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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 (R l ), areal (R a ) and using archival terrestrial (and aerial) photographs, headcut retreat rates proved to be significantly higher than those in the short-term. The averages for R l , R a and V e are 3.8 m y -1 , 31.5 m² y -1 and 47.7 m³ y -1 , respectively. Retreat rates are up to 10 times higher after road construction. On the medium-to long-term time scale, headcut retreat rates were positively related to the catchment area (A). A power relationship that best describes the relation between V e and A is V e = 0.

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15The ability to understand gully erosion development is closely related to our ability to quantify 16 the morphology of gullies. At present, various technologies are at hand to collect data at 17 increasing levels of detail. However, many of the developed technologies are time-consuming, 18 difficult to apply or expensive. As an alternative, image-based modelling offers a cost-efficient, 19 flexible and rapid method to quantify gully morphology from photographs taken in the field. In 20 this study, the use of image-based modelling was tested and fine-tuned to quantify the 21 CITE AS Frankl, A., Stal, C., Abraha, A., Nyssen, J., Rieke-Zapp, D., De Wulf, A., Poesen, J., 2015. Detailed recording of gully morphology in 3D through image-based modelling. Catena 127, 92-101. 2 morphology of four gully heads in contrasting biophysical environments prone to gully erosion: 22 two bank gullies in Central Belgium and two permanent gullies in Northern Ethiopia. Ground 23 photographs (n = 88-235) were taken with a reflex Canon EOS 450D camera having a 20 mm 24 wide-angle lens with a fixed focal length. The data collection occurred during days of 30-100% 25 cloud cover and after removing excessive vegetation in the gullies. Processing of the 26 photographs occurred in PhotoScan 1.0.2. software using the semi-automated Structure from 27Motion-Multi View Stereo (SfM-MVS) workflow, and allowed to produce 3D Digital Elevation 28Models with accuracies that range from millimetres to centimetres. In addition, for the same 29 surface, 2.5D models were created in ArcGIS. Gully morphological properties were derived and 30 include cross-sections, total volume and volume of undercut walls and soil pipe inlets. For the 31 volume calculation, OPTOCAT software was used. Cross-sections were also quantified by tape 32 meter measurements. When compared to 3D models, cross-sections quantified from tape meter 33 measurements and from 2.5D models underestimate the cross-sectional area by <1-14% and 0-34 2.5% respectively. Considering gully volume, 2.5D and 3D approximations show only small 35 differences, related to the volume of soil pipe inlets and undercutting areas. These differences, 36 however, highlight the erosive activity of the gullies, and are important to understand gully 37 dynamics in detail. Geomorphologically spoken, areas where undercutting or soil piping occurs 38 are among the most dynamic and reveal where important morphologic changes are about to occur. 39The accuracies reported in this study are similar to those obtained in other studies that consider 40 surfaces of similar scales. In sum, image based modelling is a promising tool to study in detail 41 gully morphology in 3D, which is the closest approximation of the surface morphology. 42 Frankl, A., Stal, C., Abraha, A., Nyssen, J., Rieke-Zapp, D., De Wulf, A., Poesen, J., 2015. Detailed recording of gully morphology in 3D through image-based modelling. Catena 127, 92-101. 3 43 CITE AS

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As quantitative or spatially distributed studies of environmental change over truly long-term periods of more than 100 years are extremely rare, we re-photographed 361 landscapes that appear on historical photographs (1868-1994) within a 40,000 km² study area in northern Ethiopia. Visible evidence of environmental changes apparent from the paired photographs was analyzed using an expert rating system. The conditions of the woody vegetation, soil and water conservation structures and land management were worse in the earlier periods compared to their present conditions. The cover by indigenous trees is a notable exception: it peaked in the 1930s, declined afterwards and then achieved a second peak in the early 21 st century. Particularly in areas with greater population densities, there has been a significant increase in woody vegetation and soil and water conservation structures over the course of the study period. We conclude that except for an apparent upward movement of the upper tree limit, the direct human impacts on the environment are overriding the effects of climate change in the north Ethiopian highlands and that the northern Ethiopian highlands are currently greener than at any other time in the last 145 years.