To ensure sustainable economic growth and poverty reduction, it is argued that effective water resource development is very crucial. Therefore, the main objective of this study is to assess water demand and resource reliability in the Tana subbasin. It aims to assess the current and future water demand and reliability of water resource supply and water balance components in the Lake Tana subbasin. In addition, to evaluate the effect of future water demands on Gilgel Abay, Gumara, Megch, and Ribb Rivers flow by using the WEAP model.This scenario analysis approach was used in conjunction with the WEAP model. The annual flow of Gilgel Abay, Gumara, Megch, and Ribb rivers was estimated at 57.97 MCM, 36.11 MCM, 596.15 MCM, and 53.42 MCM under the CC, IIA, and PI, respectively, due to increasing scenarios demand for water volume inflows deferented from REF due to a decrease. From the major rivers, inflow will reduce to enter Lake Tana totally by the volume of water, about 743.64 MCM. The effect of future water demands on the inflow of major rivers to Lake Tana was assessed.The result would be an increase from REF 120.95 of inflow rivers to Lake Tana of 127.26 MCM, 525.58 MCM, and 123.16 MCM under REF, CC, IIA, and PI scenarios for the years 2015-2030 of total annual volume of flow demand by about 413.16 MCM. In addition, based on scenario analysis, the annual flow unmet demand to estimated at 71.18, 76.99, 475.87, and 71.19 MCM under the REF, CC, IIA, and PI scenarios. Based on the scenario analysis, by 2030, the annula volume of flow supply water delivery from the reference 49.77 MCM assessed is estimated at 50.27 MCM, 50.71 MCM, and 51.97 MCM under the CC and IIA PI scenarios. The current water balance of the catchment means monthly precipitation in all major river gets 12.98 BCM of rainfall and for irrigation of about 0.01 BCM. They also, surface runoff and evapotranspiration were estimated at 3.35 BCM and 6.33 BCM, respectively. From the research, it has been recommended that both water supply and water management measurements will be well implemented. Key word: Lake Tana, Water Balane, Water Demand, WEAP