Population growth, and future developments, are occurring at an ever-increasing phase. These process results in an increased demand for water sectors. Water is not only influenced by human activities, but also by natural factors, such as climate change. The objective of this thesis is to model the water allocation system of the Ribb River Catchment and develop a water allocation vii system for the catchment under future developments and climate change scenarios using a computer-based modelling tool, Water Evaluation and Planning System (WEAP) Model. Records of hydrology, meteorology, and water demands for the study area were statistically tested and arranged as an input file source to suit the model. Future meteorological data of CORDEX-Africa RCM of RCP8.5 climatic data were downloaded and corrected with multi-regression and distribution mapping (DM) methods respectively, and the two data were also correlated with each other. The model calibration, validation, and its statistical measure were seen and therefore the result shows that it's good and the model can simulate the current and future scenarios. The results of this analysis should that no quantity of unmet water demand in the current account year (2020) and reference years of 2021-2050 for all selected demands. In the coming future population growth, there was no unmet demands from 2021 to 2030, but 1.21% unmet water demand was observed from 2031 to 2050. Under irrigation area expansion, 2.52% and 4.77% unmet water demand were occurred from 2021 to 2030 and 2031 to 2050 respectively. Under climate change with future development scenario, mean annual surface runoff would decreased from 26.668 to 24.758 MCM, No unmet water demand would occurred from 2021 to 2030, while from 2031 to 2050, 11.144 MCM (3.63%) unmet under scenario-I. For scenario-II, 23.932 MCM (4.14%), and 63.61 MCM (4.96%) water demand were unmet. KEY WORDS: Ribb, WEAP, Demand, RCP, Scenario