Gully and soil and water conservation structure densities in semi-arid northern Ethiopia over the last 80years

Gullies have been a common phenomenon in semi-arid northern Ethiopia for the last centuries. However, soil and water conservation (SWC) structures have been implemented for a long time to curb soil erosion. Though, like most of the affected areas worldwide, density and distribution of gullies and SWC structures, their causes and interrelations are poorly understood. The aims of this study were to develop a technique for mapping these densities of gullies and SWC structures, to explain their spatial distribution and to analyze changes over the period 1935-2014. Aerial photographs from 1935 to 1936 and Google Earth images from 2014 of the 5142km(2) Geba catchment were used. Transect lines were established to count gullies and SWC structures in order to calculate densities. On average, a gully density of 1.14kmkm(-2) was measured in 1935-1936 of which the larger portion (75%) were vegetated, indicating they were not very active. Over 80years, gully density has significantly increased to 1.59kmkm(-2) with less vegetation growing in their channel, but 66% of these gullies were treated with check dams. There was c. 3kmkm(-2) of indigenous SWC structures (daget or lynchets) in 1935-1936 whereas a high density (20kmkm(-2)) of introduced SWC structures (mainly stone bunds and terraces) were observed in 2014. The density of gullies is positively correlated with slope gradient and shrubland cover and negatively with cropland cover, whereas the density of SWC structures significantly increased with increasing cropland cover. Density maps of gullies and SWC structures indicate sensitive areas to gully formation and priority areas for the implementation of SWC structures in Geba catchment. The obtained results illustrate the feasibility of the methods applied to map the density of gullies and SWC structures in mountainous areas. Copyright (c) 2018 John Wiley & Sons, Ltd.