This study  Simulate the cultivation of legumes instead of wheat and barley and examining  the impacts it on aggregate water use, output , land rent , land under culture , total rent and shadow price of water in downstream lands of Yengejeh Dam in Neyshabur. This is an exploratory study in nature which was conducted through a questionnaire survey in 2015-2016. The population of this research is all farmers who use the water of Yengejeh dam to irrigate their lands . By Using Cochran formula, 139 wheat and barley farmers selected as the sample of this research randomly. A scenario of legumes crops such as peas, beans and lentils that have different price in the competitive and monopolist water market was developed by assimilation algorithm. According to the market situation of water in the region which is almost similar to monopoly water market, the results showed that cultivation of legume  instead of wheat  increase amount water used , production , total rent and  shadow price  of water. But it  decreases land under culture in the study area. Scenario results of cultivation of legume  instead of barley showed that  with cultivation of legume  instead of barley  the amount of product produced , land rent under , total rate and shadow price  of water reduces. But  it increased the land under culture in the study area. Therefore, the cultivation of legumes instead of wheat has better economic effects because it provided optimal resource allocation.