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Abstract

In the Otjiwarongo region (Northern Namibia), Vertisol- Kastanozem-Calcisol soil associations occur as patches of several hundred hectares in extent. They have formed in fine- grained Mid-Holocene sediments which accumulated on both sides of the subcontinental watershed between the Ugab River draining into the South Atlantic and the Omatako Omuramba draining into the Kalahari Basin. Kastanozem formation cannot be explained by the environments that exist at present. The humification suggests open savanna environments in the past and does not accord with the shrublands and thornbush savanna at present. Using AMS C-14 and OSL data, it is possible to distinguish two periods of soil degradation during the recent past. Initially, most of the Kastanozems and Vertisols were buried by slope wash sediments to a depth of several decimetres. This process started in the mid-19th century at the latest. In a second phase, the soils were affected by rill and gully erosion, indicating increased runoff. This occurred during the last decades of the 19th and the first decades of the 20th century, probably as a result of intensified cattle farming. In contrast to other parts of Namibia, the prominent river channels of the Otjiwarongo region, most of them up to 20 in wide and 3-4 in deep, are a result of recent erosion. Degradation of vegetation and soils, and river channel formation, seem to be the main causes of farmland aridification. (C) 2002 Elsevier Science B.V. All rights reserved.