Water is the single most important element of the environment and plays an important role in supporting productive human activities, But the global water demand will primarily grow due to population and economic growth, rapid urbanization, and the increasing demand for food and energy. The study aimed to assess the surface water potential and evaluate current and future water demand under developmental conditions in the Gumara watershed. GR4J model was used to model surface water potential; daily rainfall and daily evapotranspiration are the main data input of GR4J to model watershed runoff and the WEAP model was used to perform the analysis for the demand evaluation, and water management scenarios the demand computation is done up to the year 2050. The performance of the model was evaluated using statistical and graphical methods to assess the capability of the model in simulating the runoff potential for the study area. The coefficient of determination (R 2 ) and NSE values for the daily runoff was obtained as 0.82 and 0.79 respectively for the calibration period and 0.82 and 0.79 respectively for the validation period. The estimated total surface water potential was 1109.5Mm 3 from this only 14.58% is available in the dry month in the Gumara watershed and the current utilization of this potential was about 35.95Mm 3 for consumptive use. The water availability and demand computation are done on three different developmental scenarios conditions (population projection, dam, and irrigation projection scenario). Even though the demand in the study area is fully covered by the potential of the watershed for the population projection scenario but there is also unmet (deficit) demand for the Dam scenario and irrigation projection command area scenario respectively 0.31Mm 3 and 31.05Mm 3 in the dry month, so if such a scenario may happen in the future another supply option has to be considered. Finally, storage has to be provided and rainwater harvesting should be promoted in order to improve water availability for productive use additionally groundwater should also consider as a source. Key Word: - GR4J, WEAP, Gumara Watershed, Unmet demand