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Abstract: In the semi-arid Main Ethiopian Rift Valley, ecological succession is related to continuous lake retreat. Human activity, through its impact on land use and cover, affects this ecological succession at various degrees. Through a remote sensing study, we explored how the drivers for land use and cover changes (LUCC) have changed over the last decades and which impact this has on ecological succession. Remote sensing data used include a Landsat MSS from 1973, a Landsat TM from 1986 and Landsat ETM+ from 2000; a conventional type of classification was used whereby supervised classification of the 2000 image was supplemented by unsupervised classification of the older images. Due to decreased rainfall and water abstraction for intense irrigated agriculture in its catchment, Lake Abijata lost 46% of its area between 2000 and 2006. On the emerged land, an ecological succession was observed along the environmental gradient of the retreating lake: emerged bare land, grassland, land with few scattered Acacia shrubs and open woodlands. Between 1986 and 2000, LUCC tendencies were totally reversed and woody vegetation decreased strongly, indicating increased human impact. This land degradation took place in a context of instable political situation, fuelwood extraction, higher population density and better communications. [Copyright &y& Elsevier]

Abstract: In the highlands of northern Ethiopia, degraded hillslopes are closed for grazing and agriculture for environmental rehabilitation. In order to assess the effect of these exclosures on water balances, a correct assessment of evapotranspiration is needed. A common method for calculating evapotranspiration employs the reference evapotranspiration in combination with a crop coefficient. However, for natural vegetation, these crop coefficients are not readily available. Therefore, the objective of this study was to derive crop coefficients for semiarid natural vegetation using two different approaches. The crop coefficients were assessed with the dual crop coefficient approach as outlined in the FAO Irrigation and Drainage Paper n°56 and compared with crop coefficients derived from a simple soil water balance. A good agreement between both methods was found. The mid-season crop coefficients ranged from 0.50 to 0.85 for open vegetation with small shrubs and from 0.85 to 0.95 for well developed shrubland. Vegetation with a closed tree canopy was characterized by a mid-season crop coefficient between 0.95 and 1.1. Additionally, a significant logarithmic relation between vegetative soil cover and crop coefficient was found. Using this relation, crop coefficients can be easily derived for semiarid shrub savannahs in similar environments and used for the calculation of evapotranspiration. [Copyright &y& Elsevier]

The Ethiopian ard plough ‘Maresha’ is a tillage tool that most farmers still use for land preparation. The production of the wooden structure is based on experience, culture, and trial and error methods. In this paper, the basic design of the ard plough is presented. The mathematical descriptions (traditional force analysis) are based on static analysis at equilibrium of the structure. The forces considered were the pulling forces provided by a pair of draught animals, the operator input force, gravitational weight of the implement, the normal and tangential interfacial forces acting on the ploughshare and wooden side-wings and the inertial force. The draught, vertical, tangential interfacial and normal interfacial forces of the implement were determined. Moreover, sensitivity of draught and vertical forces to different pulling angles and sensitivity of normal and tangential interfacial forces to different rake angles were investigated. The force analysis was validated by means of the finite element (FE) analysis using the ABAQUS package. It was confirmed that draught force on the ploughshare increased with pulling angle. Similarly, the tangential interfacial force of the implement was higher than the normal interfacial capacity at lower pulling angles. The output of the FEM and traditional calculation resulted in small errors of less than 3% for draught and 5% for vertical forces for small pulling angles ⩽30°. This study integrates the previous research experiences with theory and computer-based analysis and simulations. The design guidelines and considerations for improving or developing small-scale tillage implements are presented. [Copyright &y& Elsevier]

Abstract: As a response to land degradation in Tigray, Northern Ethiopia, regional authorities have promoted rehabilitation of degraded lands through closed areas since 1991. It is generally assumed that exclosures lead to restoration of natural resources such as soil fertility, vegetation biomass and composition, fauna, and water storage; however, this is not supported by studies. In the present study, our objective was to assess the impact of exclosures on soil properties, and relate the age of exclosures to their effectiveness in improving soil chemical and physical properties. Five and ten years closed areas had significantly () higher levels for soil organic matter, total nitrogen, and available phosphorus compared to free grazing lands, and the highest levels in were observed in closed areas at footslope position. Our results show that exclosures are not only effective in restoring vegetation, but also in improving soil nutrient status, and reducing erosion. From a technical point of view, it is therefore recommendable to convert grazing areas into exclosures. However, implementing exclosures leads to an even smaller communal area left for grazing and the socio-economic consequences of this should also be considered. [Copyright &y& Elsevier]