Three-dimensional groundwater flow modelling of the Lower Mahanadi River Basin, Nayagarh, Odisha, India Using MODFLOW-GMS–NWT

Groundwater is the primary source of water for agricultural and domestic use in the lower part of the Mahanadi River Basin, particularly in Nayagarh district, Odisha. Increasing groundwater abstraction, coupled with spatial variability in recharge and changing rainfall patterns, has intensified pressure on aquifer systems, highlighting the need for scientifically robust groundwater assessment tools. This study develops a three-dimensional transient groundwater flow model for the Nayagarh region using MODFLOW-NWT in GMS 10.7.7. The model integrates key hydrological processes, including spatially distributed recharge, evapotranspiration, river–aquifer interaction, drainage, and pumping. A two-layer aquifer system consisting of an unconfined upper layer and a confined lower layer is considered. Hydrogeological parameters were assigned using raster-based datasets and literature-supported ranges, with horizontal hydraulic conductivity varying from 0.094 to 5.0 m/day, vertical anisotropy of 3, specific yield of 0.20, porosity of 0.30, and specific storage of 1.0 × 10⁻4. The model domain was discretised into 4,752 grid cells, of which 2,385 were active. Model calibration and validation demonstrated strong agreement between observed and simulated groundwater heads, and the simulation achieved excellent numerical stability with a flow-budget error of only 9.86 × 10⁻⁷ %. Flow-budget analysis revealed that recharge is the dominant inflow component, while ET, river leakage, and drainage constitute the major outflows. Simulated groundwater heads are highest in the basin's northern (upland areas) and decrease toward the southern (low-lying areas), reflecting natural hydraulic gradients. The confined aquifer exhibited greater sensitivity to pumping, indicating higher vulnerability to over-extraction. The findings of this study provide practical guidance for groundwater governance by identifying safe abstraction zones, regulating over-exploited regions, and prioritising artificial recharge interventions. The model can helps to policymakers aligned with Sustainable Development Goal 6 (Clean Water and Sanitation) in the lower Mahanadi River Basin.