APPLICATION OF THE CAESIUM-137 TECHNIQUE TO SOIL DEGRADATION STUDIES IN THE SOUTHWESTERN HIGHLANDS OF ETHIOPIA A. MEKURIA 1 * , P.L.G. VLEK 2 AND M. DENICH 2 1 Addis Ababa University, Environmental Science Programme, PO Box 15610, Addis Ababa, Ethiopia 2 Center for Development Research, University of Bonn, Walter-Flex Str. 3, D 53113 Bonn, Germany Received 25 June 2010; Revised 6 December 2010; Accepted 7 December 2010 ABSTRACT Soil degradation is a serious problem in the central and northern Highlands of Ethiopia. It has been so for several decades as a result of over exploitation and mismanagement. Relocation of a portion of the population from these regions to the relatively less populated Southwestern Highlands has taken place for decades to try to address the problem. However, such mass resettlements have caused severe soil degradation problems in many destination areas in the Southwestern Highlands. The aim of this study was to assess the problem of soil degradation using the caesium-137 isotope and to test its value for erosion study in the region. The adapted USLE was applied to compare results from the caesium-137 isotope studies. Along a deforestation continuum, fields cultivated for various years were studied for erosion. From a reference grazing land plot, total caesium-137 fallout of 2026 Æ 176 Bq m À2 with a CV of 24Á6 per cent was recorded showing the presence of sufficient fallout to apply the technique. Erosion in cultivated fields was estimated against this reference using conversion models. Results from the Proportional Model jÀ13Á9 Æ 2Á7jand the adapted USLE j12Á3 Æ 2Á6j were not significantly different ( p < 0Á05), meaning the technique provides reliable results. A positive relationship was observed between severity of erosion and time of cultivation after forest clearing (R 2 ¼ 0Á78). The mean annual loss of soil from cultivated land, 14Á9 Æ 2Á9 t ha À1 y À1 , is already beyond the tolerable threshold and might exacerbate further clearing of forests for cultivation if the land is not properly managed. Copyright # 2011 John Wiley & Sons, Ltd. key words: soil erosion; caesium-137; Mass Balance Model; Proportional Model; USLE; Ethiopia INTRODUCTION The problem of soil degradation in the Highlands of Ethiopia is one of the most severe in the sub-Saharan Africa region (Young, 1998). Forest clearing for agricultural use is the prime cause of the problem (Melaku, 1992; Fitsum et al., 1999; Feoli et al., 2002). The Highlands are home for 88 per cent of the populace and contribute 90 per cent of the total agricultural production. Increasing population density and reckless land clearing for agriculture has reduced the original forest cover from 40 per cent to 2Á7 per cent in less than half a century (Reusing, 1998). Apart from the forest cover loss, a number of factors such as the cereal-based farming system, lack of awareness, poor land management practices and poverty are among the main factors for the continued degradation of the land. As a result, many parts of the highland agro-ecosystems are destabilized. The most affected are the northern and central parts from which about 42 metric tons of fertile topsoils are eroded in every single hectare of cultivated land each year, amounting to a loss of 4 mm depth of top soil (Hurni, 1988). Because of this, the land has lost much of its production potential and degradation in some areas is beyond the threshold of reversibility (Constable and Belshaw, 1989). On-the-other-hand, the Southwestern Highlands where this study was conducted is relatively less populated and deforestation for agriculture has been insignificant for many decades (Russ, 1945). In an effort to relax the pressure in the degraded Highlands, the relocation of people and their resettlement in the less densely populated Southwestern Highlands has been adopted as a policy by the government, specifically after the 1984 drought. In effect, several thousand households have been resettled in the South- western Highlands in the last few decades. Because of the better forest cover and the traditional perennial crop-based farming system, the Southwest region has been perceived as a ‘free zone’ with much less soil degradation (Solomon, 1994). One of the impacts of the resettlement is expansion of cereal crop-based farming (which is erosion prone and involves forest clearing) to new forest frontiers causing soil erosion problems in the region (Alemneh, 1990; Melaku, 1992). The gravity of soil erosion problem triggered by the resettlements has not been addressed by research and it is imperative to address the problem and generate information land degradation & development Land Degrad. Develop. (2011) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/ldr.1088 *Correspondence to: A. Mekuria, Addis Ababa University, Environmental Science Program, PO Box 15610, Addis Ababa, Ethiopia. E-mail: mekuriaa@hoarec.org Copyright # 2011 John Wiley & Sons, Ltd.