Regional Simulation of Coupled Surface and Groundwater Processes: A Process-Based Approach

Groundwater plays a crucial role in sustaining water resources in monsoon-dominated river basins, particularly during dry periods when surface water availability declines. In such systems, groundwater storage and its interaction with surface water help regulate seasonal water availability and maintain river flows. However, the mechanisms through which subsurface heterogeneity influences recharge processes, evapotranspiration dynamics, and groundwater contributions to basin-scale hydrology remain insufficiently understood. This study investigates the role of groundwater in sustaining basin-scale water resources in the Netravati River Basin, a humid tropical catchment located along the west coast of India. To analyze these processes, an integrated surface–groundwater modelling framework was developed using MODFLOW 6. The modelling approach explicitly represents interactions among the unsaturated zone, the groundwater system, and the river network. Multiple conceptual representations of the subsurface system were explored by developing three model configurations: a single-layer model representing a simplified aquifer system; a two-layer model distinguishing between the weathered and fractured aquifers; and a four-layer model providing finer vertical discretization of the weathered zone into shallow, intermediate, and deeper intervals above the fractured-rock aquifer. These models were calibrated and validated using observed groundwater heads, streamflow records, and remotely sensed evapotranspiration estimates for the period 2009–2014. A detailed water-balance analysis was carried out using a modelling framework calibrated with available data for the unsaturated zone, the groundwater system, and the entire catchment to quantify the partitioning of precipitation into recharge, evapotranspiration, changes in groundwater storage, and streamflow. The results highlight the important role of groundwater in sustaining river flows, particularly through groundwater discharge to streams during the dry season. Subsurface heterogeneity was found to influence recharge dynamics and groundwater–surface water interactions across the basin. Finally, the calibrated model framework was used to assess the hydrological system’s sensitivity to future climate change and to increasing groundwater abstraction. Scenario analyses show that changes in rainfall patterns and intensified groundwater pumping can significantly affect groundwater storage and streamflow dynamics, with potential implications for long-term water availability. Overall, this study provides a process-based understanding of groundwater’s role in sustaining basin-scale water resources in monsoon-dominated catchments and offers insights for improving water resources management under changing environmental conditions.