To ensure sustainable economic growth and poverty reduction, it is argued that effective water resource development is very crucial. Owing to this fact different water resource development interventions are implemented and planned for the future to use the water resource of the Ribb catchment. Therefore, this study has assessed the water demand and water resource reliability of the Ribb catchment, Tana sub-basin. The study aims at estimating the current and future demand and supply of water, unmet demand, reliability of water supply, and water balance components in the Ribb catchment and evaluating the effect of future water demands on Ribb river flow and its influence on Lake Tana. WEAP model was then configured for the Ribb catchment at its near Lake Tana and two demand nodes (water supply and irrigation) were added and connected with water sources. The output of the model was analyzed which includes stream flow, demand, supply requirement, unmet demand, and reliability of demand and two future demand scenarios (increase irrigation area and population increase). The annual average stream flow of Ribb was estimated at 543.3, 512.3 and 516.8 MCM under the reference, Increase irrigation area and population increase demand scenarios respectively at 2035 G.C. The demand has shown fast growth where the supply requirement including the losses was estimated as 103.1, 161.2, and 152.2 MCM for 2035 under reference, Increase Irrigation area and Population increase demand scenarios respectively. Though the amount of annual flow volume is by far greater than the annual demand and unmet demand of 76.1, 85.4 and 80.3 MCM has been determined at 2035 G.C under reference, Increase Irrigation area and Population increase scenarios. The demand that does have low reliability is irrigation sites, the reason behind this is that water demand at these sites is very high during the low river flow months, where water is scarce because mainly dependent on the natural flows of the rivers. Irrigation site becomes low during the periods when water resource availability becomes high. Based on the result of this research it has been recommended that both water supply and demand side management measures should be well implemented in an integrated manner. ACKNOWLEDGEMENT First of all, I would like to thank the almighty GOD and his mother St. Marry for helping to accomplish my study. I would like to express my wholehearted gratitude to my advisor Dr. Silesh Mamo, for his precious advice, suggestions, and constructive comments during the research period. I have benefited a lot from his experience and knowledge during my stay at the department of Hydrogeology. I wish to also appreciate all my friends and course mates for their support in one way or the other. I am greatly indebted to my family, for their prayers, approval, and support, understanding financial support, and always being there for me. Finally, I am also thankful to those not mentioned here but who have helped and supported me to achieve this research work successfully.