Budgeting soil erosion from floodplain and alluvial fan sediments in the western Palatinate Forest (Pfalzerwald, Germany)

摘要

Every approach to erosion budgeting has to start from intensive field work. Following such field work in the floodplains of the Schwarzbach catchment of the Palatinate Forest (Pfalzerwald; 1,151.55 km~2), we attempt to determine the amount of Holocene flood-plain sediments deposited there. To that end several cross sections were augered and dug in all reaches of the Schwarzbach and some of its tributaries. From the amount of floodplain sediment calculated from the field data, of 35.1 million m~3 or 52.5 million tons the amount of soil erosion necessary for it to have occurred has been calculated. With reference to only those sediments still present in the floodplains, an eroded layer of 31 mm, equivalent to 465 t/ha, was calculated. From radiocarbon dating of in situ organic material and the stratigraphic position of peats, two periods of different intensity of soil erosion could be distinguished for the whole catchment: from BC 1500 to AD 1000 the average erosion rate was 0.55 mm/100 years, and from AD 1000 to 1850 the much higher rate was 2.02 mm/100 years. The principal source of the floodplain sediments has been the youngest and almost ubiquitous Late Pleistocene upper layer of periglacial solifluction. Therefore also some typical cover-bed profiles of the Palatinate Forest are described. They become thicker and increasingly silty towards the west, with increasing suitability for farming and thus were more affected by soil erosion than in the upper reaches of the catchment. Two pollen profiles are discussed in relation to the settlement history of the western Palatinate Forest. A local time marker is the small alluvial fan of a subcatchment, in which has been preserved evidence of an extreme local erosion event of High or Late Medieval age. Studies of similar small Upland catchments by the author in Rhineland-Palatinate and Hesse show that the erosion rates of the Schwarzbach valley are comparatively low, less than a third of the highest rate calculated for the Aar valley of the Taunus Mts. This may be explained by the reduced suitability of the Palatinate Forest for agriculture, and consequently a later onset of forest clearing, mostly followed by merely extensive land use. The soil erosion calculations by other researchers are reviewed.