study was the methodical measurement of changes in the microrelief characteristics of the surface caused by Caterpillar harvesters in steep slope forest areas and the measurement of differences in drop size distribution of rainfall under the influence of forest canopy. The results should permit predication of the rainfall erosion potential under forest canopy and the influence of microrelief on soil erodibility. To put this into practice, suitable methods of measurement, recording and evaluation had to be developed.

On a steep slope, three new lanes were driven by a Caterpillar harvester in the line of fall. Microrelief measurement was carried out with a free-positioning laser Scanner, which records polar coordinates. The measurements were taken within four areas of about 20 m2. The areas were scanned before harvesting, immediately after harvesting and cable logging and after the influence of one natural summer rainfall. In order to describe the microrelief, the quotient of total surface area divided by the map area (TSA/MA) and the effective rainfall energy was determined. Since the measurements were made as polar coordinates distanced with regard to angles, the data points from the ground scan were irregularly spaced. Therefore, for further calculations in a raster-GIS, conversion into a regulär grid was necessary. For this, the strong dependence of the microrelief index on the selected method of interpolation had to be minimized with regard to erodibility factors.

The microrelief index TSA/MA showed that the four areas had a relatively high roughness, even before harvesting. Harvesting and cable logging resulted in a small mean increase of the roughness index. The following natural rainfall decreased the roughness index to approximately the original level. These results indicate a Iow tendency towards sealing processes on the surface. A roughness oriented in a parallel direction along the slope appeared within the Caterpillar tracks.

The drop size distribution and the kinetic energy of rainfall under canopy was measured under a relatively closed crown of Norway Spruce (Picea abies (L.) Karst.). The Iowest branches of the crown were at a height of roughly 10 meters. The measurement of the drop size distribution was carried out with an optical disdrometer, which simultaneously measures size and velocity of hydrometeors. In order to integrate the inhomogeneous drop distribution on the area, the disdrometer was mounted on a controllable sledge to slide slowly through the wood during rainfall.

Drop size measurement under the crown revealed a different distribution to that from rainfall in the open area. In contrast to the open area distribution which showed one peak, the distribution under the crown formed two peaks. The first of these is within the drop diameter area of less than 1 mm, the greater is at a drop diameter of about 4 mm. The Iower peak originates from bursting raindrops, the greater from the accumulation and dripping off from the crown of large, energy-rich raindrops. Therefore, it is possible that in a given Situation the rainfall under the crown is more erosive than the corresponding rainfall on the open area. This Situation occurs when the crown is saturated and the influence of interception tends to zero. From 10 independently observed rainfall events under the crown a mean rainfall mass of 83.3 %, a mean kinetic rainfall energy of 88.4 % and an erosivity index El5 of 59.4 % of the corresponding open area rainfall were obtained. Thus, a layer of crowns reduces the mean erosivity of rainfall, although Single events could be higher. An empirical derivative from the relationship between the height of the dripping points, drop diameter and drop energy clearly shows the influence of crown height on the kinetic energy of rainfall under canopy.

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