Plough marks as a tool to assess soil erosion rates: A case study in Axum (Ethiopia)

From the antiquity, the hilly landscape around the city of Axum in northern Ethiopia has been terraced and extensively used for agriculture purposes. Impacts of the “ard” plough on protruding and/or buried boulder at a depth less than the tillage depth produce scratches on their sides or their top. The spatial relationship among plough marks and rock surface orientations gives information on the ploughing direction, whereas the relative height from the present ground surface to the top of the marks and information on the age of cultivation can be used to assess soil loss rates. Parameters to define plough mark orientation were defined and measured in the field on boulders with plough marks within a study terrace located in an archaeological area with an approx of 2040 year age. These parameters are height from the ground, dip, slope and frequency. asurements also show that the initial terrace slope was steeper than the present one confirming the effectiveness of the traditional terracing practice in soil conservation because of the low soil erosion rates assessed over a so large time interval. For the main time intervals of the Axumite history, corresponding to the rise and fall of the Axumite civilization and to variable conditions of human pressure on the land, the maximum erosion rates inferred are of the order of 3.4 Mg ha− 1 y− 1. This result is compared with erosion rates calculated with other methods reported in literature (e.g. PSIAC) and field measurement on terraces in the same region leading to values coinciding with those obtained by the plough marks method.