Assessment of environmental flow requirement effects at an estuary

This paper describes the application of dynamic system modelling to investigate the consequences for water supply of maintaining environmental flow requirements in the Gorganrud River Basin in Iran. In this study the output results of Vensim and Weap models were compared and found to match each other closely. However, for a specific scenario on the same computer, Vensim was more than 100 times faster than Weap. Currently, about 55% of the time the Gorganrud Estuary is dry without any in-stream environmental flow, causing grave environmental problems, particularly in relation to fish habitat. If the reservoirs are operated to maintain environmental flow requirements, there is less than 10% supply–demand deficit for agricultural uses but flows are maintained to the estuary throughout the year at about 1 m3/s, which benefits the river ecosystem; in addition there are flood risk management benefits about 35% of the time. Accepting these minor deficits and maintaining the environmental flow requirements at the Gorganrud Estuary thus achieves a valuable trade-off.