Performance Assessment of Nohob Dam Reservoir under Drought Conditions Using MODSIM Model

Due to lack of available water resources and the continuous increase in the water demands, construction of new dams seems inevitable. Dams are such infrastructures for water storage and supply, especially in drought periods. If the operation of the reservoir is optimized, the purposes of dam construction is efficiently supplied. Since the optimum operation of large water resource systems are time consuming, complex and sometimes unattainable, the use of alternative methods of optimization models seems a good solution which are accurately acceptable and give near-optimum answer. Nohob Dam is one of these dams which are implementing at Khar-Rud River at the Qazvin province. The main aim of the operation of the dam is to store Khar-Rud river flows and adjust to meet water rights for agricultural lands downstream and artificial recharge of Qazvin plain. The aim of this study was to investigate the water allocation to agriculture and environmental uses and artificial recharge of aquifers in drought conditions using MODSIM model. The operation of the dam was simulated using the MODSIM model for a long period of 50-year and during the periods of 1965 to 2015, as well as a 17-year hydrological drought period during the periods of 1998 to 2015, which is determined on the basis of average changes in river discharge relative to the long-term average. Simulation was conducted monthly for four options of “before the sedimentation”, “20-year sedimentation”, “50-year sedimentation”, and “no-reservoir conditions”. Water allocation priorities are as follows: 1- environmental requirements, 2- agricultural water rights, and 3- artificial recharge. Reducing Khar-Rud water volume at the dam site from the years 1998 to 2015, has caused the loss of Nohob dam performance for various reasons, including drought and water withdraw from upstream, so that the needs of any sections is not supplied. By comparing the results of sedimentation options, it is identified that in 20-year sedimentation conditions, the reservoir has the best performance, which is due to the low dead volume compared to the useful volume of the reservoir in these conditions. By comparing reservoir performance with no-reservoir conditions with the other sedimentation options, it can be concluded that the dam construction will not have much effect on improving the water distribution conditions and the amount of supply in these conditions will not be significantly improved. In order to evaluate the simulation results of the operation of the reservoir, various parameters, such as, supply percentages, reliability, resilience and vulnerability were used. Also, the simulation results showed that in long-term simulation (50-year) and drought conditions, the dam construction would slightly change the increase in the volume and temporal water supply and demand in the region, so that the percentage of provision of the total demands of water in 20-year and 50-year sedimentation conditions are reduced from 77.4 to 37.6% and 73.7 to 35.8%, respectively, and for no-reservoir conditions, it is decreased from to 58.8 to 34.1%. By continuing the drought conditions and increasing water supply up to 1-2% as compared to the no-reservoir conditions, the dam construction and the continuation of its implementation is completely unjustified. In general, it can be concluded that in the drought conditions, the dam construction to improve the water distribution conditions is not justifiable, and perhaps its application is only to control the sudden flooding and prevent the damage caused by the flood at downstream of the dam.