Eucalyptus is favoured because of its quick biomass accumulation and ability to grow under harsh environmental conditions. It is used for supply of fuel wood, poles, timber and oil. But Eucalyptus is often considered a threat for water availability of downstream areas, and its environmental sustainability has often been questioned. This study contributes to this discussion by making an estimation of the potential evaporation of Eucalyptus at catchment level. A water balance was set up for two catchments in Southern Rwanda. The transpiration by Eucalyptus was determined making use of an equation developed by Radersma et al. (2006) which relates potential transpiration to leaf area. In the two catchment the DBH of individual trees was sampled and the total coverage determined making use of Google earth images. Making use of allometric equations for DBH and leaf biomass, the leaf area and associated potential transpiration for individual trees was determined and extrapolated to catchment level. With some adaptations for the dry season a transpiration of 869 mm/year was found, which is 70% of annual precipitation. The conditions under which Eucalyptus is grown in Rwanda and the associated water consumption, did not give strong evidence that Eucalyptus is a threat for water availability. Compared with seasonal key crops in the area the potential transpiration of Eucalyptus was a little higher on a yearly basis. Compared with perennial crops however, the potential transpiration was lower.