J. Hydrol. Hydromech., 61, 2013, 1, 21–29 DOI: 10.2478/johh-2013-0004 21 Eco-hydrological impacts of Eucalyptus in the semi humid Ethiopian Highlands: the Lake Tana Plain Tilashwork Chanie 1 , Amy S. Collick 2 *, Enyew Adgo 3 , C. Johannes Lehmann 4 , Tammo S. Steenhuis 5 1 Cornell University Master’s Program in Integrated Watershed Management and Hydrology Cornell University at Bahir Dar. Currently at: Amhara Regional Agricultural Research Institute Debre Berhan, Ethiopia. 2 Pasture Systems and Watershed Management Research Unit, Agricultural Research Service, USDA, University Park, Pennsylvania, 16802, USA. 3 Faculty of Agriculture, University of Bahir Dar, Bahir Dar, Ethiopia. 4 Department of Crop and Soil Sciences, Cornell University, Ithaca, NY, 14853, USA. 5 Department of Biological and Environmental Engineering, Cornell University, Ithaca, NY, 14853, USA, and Department of Civil and Water Resources Engineering, Institute of Technology, Bahir Dar University, Bahir Dar, Ethiopia. * Corresponding author. E-mail: Amy.Collick@ars.usda.gov Abstract: Eucalyptus is the tree of choice for wood production by farmers in Ethiopia. Although there are many claims about its harmful effect on ecology and water availability, little actual research exists. The main objective of this study was, therefore, to study the extent of harm of Eucalyptus on the ecosystem. This study was conducted at the Koga Water- shed near Lake Tana in Ethiopia. Twenty-five farmers were interviewed and a field experiment with three replications was carried out to quantify the effect of Eucalyptus on various soil physical and chemical properties and maize crop measurements and to compare bulk density, soil moisture contents, maize crop counts and shading effects in fields bor- dered by Eucalyptus and Croton macrostachyus. Our results show that Eucalyptus decreased both soil nutrients and maize yields within 20 m of the trees. Although moisture content was not affected during the monsoon, it decreased fast- er within 30 m of the Eucalyptus trees than elsewhere. Soils become water repellent, too. Local farmers’ perception agreed with our experimental findings and indicated that Eucalyptus trees are exhausting the once productive land. They also reported that Eucalyptus dries up springs. Despite this, the growers insist on planting Eucalyptus because of its cash income. Keywords: Eucalyptus; Soil water repellency; Croton machrostachyus; Koga watershed. INTRODUCTION Globally, more than 80 countries have planted more than 4 million hectares of Eucalyptus. It is fast growing, requires minimal upkeep, grows up from its roots and has easily collect- ed seeds and is desirable for lumber, construction and fuel- wood. Therefore, Eucalyptus has become one of the most planted tree species in Africa in recent years. In Ethiopia Eucalyptus was introduced in either 1894 or 1895 because of the massive deforestation around Addis Ababa for firewood (Pankhurst, 1961). Since then, shortages in fuel wood supplies and a need for long-term economic returns on farmers’ land have made Eucalyptus very attractive. Eucalyptus is commonly planted in a farmer’s cropland plot or on bounda- ries of cropland. In addition, they are grown on marginal lands and in some cases they are planted to stabilize gullies in wet areas. Although quantitative evidence is scanty, there has been a perception that planting Eucalyptus adversely affects crop productivity (Kidanu et al., 2005). Lane et al. (2004) described that in China, the expansion of Eucalyptus plantations on lands previously used for crops and occupied by indigenous trees and grass lowered water tables and reduced water availability for irrigation due to soil hydrophobicity (water repellency) and its deep and dense root network. Eucalyptus seedlings are vulnera- ble to severe water stress unlike the seedlings of indigenous deciduous tree species in Ethiopia (Gindaba et al., 2004). This shows that Eucalyptus trees need more water and compete with neighboring plants for the available water in the soil. El-Amin et al. (2001) in Sudan reported that Eucalyptus caused crop yield reduction due to nutrient depletion and production of toxic exudates (allelochemicals). Finally, nutrients are exported out from the plantation’s soil system by removing trees for timber sales and fuel wood (Holgén and Svensson, 1990). The environmental impacts of vegetation on the hydrology have been studied only to a limited extent in Ethiopia and east- ern Africa (Bayabil et al., 2010). This is especially true for Eucalyptus trees as discussed above. Therefore, this study examines the effect of Eucalyptus on soil physical and chemical properties, light intensity, and root distribution. The study also compares the density of undergrowth, moisture content and crop performances up to 40 m from the tree stands of two common plantation types, Eucalyptus stand and C. macrostach- yus, trees used for shade coffee. MATERIALS AND METHODS Study area The Koga Watershed, a 28,000 ha watershed, was selected for this study because of the presence of the Koga Watershed Irrigation and Watershed Management project. It consists of a 7,000 ha command area for irrigation agriculture and an envi- ronmental management plan for reducing upstream erosion, and it has been supported by the African Development Bank (ADB) and the Ethiopian government. The catchment area is located between 11 o 10’ N to 11 o 25’ N latitude and 37 o 02’ E to 37 o 17’ E longitude and ranges from 1800 to 3200 m in elevation. It