Jan Nyssen

Human impact on the environment in the Ethiopian and Eritrean highlands¿a state of the art

Abstract This review analyses the environmental evolution of the Ethiopian highlands in the late Quaternary. The late Pleistocene (20,000–12,000 14C years BP) was cold and dry, with (1) low lake levels in the Rift Valley, (2) large debris fans on the flanks of Lake Abhe basin, and (3) the Blue Nile transporting coarse bedload. Then, a period with abundant and less seasonal rains existed between 11,500 and 4800 14C years BP, as suggested by increased arboreal pollen, high river and lake levels, low river turbidities and soil formation. Around 5000–4800 14C years BP, there was a shift to more arid conditions and more soil erosion. Many phenomena that were previously interpreted as climate-driven might, however, well be of anthropic origin. Thick sediment deposits on pediments as well as an increase of secondary forest, scrub and ruderal species in pollen diagrams are witnesses of this human impact. One important aspect of the late Quaternary palaeoenvironment is unclear: changes in Nile flow discharges and Rift Valley lake levels have been linked to changes in precipitation depth. Most authors do not take into account changes in land use in the highlands, nor changes in the seasonality of rain, both of which can lead to a change in runoff coefficients. Tufa and speleothem deposition around 14,000 years ago tend to indicate that at the end of the Last Glacial Maximum (LGM), conditions might have been wetter than generally accepted. The most important present-day geomorphic processes are sheet and rill erosion throughout the country, gullying in the highlands, and wind erosion in the Rift Valley and the peripheral lowlands. Based on existing sediment yield data for catchments draining the central and northern Ethiopian highlands, an equation was developed allowing to assess area-specific sediment yields: SY =2595A −0.29 (n=20; r 2 =0.59) where SY=area-specific sediment yield (t km−2 year−1), and A=drainage area (km2). With respect to recent environmental changes, temporal rain patterns, apart from the catastrophic impact of dry years on the degraded environment, cannot explain the current desertification in the driest parts of the country and the accompanying land degradation elsewhere. Causes are changing land use and land cover, which are expressions of human impact on the environment. Deforestation over the last 2000–3000 years was probably not a linear process in Ethiopia. Studies on land use and land cover change show, however, a tendency over the last decades of increasing removal of remnant vegetation, which is slowed down or reversed in northern Ethiopia by a set-aside policy. Ongoing land degradation requires urgent action at different levels of society. Soil and water conservation (SWC) structures are now widely implemented. Local knowledge and farmers initiatives are integrated with introduced SWC techniques at various degrees. Impact assessments show clear benefits of the soil conservation measures in controlling runoff and soil erosion. In high rain areas, runoff management requires greater emphasis during the design of soil conservation structures. In such areas, investment in SWC might not be profitable at farm level, although benefits for society are positive. This pleads in favour of public support. The present land degradation in the Ethiopian highlands has a particular origin, which includes poverty and lack of agricultural intensification. Causes of these are to be found in the nature of past and present regional social relations as well as in international unequal development. This review strengthens our belief that, under improved socio-economic conditions, land husbandry can be made sustainable, leading to a reversal of the present desertification and land degradation of the Ethiopian highlands.

Desertification? Northern Ethiopia re-photographed after 140 years

A collection of sepia photographs, taken during Great Britains military expedition to Abyssinia in 1868, are the oldest landscape photographs from northern Ethiopia, and have been used to compare the status of vegetation and land management 140 years ago with that of contemporary times. Thirteen repeat landscape photographs, taken during the dry seasons of 1868 and 2008, were analyzed for various environmental indicators and show a significant improvement of vegetation cover. New eucalypt woodlands, introduced since the 1950s are visible and have provided a valuable alternative for house construction and fuel-wood, but more importantly there has also been locally important natural regeneration of indigenous trees and shrubs. The situation in respect to soil and water conservation measures in farmlands has also improved. According to both historical information and measured climatic data, rainfall conditions around 1868 and in the late 19th century were similar to those of the late 20th/early 21st century. Furthermore, despite a ten-fold increase in population density, land rehabilitation has been accomplished over extensive areas by large-scale implementation of reforestation and terracing activities, especially in the last two decades. In some cases repeat photography shows however that riparian vegetation has been washed away. This is related to river widening in recent degradation periods, particularly in the 1970s-1980s. More recently, riverbeds have become stabilized, and indicate a decreased runoff response. Environmental recovery programmes could not heal all scars, but this study shows that overall there has been a remarkable recovery of vegetation and also improved soil protection over the last 140 years, thereby invalidating hypotheses of the irreversibility of land degradation in semi-arid areas. In a highly degraded environment with high pressure on the land, rural communities were left with no alternative but to improve land husbandry: in northern Ethiopia such interventions have been demonstrably successful.

The environmental significance of the remobilisation of ancient mass movements in the Atbara–Tekeze headwaters, Northern Ethiopia

Abstract Old landslides are prominent features in the landscape around Hagere Selam, Tigray Highlands, Ethiopia. The available evidence suggests their Late Pleistocene to Middle Holocene age and conditions of soil humidity. The affected geological layers, often silicified lacustrine deposits prone to sliding, rest upon or above the water holding Amba Aradam sandstone aquifer. Three examples of present-day (remobilisation of old) mass movements are illustrated and discussed. The aims of the study were to unravel the environmental conditions of the present-day remobilisation of ancient flows, as well as those of first-time landslides. The first two mass movements discussed are slumps, located in areas with vigorous regeneration of (grassy) vegetation. Their activation is thought to be the consequence of an increase in infiltration capacity of the soils under regenerating vegetation. One of these slumps had a horizontal movement of the order of 10–20 m in 1 day. The other case is the remobilisation of the May Ntebteb debris flow below the Amba Aradam sandstone cliff. The debris flow presently creeps downslope at a rate of 3–6 cm year−1. Palynological evidence from tufa shows that the reactivation of the flow started 70 years ago. Shear resistance measurements indicate the danger for continuous or prefailure creep. From the soil mechanics point of view, the reactivation of the debris flow is due to the combination of two factors: (1) the reduction of flow confining pressures as a result of gully incision over the last hundred years, and (2) the increase of seepage pressure as a consequence of the cumulative effect of this incision and the increase in infiltration rates on the lobe since grazing and woodcutting have been prohibited 8 years ago. The role of such exclosures as possible landslide triggers is discussed. From the geomorphological point of view, the ancient movements and their present-day reactivation cannot be compared: the ancient movements led to the development of debris flows, whereas the reactivations relate to the dissection of these mass movement deposits.

Soil and water conservation in Tigray (Northern Ethiopia): the traditional dagat technique and its integration with introduced techniques

Between the cultivated fields in the highlands of Tigray, one finds, besides the recently introduced stone bunds, many lynchets, with a height ranging from 0·3–3 m. Grasses occupy the riser and a more or less large strip on the shoulder. Traditionally, farmers established an untilled strip of about 2 m wide at the lower plot limit. This grass strip reduced runoff velocity, allowed for water to infiltrate and trapped sediment. Year after year, these lynchets, locally called dagat, continued to grow. In this study, the dagat technique is characterized and its evolution and reasons for partial destruction are outlined. Stereoscopic aerial photo analysis shows for the study area near Hagere Selam (13 °39′N, 39 °10′E, 2650 m a.s.l.) that 20·7 per cent of the major dagats (more than 1 metre high) have disappeared between 1974 and 1994. These lynchets, however, remain an important linear element in the landscape (22·7 m ha−1, i.e. their density on cultivated fields is much higher). Of the smaller lynchets, a great proportion has been levelled in order to increase plot surfaces and spread fertile soil over the field. Famines and impoverishment caused the farmers to increase short-term agricultural production in this way. Since the 1980s, the farmers built stone bunds on most of the cultivated land. Their average length equals 56·1 m ha−1 in the study area. The establishment of stone bunds results in the development of small terraces. Especially during recent years, there is a tendency to integrate the traditional knowledge of dagat with the building of stone bunds. A quantitative assessment of the effectiveness of both techniques must be made. Copyright

Effects of land use, slope gradient, and soil and water conservation structures on runoff and soil loss in semi-arid Northern Ethiopia

Land degradation and recurrent drought are the major threats to rain-fed agriculture in the semi-arid Ethiopian highlands. Water harvesting has become a priority in the Tigray region since 1990. However, the success of water harvesting in reservoirs is limited due to reduced inflow. The aim of this study was to investigate the effects of typical land-use types, slope gradients, and different soil and water conservation (SWC) structures on runoff and soil loss at the runoff-plot scale. Six runoff measuring sites, corresponding to three slope gradients, were established for cropland (cultivated land for annual crop production) and rangeland (heavily grazed land on hillslopes with high rock-fragment cover) at Mayleba catchment in Tigray, Ethiopia. SWC structures tested were stone bunds, trenches, and stone bunds with trenches, in addition to control plots. In total, 21 large runoff plots (with lengths of 60 to 100 m) were monitored daily for runoff production and soil loss during the main rainy season (July–September) in 2010. The results show that the seasonal runoff coefficient (RCs) representing the fraction of rainfall measured as runoff was much higher for rangeland (0.38 < RCs < 0.50) compared to that for cropland (0.11 < RCS < 0.15). Seasonal soil loss (SLs) values were five to six times larger on rangeland (28.6 < SLs < 50.0 ton ha−1) compared to that for cropland (4.6 < SLs < 11.4 ton ha−1). Stone bunds with trenches were the most effective SWC structures in reducing runoff and soil loss. With the same SWC structures installed, RCs and SLs for both rangeland and cropland tend to decrease with increasing slope gradient mainly due to a corresponding increase in rock-fragment cover. The effects of SWC structures on runoff production and soil loss are considerable; hence, it is crucial to consider these effects for optimal design of water-harvesting schemes such as micro-dams that collect and store surface runoff for irrigation development in the Ethiopian highlands.

Soil erosion and conservation in Ethiopia: A review

This paper reviews Ethiopia’s experience and research progress in past soil and water conservation (SWC) efforts and suggests possible solutions for improvement. Although indigenous SWC techniques date back to 400 BC, institutionalized SWC activity in Ethiopia became significant only after the 1970s. At least six national SWC-related programs have been initiated since the 1970s and their focus over time has shifted from food relief to land conservation and then to livelihoods. The overall current soil erosion rates are highly variable and large by international standards, and sheet, rill, and gully erosion are the dominant processes. The influence of human activities on the landscape has traditionally been deleterious, but this trend seems to have recently reversed in some parts of the country following the engagement of the communities in land management. The efficiency of SWC measures show mixed results that are influenced by the type of measures and the agro-ecology under which they were implemented; in general, the relative performance of the interventions is better in the drylands as compared with humid areas. Methodological limitations also occur when addressing the economic aspects related to benefits of ecosystem services and other externalities. Although farmers have shown an increased understanding of the soil erosion problem, SWC efforts face a host of barriers related to limited access to capital, limited benefits, land tenure insecurity, limited technology choices and technical support, and poor community participation. In general SWC research in Ethiopia is fragmented and not comprehensive, mainly because of a lack of participatory research, field observations, and adoptable methods to evaluate impacts. A potentially feasible approach to expand and sustain SWC programs is to attract benefits from global carbon markets. Moreover, a dedicated institution responsible for overseeing the research–extension linkage of SWC interventions of the country should be established.

Environmental conditions and human drivers for changes to north Ethiopian mountain landscapes over 145 years

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 km(2) 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 21st 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.

A Political Ecology Perspective of Land Degradation in the North Ethiopian Highlands

Severe environmental degradation in the north Ethiopian Highlands is among others the result of mismanagement, overpopulation and droughts. However, here, we investigate the linkages of land degradation with the historical dynamics of the political–ecological system and regional land policies. We performed semi-structured interviews with 93 farmers in eight villages in the Tigray region (north Ethiopia) and conceptualised a political–ecological model of land tenure and degradation changes for the region. Results show that different land policies caused and still cause land degradation in several ways. Interviews reveal that the unequal character of land rights during feudal times played an important role in 19th and 20th century land degradation. In particular, poor farmers were forced to construct their farms on marginal terrains, such as steep slopes in dry areas and marshes in cold and humid areas, increasing the catchment water runoff and degradation. The interviews further suggest that after the Derg regime (1974–1991), environmental conservation strategies were successfully implemented at larger scales. Overall, feudal, Derg and contemporary land policies have all had impacts on environmental degradation and have left their fingerprints on the physical landscape of northern Ethiopia. Copyright

Tree species selection for land rehabilitation in Ethiopia: from fragmented knowledge to an integrated multi-criteria decision approach

Dryland regions worldwide are increasingly suffering from losses of soil and biodiversity as a consequence of land degradation. Integrated conservation, rehabilitation and community-based management of natural resources are therefore of vital importance. Local planting efforts should focus on species performing a wide range of functions. Too often however, unsuitable tree species are planted when both ecological suitability for the targeted area or preferences of local stakeholders are not properly taken into account during selection. To develop a decision support tool for multi-purpose species selection, first information needs to be pooled on species-specific ranges, characteristics and functions for a set of potentially valuable species. In this study such database has been developed for the highly degraded northern Ethiopian highlands, using a unique combination of information sources, and with particular attention for local ecological knowledge and preferences. A set of candidate tree species and potentially relevant criteria, a flexible input database with species performance scores upon these criteria, and a ready-to-use multi-criteria decision support tool are presented. Two examples of species selection under different scenarios have been worked out in detail, with highest scores obtained for Cordia africana and Dodonaea angustifolia, as well as Eucalyptus spp., Acacia abyssinica, Acacia saligna, Olea europaea and Faidherbia albida. Sensitivity to criteria weights, and reliability and lack of knowledge on particular species attributes remain constraints towards applicability, particularly when many species are jointly evaluated. Nonetheless, the amount and diversity of the knowledge pooled in the presented database is high, covering 91 species and 45 attributes.

Spatial distribution of rock fragments in cultivated soils in northern Ethiopia as affected by lateral and vertical displacement processes

Abstract Although, in semi-arid environments, rock fragments at the soil surface and within the topsoil play an important role in desertification control, little is known about their spatial distribution. Therefore, this study analyses spatial patterns of rock fragment cover along catenas, and the vertical variations in volumetric rock fragment content in soil profiles in the highlands of Tigray, northern Ethiopia. Natural and anthropogenic processes inducing these patterns are assessed. Volumetric rock fragment content (RV) was analysed in 10 soil pits. All rock fragments were extracted, and their volumes determined by pedo-stratigraphic unit, size and lithology. The rock fragment cover (RC) was determined by the point-count method using vertical photographs of the soil surface. The following processes contribute to the vertical variability of RV: (1) in Vertisols, upsqueezing as a consequence of swell-shrink cycles (argillipedoturbation) is responsible for high RC at the soil surface; (2) large rock fragments (>7.5 cm) are rapidly brought to the soil surface by kinetic sieving through tillage, even in the case of continuous fine sediment deposition, what may result in a rock fragment rich subsoil, underlying a thick soil layer (up to 80 cm) without large rock fragments and a topsoil with a high RC at the surface; (3) Skeletic Regosols at the foot of cliffs show no systematic vertical rock fragment distributions. As to rock fragment cover along the catena, some fundamental differences appear between the basalt and limestone substrate. On the basalt catena with slope gradients between 0.06 and 0.42 m m−1, RC is high everywhere (57–85%) and is unrelated to slope gradient. Vertical processes such as kinetic sieving through ploughing and argillipedoturbation determine the rock fragment distribution at the soil surface. In limestone areas, argillipedoturbation is less active and RC is positively correlated with slope gradient (R2=0.74; n=6; P 2 cm) and more kinetic sieving sensitive rock fragments lowers R2 to 0.46 (n=6; n.s.). Preconditions for the present spatial distribution of rock fragments at the soil surface in the study area are the occurrence of two geomorphic processes in the past: (1) mudflows depositing diamictons including much coarse debris and (2) intense water erosion, which occurred after deforestation and exposed rock fragments at or near to the soil surface. Generally, the balance between lateral and vertical movements of rock fragments now controls the spatial distribution of RC. With respect to the lateral displacement processes: (1) lateral transport over the soil surface by trampling, tillage and concentrated runoff, especially on steep slopes, and (2) rockfall from the cliffs. Vertical supply of rock fragments to the soil surface is caused by (1) selective runoff erosion and the development of erosion pavements, (2) tillage induced kinetic sieving, bringing preferentially large rock fragments (>7.5 cm) to the soil surface, and (3) argillipedoturbation in Vertisol areas.