Agriculture is the main source of water withdrawals which accounts for about 70% of the total freshwater withdrawals in the Hirmand catchment. In recent years, water shortages have subjected agriculture and environment sectors to difficulties in the Hirmand catchment. In this study, the aim is to present a system dynamics model (SDM) to evaluate the delivered water demand and the unmet water demand. Therefore, first, the water resources system of the Hirmand catchment was modeled in the software of Vensim, and water supply and demand were afterwar simulated. Having considered the ongoing legislation and policies, as well as regional plans, the current and future status of the system were delineated using varied scenarios. The results showed that the implementation of the 46.000-hectare Agricultural Project causes an annual water shortage of 415.930 million m3 in the agricultural lands outside the project scope. Besides, the implementation of the second line of the Water Transfer Project - aimed to deliver water to Zahedan - will intensify the water shortage in the agricultural sector specifically to the peripheral lands outside the plan by 9.324 million m3/year. The plan will also exacerbate the water shortage in the environmental sector by 6 million m3/year. Based on the results, saving up to 20% water in the drinking sector will increase water supply in the agricultural and environment sectors respectively by 3.215 and 2 million m3/year. If the current water allocation policy continues, spreading the particulate matter is inevitable, as a result. To protect the environment of the Hirmand catchment, giving the agriculture and environment sectors the same priority in the upper hand policies is emphatically recommended.