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GM4.5/SSS9.19

Global change and geomorphic processes in the Horn of Africa (co-organized)
Convener: Jan Nyssen  | Co-Convener: Paolo Billi 
Orals
 / Wed, 30 Apr, 13:30–15:00  / Room G10
Posters
 / Attendance Wed, 30 Apr, 17:30–19:00  / Blue Posters
The rugged landscapes of the Horn of Africa have been impressed and partly degraded by agriculture since 3 millennia at least. As soils are not silty and as they often bear a dense stone cover, soil profiles have not yet been fully truncated by soil erosion that is concomitant to tilled agriculture.
Over the last 20 years, we have used an array of geomorphological and environmental research methodologies in the Horn of Africa, focussing on processes, their rates and spatial variability.
Results of palaeoenvironmental studies indicate that forest clearance and regrowth have been a cyclic process over the last 3000 years. As of the late 19th century, gullies were present, though they had become stabilised by 1935; a strong incision phase started in the 1960s.
Currently, in areas with low density of soil and water conservation (SWC) structures, soil loss rates are in the order of 9 t ha-1 y-1 at catchment scale, attributable for 2/3 to sheet and rill erosion. Gully erosion rates and drainage density are decreasing since ca. 2000, in line with increased conservation activities and improved vegetation cover.
In situ water harvesting and the construction of small reservoirs have both led to strongly decreased runoff coefficients (RC) at catchment scale, and to increased levels of the water tables. Particularly in cases of wide conversions to forest, such as around Alamata, effects are very clear, particularly in terms of decreased downstream flooding and changes to hydrogeomorphology (river incision, narrowing). On-farm, conservation agriculture experiments showed promising results, particularly in terms of reduced soil loss and runoff response; it is now a matter of finding the appropriate ways of scaling it up: action research on farmer-to-farmer extension and empowerment would be most welcomed.
Further, global climate change will bring higher temperatures and, according to most models, increased yearly rainfall. Anecdotal evidence shows that this already leads to an upward movement of the agroclimatic boundary for wheat. The impacts of global warming on the upper treeline will need to be disentangled from impacts of recent human settlement near the afro-alpine areas and on runoff response (hence drainage density and soil erosion) that is anticipated to strongly increase in the openfield landscapes of northern Ethiopia. On the other hand, larger runoff volumes increase possibilities for irrigated agriculture in lower-lying areas, particularly the marginal grabens of the Rift Valley.
Cornerstones of sustainable land management include forest development in critical places, over sufficiently large areas. Soil and water conservation activities also enhance infiltration during the short but heavy storms and improve the situation with regard to flooding, soil erosion and groundwater. The current land tenure system in which an equality of land holdings is attempted, favours solidarity among the farmers to undertake communal catchment management activities. Besides the need for collecting a wide set of original data, conceptually, in all related research, a good comprehension of the hydrological balance is needed. Further, for nutrient, sediment and water-related processes, it is important to understand the occurrence of sinks and to keep the scale concept in mind. These principles are at the base of the successful implementation of catchment management activities in the Horn of Africa; in this regard the impacts of catchment management could be monitored in detail and future development scenarios could be elaborated.

Invited talks:
- Mitiku Haile (Mekelle, Ethiopia, and UNESCO; Confirmed) – Sustainable Land Management in the Ethiopian highlands (
- Francesco Dramis (Roma3) – Mid- to late Holocene stages of tufa deposition/erosion in Ethiopia: the role of climate fluctuations and human impact
- Hans Hurni (Bern)
- Martin Williams (Adelaide, Australia; Confirmed) – Distinguishing between natural and anthropogenic desertification processes in the Ethiopian Rift and the Jubba Valley, western Somalia
- Asfawossen Asrat (Addis Ababa, Ethiopia)

The conveners aim at editing a Special Issue of the journal Geomorphology, incorporating selected papers presented in the session.