Achieving a sustainable development is crucial, but is even more important in developing countries, where a wide number of people does not have a safe and secure access to water and relay on the environment to sustain their lives. The Ethiopian Central Rift Valley basin is already a degraded basin from the environmental point of view: ecosystems are endangered due to human activities there developed. Moreover, poverty is widespread all over the basin, with population is mainly living from agriculture on a subsistence economy. In order to achieve sustainable development to increase population incomes without affecting lives of those who are highly dependent on the environment and Integrated Water Resources Management approach shall be applied. First step has been to model the basin water resources, using the Soil and Water Assessment Tool (SWAT), which, after calibration and validation of the model, has given correct results. In order to follow deepening in the IWRM approach, more information on actual and future water demand and specifically water applied to agriculture will be needed.