Evaluating Minimum Environmental Flow Requirements in Rivers: A Combined Decision-Tree Approach Integrating Hydrological, Physical Habitat, and Water Quality Indexes

The present study proposes a new method to assess the minimum environmental flow regime of rivers in which hydrological, physical habitat, and water quality indexes are integrated in a decision tree. Conventional methods of environmental flow are not able to integrate environmental interactions, which is the main motivation of this research work. The base environmental flow was defined by a hydrological or desktop approach. However, adding ecological requirements of physical habitats and water quality should be considered for assessing needs of the river ecosystem as well. The physical habitat loss function was defined based on fuzzy physical habitat simulation. Moreover, a combined water quality index was applied to define the water quality loss model. All environmental requirements were integrated in a decision-tree model to protect and sustain the ecological status of the river ecosystem due to water abstraction. Based on case study results, the decision-tree model improved the ecological status of the river ecosystem, which means using the hydrological method, physical habitat method, or water quality index is not independently recommendable to assess minimum environmental flow regime. The decision-tree model proposed 40% of annual available flow as the minimum environmental flow. In contrast, the hydrological method, physical habitat function, and water quality index independently proposed 32%, 30%, and 38% of annual available flow, respectively. The results highlight that using a combined method such as the decision-tree method in which interactions of methods could be defined is necessary to assess minimum environmental flow regime in rivers reliably.