Gamasyab Basin which is located in the western part of Iran, is one of the main sub basins of the Karkheh basin. The basin has an area of 11459 km 2 . The largest part of the Gamasiab basin area is located in Kermanshah province and partly in Hamadan. The basin has an area of 11459 km 2 . Due to the fact that the level of aquifers has decreased and the Gamasiab River has been subjected to seasonal drying in the recent years, some actions are required to prevent the extinction of water resources and to deal with a water deficit or drought crisis. In this research, using WEAP software in Gamasiab basin, water resources and catchment consumption were simulated in order to assess the status of the basin and utilize and allocate optimal resources and uses of the basin. It also tries to maintain surface water and underground water resources through exploitative policies. In this research, in addition to the reference scenario, four scenarios were used to optimize harvesting and consumption in the basin including sprinkler irrigation scenarios, drip irrigation, irrigation based on water requirement and water deficit irrigation scenarios including three scenarios 15%, 30% and 45% of water deficit irrigation. Meanwhile, according to the results of WEAP software, the optimum result was obtained from the drip irrigation scenario which has the lowest non-supply requirements (average 0.5 million cubic meters for all sub basins) and the largest volume of aquifers (318 million cubic meters for the Sahne sub basin, 841 million cubic meters for the Harsin sub-basin and 3003 million cubic meters for the Kermanshah sub basin). After that, the optimum result was obtained from the irrigation scenario (The average of 0.5 million cubic meters of unmet demands for all sub basins) and the largest volume of aquifers (308 million cubic meters for the Sahne sub basin, 803 million cubic meters for the Harsin sub-basin and 2967 million cubic meters for the Kermanshah sub basin). Then, 15%, 30% and 45% water deficit irrigation have the least unmet demand and the largest increase in aquifer volume. In the 15% water deficit irrigation scenario, if we assume that the production function is linear and coefficient of yield reduction (Ky) is constant, for 85% water deficit irrigation rather than full irrigation, there is a 15.8% reduction in product yield, but this reduction is more tolerable than yield reduction in two scenarios of 30% water deficit irrigation and 45% water deficit irrigation, which The yield reduction rate is 32.1% and 48%, respectively. In the 45% water deficit irrigation scenario, the results are similar to the drip irrigation scenario, but with the assumption of the linearity of the production function, it reduces the yield by 48% and decreases the farmers' income, so it can not be a good policy for the basin. In the surface irrigation scenario based on water demand, the amount of water required for irrigation is determined by the amount of water demand from CropWat software. The results of this scenario are better than the current situation of the catchment. It saves about 70 million cubic meters of water per year for the Kermanshah sub-basin, 50 million cubic meters per year for the Harsin sub-basin and 10 million cubic meters per year for the sub-basin. However, in this scenario, the amount of water demand is in deficit, but overall, the deficit for the whole year is less than the reference scenario. Keywords WEAP, CropWat, Simulation, Scenario , Allocation