Due to increasing water scarcity in semi arid region, greater attention is being given to conjunctive use of surface water and groundwater resources. Combined simulation-optimization models are powerful techniques to achieve the optimal conjunctive use policies. In this research, the conjunctive use policies for surface water and groundwater resources of Najafabad sub basin located in Zayandehrud basin is achieved through the objective of supply the agricultural water demand for three irrigation systems commands area over planning horizon, considering three sets of constraints: available surface water, canal conveyance and cumulative groundwater level variation in each zone. So, a simulation-optimization model is developed using genetic algorithm (GA) and artificial neural networks (AANs) as an approximate simulator of the groundwater simulation model. The results of frequent execution of the groundwater simulation model (Groundwater Vistas 4) for different set to recharge-discharge value are used to train the A based simulation model. This models is then linked to the GA based optimization model to develop the conjunctive use operating policies at each season as planning periods in the planning horizon. The proposed model is applied for different scenarios that are defined at the region. Results from scenario I showed, supply total water requirement in each zone need to allocation more released water from Zayandehrud reservoir to the irrigation systems. In other scenarios, total quantity of the available surface water at each planning period has been limited according to applied management strategies in the basin. Results of different scenarios have shown the necessity of reduce demands to improve reliability of the water supply and groundwater drawdown in long-term use of water resources. Based on the result of this study, the developed policies are able to control groundwater fluctuations in the study area. They also show importance of integrated approach for allocation surface water and groundwater resources in the Najafabad plain.
