Gully head retreat rates in the semi-arid highlands of Northern Ethiopia

Abstract

Due to overgrazing and agricultural intensification, gully erosion severely affects sub-Saharan countries; however, insufficient quantitative studies exist for this part of the world. This paper presents data on gully head retreat rates in Northern Ethiopia and relates these rates to gully and environmental characteristics. The monitoring of headcuts over one rainy season (2010) revealed that present-day retreat rates are low, with average annual linear (Rl), areal (Ra) and volumetric (Ve) retreat rates of 0.34my−1, 1.70m2y−1 and 5.2m3y−1, respectively. These results express the positive effects of recent soil and water conservation practices on gully stabilization. Significantly higher values of Rl (up to 1.93my−1) occurred in the Vertisol areas affected by soil piping. When considering the medium- to long time scale (1–47years) using archival terrestrial (and aerial) photographs, headcut retreat rates proved to be significantly higher than those in the short term. The averages for Rl, Ra and Ve are 3.8my−1, 31.5m2y−1 and 47.7m3y−1, respectively. Retreat rates are up to 10 times higher after road construction. For the medium to long term, headcut retreat rates were positively related to the catchment area (A). A power relationship that best describes the relation between Ve and A is Ve=0.53A0.31 (r2=0.27, n=18). Compared to other areas worldwide, regressive erosion has been rapid in Ethiopia as a result of the degraded and steep landscape combined with erosive rains and the occurrence of Vertisols. In Vertisols, headcut retreat is controlled by soil piping. Because no adequate techniques exist to control gully initiation and development in Vertisols, alternative techniques should be developed that deactivate soil pipes.