As the impact of climate change with increased growth and development of the world, causing major problems in the field of available water resources. Under such conditions modeling related to the allocation of water resources must be considered based on accurate predictions of the all future factors affecting water resources and consumptions, including climate change. This study investigated the effects of climate change on water resources and consumptions in the downstream of the Zayandeh-Rud dam as one of the areas involved in Iran's water resources problems. 15 GCM models were selected to investigate the climate change in the 30-year period from 2015 to 2044 under the A2 and B1 emission scenarios. Based on the performance of the GCMs to prediction of temperature and precipitation parameters, these models weighted and combinations of climate change patterns, including the ideal, medium and critical were defined. Daily temperature and precipitation data were generated with the LARS-WG model and were converted to the regional scale using the new method of Modified Weighting Approach Using Actual Value (MWM-AV). Then, the impacts of climate change were investigated on the agriculture, input runoff to the dam and limit withdrawals from groundwater. Finally, the available water in the future was allocated between different applications using the GAMS software. Water allocated based on 6 different management scenarios, which included various scenarios, such as increased water efficiency in agriculture, elimination of crops with high water use, reduction in the rate of drinking and industrial water increases and combined scenarios. The results showed that the temperature will increase in all climate change patterns and precipitation often will reduce in the region. The temperature changes were estimated between 0.55 to 1.34°C and precipitation changes were estimated between +1.8% to -20.7%. The maximum precipitation decrease will occur in the West and the maximum temperature increase will occur in the East of the basin. The reduction in the dam inflow is 80 million cubic meters for the most ideal condition and 274 million cubic meters in the most critical condition. The evapotrairation in the study area between would increase between the 1.3 to 8.4 % that would increase the agricultural water requirement. The water requirement of the rice and corn crops, would increase more than other plants in the region. Climate change will also change the recharger parameters of the aquifer and will reduce the extraction of water to 1929 million cubic meters in the beginning of the allocation period. Results of water allocation management scenarios showed that without a specific management on water consumption, annual water shortage will occur from 610 to 1031 million cubic meters in the area. The minimum water shortage occurs in the case of apply the increase in irrigation efficiency, elimination of crops with high water use, reduction in the rate of drinking and industrial water increases at the beginning of modeling, simultaneously. Under this condition, the water shortage will be minimized, but the water shortage is stable. On the other hand, due to the impossible states of immediate implementation of the above changes, the optimal scenario was diagnosed based on these changes in a 30 years period. Results showed that under this management scenario, the water shortage can be controlled by 516 to 816 million cubic meters. In general, we should not expect that the water resources shortage reduced only based on the water consumption management in the region. Excessive load on water resources and water-transfer to the outside areas of the river basin, will increase the water demand. So, despite the decline in cultivation and groundwater extraction control, the water shortage will be intensified in this area. A part of water shortage in the future, will occur because of the low water inflow into the dam that the decrease in precipitation is the main reason of this change. So to fix the water shortage in the basin, the other water supply projects should be considered such as inter-basin water transfer. The volume of required water for inter-basin water transfer should be considered based on the maximum shortage occurring in the optimal management during the next 30 years, by 500 to 800 million cubic meters. Keywords: Zayandeh-Rud River Basin, Climate change, Temperature, Precipitation, Water resources management, Water shortage, Optimized water resources allocation