Assessment of environmental flow requirements using a coupled surface water-groundwater model and a flow health tool: A case study of Son river in the Ganga basin

Rapidly increasing population, industrialisation, and unsustainable consumption have resulted in deterioration of water quantity and quality, thus affecting the health of the river. One of the essential components for ensuring river health is environmental flow. Nevertheless, the environmental flow is neglected or not appropriately accounted in most of the water management assessments. The objective of this study was to better quantify and assess the environmental flow requirements and developing management practices for ensuring the ecological health of the river. Soil and Water Assessment Tool (SWAT) coupled with a finite element groundwater model, was used for hydrological modelling. The environmental flow regime was evaluated using an index 'Flow Health Score' (FHS), based on a Flow Health Tool. The model was simulated for three conditions - Virgin condition, Managed condition, and Modified condition for Upper and Lower Son separately. Virgin condition assumed no hydraulic interventions, while managed condition incorporated the existing management practices, and modified condition corresponded to the management practices modified in this study for ensuring environmental flow. This approach was demonstrated for the Son River in the Ganga basin. The modelled streamflow agreed with the observed discharge data (R-2 = 0.78-0.85). Further, SWAT-FEM coupled model better estimated the low flows than the SWAT model, thus demonstrating the importance of accounting for surface water - groundwater interaction in such environmental flow assessments. Flow health assessment indicated that reservoirs and irrigation diversions have severely affected the high flows in the Upper Son (high flow and highest monthly flow index less than 0.2), and low flows in the Lower Son (low flow index less than 0.2). The model simulation with the modified management practices increased the Flow Health Score (FHS) from 0.4 to 0.6 in Upper Son, and 0.4 to 0.7 in Lower Son. This study showed that coupled SWAT-FEM model, along with the flow health tool, could be effectively used for assessing the in-stream flow requirements and for designing alternative management plans to meet the in-stream flow requirements.