Irrigated agriculture is the largest water user in Iran. According to importance of promote of water conservation in agricultural, it is necessary to identify of the outcomes of paying subsidy to drip irrigation systems in farm and region levels. The aim of this research is to analysis the effectiveness of water conservation policy by paying subsidy for changing from flood to drip irrigation. Positive mathematical programming model (PMP) was developed, in order to investigation of effectiveness of economic encouragement in farmer decision making for choosing types of irrigation technology, cropping pattern, and applied and consumption water in rural of GarkanShomali district as a part of Najafabad aquifer. The purpose of this model was maximizing of economic profits subject to two scenarios: water allocation under present conditions, and volumetric water allocation. Data were collected by survey methods, documentary studies, and expert’s opinions. Results showed farmers will invest in technologies that reduce water applications when faced with lower financial costs for converting to drip irrigation. In water allocation scenario under present conditions, with increasing of subsidy, exploitation costs are decreased, and farm net income and production value are increased because performance and cultivation area were increased that lead to increasing of water consumption while applied water is fixed. Therefore, in these conditions irrigation efficiency in farm level in contrast with region level is increased. In volumetric water allocation scenario, with increasing of subsidy, exploitation costs are decreased, and net farm income and production value are increased with increasing performance without increasing of cultivation area. At this point, amount of water consumption is fixed but applied water by increasing efficiency of drip irrigation system is increased. Thus, paying of subsidy leads to increasing of irrigation efficiency in farm and region levels, and it is effectiveness as a policy to coping with drought conditions. According to results, suggested paying encouragement subsidy to farmer for changing type of irrigation to drip irrigation system realize with management of volumetric water allocation. Keywords: Subsidy, Drip Irrigation, Water Use, Applied Water, Volumetric Water Allocation, Positive Mathematical Programming Model, Najafabad Aquifer