It is now well recognised that invasive alien species, particularly tree species, often have much increased water usage compared with native vegetation. Perhaps less well understood are the reasons for this increased water use and whether such increases should be expected from all species of invading alien trees under all environmental conditions. This paper examines the reasons for increased water use from trees as compared with short crops. From a knowledge of these reasons and a knowledge of the limiting processes (the Limits Concept) governing alien tree and native tree and short crop water use, (derived from case studies in India and RSA), we suggest that it is now possible to assess under what conditions high water use by aliens may occur. Inverse solutions based on knowledge of growth rates are also suggested as another approach for assessing alien and native tree water use under water limited conditions. We conclude that in dry climates the greatest increase in water use from aliens, in both absolute and percentage terms, may occur in water limited rather than riparian (water unlimited) conditions. Hydrological models which can predict the spatially distributed increase in water use by aliens within catchments, coupled with ecological models which can predict controlled and uncontrolled invasion, can assist the evaluation and design of improved cost-effective eradication programmes. Such coupled models, linked with an economic evaluation component, should indicate in what circumstances the value of the extra streamflow released may alone be sufficient to cover the costs of the eradication programme and under what circumstances the ecological (protection of indigenous communities) and other socio-economic benefits also need to be taken into account to justify the costs of the programme.