Sediment‐bound nutrient export from micro‐dam catchments in Northern Ethiopia

The losses in soil nutrients (nitrogen (N), available phosphorus (P_av), organic carbon (OC), potassium (K), calcium (Ca) and magnesium (Mg)) in the catchment and the storage in the reservoir as a result of sediment delivery were assessed in 13 catchments/reservoirs in Tigray, Northern Ethiopia. This specifically dealt with factors controlling the losses, the fertility status of the deposited sediment, the nutrient export (NE) rates and associated costs. Enrichment ratio (ER) values >1 were observed for the plant nutrients and the finer soil fractions. The high ER is associated with the preferential transport of nutrients bound to finer soil fractions and the parent material dissolution and its transport via runoff. However, the fertility status of the deposited sediment is not sufficient by itself to support a sustainable crop growth and hence external addition is necessary, mainly for N and P_av fertilizer. Generally, rates of NE were high. The high OC export on the other hand dictates the potential of reservoir sediments for OC sequestration. The cost price of loss of only N and P_av, eroded from the catchment slopes, was estimated at €34·2 million (Euros) in March 2006 for the Tigray. Pity enough, policy makers and beneficiaries do not realize the magnitude of the problem, which forms a major threat for the crop production in the country. Therefore, it is important not only to make the public aware of the problem but also of implementing integrated soil fertility management practices. Copyright © 2007 John Wiley & Sons, Ltd.     

Factors Affecting Small-Scale Farmers’ Land Allocation and Tree Density Decisions in an <Emphasis Type="Italic">Acacia decurrens</Emphasis>-Based <Emphasis Type="Italic">taungya</Emphasis> System in Fagita Lekoma District,North-Western Ethiopia

The development of woodlots as an additional source of livelihood and as a land management option for small-scale farmers is a challenging issue in the cereal-based subsistence farming systems of north-western Ethiopia. There is a need to understand why and which factors determine the decisions of small-scale farmers to grow short-rotation woodlots on their land. Data used in this study were collected from a survey of 200 randomly selected households in the region. A Tobit regression model was used to determine predictor variables for farmers’ decisions to allocate land to planting Acacia decurrens (J.C. Wendl.) Willd. and at what density trees are planted on the respective plots. The most important motivations for planting A. decurrens were income, soil fertility management, and soil and water conservation. Having a male head of household, long distance to markets and plots being on marginal land, among other factors, increased the allocation of land to A. decurrens woodlots. Having a male head of household, access to credit and plots being on marginal land, among other factors, increased tree planting density. Age had a negative effect on both allocation of land to woodlots and tree density, whereas farm size had an inverted U-shaped relationship with both decisions. These results suggest that wider expansion of A. decurrens-based plantation systems could be achieved through improving extension, credit access and road infrastructure to connect small-scale farmers to markets and finance.     

Reservoir sedimentation and its mitigating strategies: a case study of Angereb reservoir (NW Ethiopia)

Purpose  

The Angereb dam in northwestern Ethiopia was commissioned in 1997 to serve as a domestic water supply for 25 years. However, its sustainability is being threatened by rapid sedimentation. The overall objective of this study was to understand reservoir sedimentation in this tropical highland watershed and to propose its mitigating strategies that would contribute to the improved planning and management of reservoirs in similar regions.     

Mitigating the anti-nutritional effect of polyphenols on in vitro digestibility and fermentation characteristics of browse species in north western Ethiopia

Tropical Animal Health and Production - Browse species are important sources of forage for livestock in Ethiopia, especially during the dry season, when the quality and quantity of green herbage is...     

Environmental change,geomorphic processes and land degradation in tropical highlands

The international symposium ‘HighLand2006’, held in Mekelle (Ethiopia), from 21 to 25 September 2006, was the second time that Ethiopia could host a major international scientific event on geomorphological processes. HighLand2006 created a forum for those conducting research on the impacts of environmental change on geomorphic processes and land degradation in East African highlands as well as in similar regions around the globe. Discussions of results and exchange of experiences during indoor sessions as well as excursions have resulted in this distinctive set of scientific papers.     

ASSESSING THE PERFORMANCE OF A SPATIALLY DISTRIBUTED SOIL EROSION AND SEDIMENT DELIVERY MODEL (WATEM/SEDEM) IN NORTHERN ETHIOPIA

Most regional‐scale soil erosion models are spatially lumped and hence have limited application to practical problems such as the evaluation of the spatial variability of soil erosion and sediment delivery within a catchment. Therefore, the objectives of this study were as follows: (i) to calibrate and assess the performance of a spatially distributed WATEM/SEDEM model in predicting absolute sediment yield and specific sediment yield from 12 catchments in Tigray (Ethiopia) by using two different sediment transport capacity equations (original and modified) and (ii) to assess the performance of WATEM/SEDEM for the identification of critical sediment source areas needed for targeting catchment management. The performance of the two model versions for sediment yield was found promising for the 12 catchments. For both versions, model performance for the nine catchments with limited gully erosion was clearly better than the performance obtained when including the three catchments with significant gully erosion. Moreover, there is no significant difference (alpha 5 per cent) between the performances of the two model versions. Cultivated lands were found to be on average five times more prone to erosion than bush–shrub lands. The predicted soil loss values in most parts of Gindae catchment are generally high as compared with the soil formation rates. This emphasises the importance of implementing appropriate soil and water conservation measures in critical sediment source areas prioritising the steepest part of the catchment (i.e. areas with slope >50 per cent). The applicability of the WATEM/SEDEM model to environments where gully erosion is important requires the incorporation of permanent gully and bank gully erosion in the model structure. Copyright © 2011 John Wiley & Sons, Ltd.     

Sediment yield variability in Northern Ethiopia: A quantitative analysis of its controlling factors

In Tigray-northern Ethiopia, lack of sediment yield data and appropriate methodologies for predicting sediment yield have contributed to poor planning resulting in rapid sedimentation in reservoirs and storage capacity loss. The objectives of this study were: (1) to assess the spatial variability of absolute sediment yield (SY) and area-specific sediment yield (SSY) and to identify their controlling factors for 11 representative catchments and (2) to develop models to predict sediment yield. We quantified sediment yield from reservoir sediment surveys and studied the role of bio-physical characteristics of the catchments and their interactions in controlling SY and SSY variability.