Tillage erosion on slopes with soil conservation structures in the Ethiopian highlands

Soil translocation due to tillage by the ox-drawn ard plough appears to be an important source of colluviation behind stone bunds and lynchets in the Ethiopian highlands. To quantify erosion rates caused by this plough in Ethiopia, painted and numbered rock fragments, 3-5 cm in intermediate diameter, were used as tracers to monitor soil movement on 16 sites, each having a different slope gradient, in the district of Dogu'a Tembien, Tigray Region, Ethiopia. Average tillage depth was 8.1 cm and the net mean downslope displacement distance per tillage operation ranged from 4.7 cm for a 0.03 m m(-1) slope to 34.4 cm for a field with a gradient of 0.48 m m(-1). There was a strong correlation (R-2 = 0.84, P < 0.001) between slope gradient and downslope displacement. Where present, large rock fragments (>15 cm intermediate diameter) are obstacles to the downslope movement of tilled soil. The unit soil transport rate (Qs) per tillage operation ranged from 4.8 kg m(-1) on the 0.03 m m(-1) slope to 38.7 kg m(-1) on the 0.48 m m(-1) slope. These values represent the mass of soil deposited by tillage behind 1 m of lynchet or stone bund. During each tillage operation the same mass of soil is also removed from the foot of the upper stone bund or lynchet. For the first tillage opc:ration, before the onset of the rainy season, the tillage transport coefficient (K) was 68 kg m(-1). As farmers till 1-4 times per year, annual K values can be assessed to range from 68 to 272 kg m(-1). These values are less than those observed for mechanised tillage, which however, is usually conducted on less steep slopes. On average, tillage erosion can be held responsible for half of the sediment deposited behind newly constructed stone bunds in the Tigray highlands. (C) 2000 Elsevier Science B.V. All rights reserved.