Modelling vadose zone flows and groundwater dynamics of alluvial aquifers in Eastern Gangetic Plains of India: evaluating the effects of agricultural intensification

Groundwater-dependent agricultural intensification has led to unsustainability of groundwater systems in many parts of the world. The unconfined aquifers in the Eastern Indo-Gangetic Plains (EIGP) of India are one of the most extensive aquifer systems in South Asia and are prone to ever intensifying agricultural systems. This paper aims at evaluating the long-term impacts of agricultural intensification on groundwater dynamics in an agriculturally important sub-region of EIGP. The study proposes a multi-model approach, combining the capabilities of vadose zone model (HYDRUS-1D) and aquifer simulation model (MODFLOW) to analyse the recharge and discharge mechanisms in the alluvial Gangetic Plains. The study mathematically reconstructed the vadose zone and aquifer geometry and implemented them into flow models. We demonstrate that the loosely coupled vadose zone and groundwater flow modelling framework is highly suitable for simulating the impact of changed cropping intensity on groundwater dynamics. The recharge estimates from HYDRUS-1D and calibration of regional groundwater flow model offered a robust set of parameter values for the prevalent conditions of EIGP. The results show good correspondence between the observed and simulated water table levels during calibration, with RMSE = 0.56–0.59 m, NSE = 0.76–0.99 and R2 = 0.83–0.91, all within acceptable limits. Projections showed that although the water table in the region would remain fairly stable under lower levels of cropping intensity (135–150%), further increase to 200 and 300% would lead to water table decline at the rate of 0.87 and 1.83 m per year, respectively. These changes could therefore impact future groundwater risk management in EIGP. This reinforces the view that groundwater-dependent intensification may be sustainable in long term only if adaptation strategies and compensatory measures are adopted.