Assessing impacts of climate change induced increased groundwater abstractions on the aquifer storage in Noyyal River basin, India

Groundwater plays a significant role in the global hydrological cycle and exhibits a close connection with climate change. It plays vital role in maintaining stream flows, supporting vegetation, moderating energy exchanges across land atmosphere interface. Additionally, it serves as a valuable storage reservoir during drought periods in areas where rainfall is unpredictable, or surface water sources are insufficient to meet anthropogenic water requirements. In this study we examined impacts of groundwater abstractions on aquifer storage in the Noyyal river basin (3,600 km2), which is a sub-basin of Cauvery river in Southern India. The period from 2012 to 2014 experienced drought conditions in the region, leading to reduced surface water resources that subsequently affected aquifer storage due to increased draft. During this time, the mean groundwater depth dropped from 9m (2012) to 22m (2017). Numerical models offer effective methods for simulating and analysing groundwater dynamics under varying conditions, and they have found widespread worldwide application. In this study, we performed multiple automated PEST simulations to obtain the parameters for the aquifer and used MODFLOW model to derive the initial condition from a well-calibrated steady-state model. Calibrated model produced realistic GW scenario in basin for the year 2012 with an impressive R-squared = 0.99 (simulated vs Observed heads), RMSE (heads) = 2.36m, RMSE (flow) = 1045 m3/year (< 0.6% of observed flow). These encouraging results from the numerical model demonstrate the promising potential of PEST tool in estimating parameters of the aquifer. Subsequently, a transient phase simulation with monthly time steps (Jan 2012 to Dec 2017) was setup to evaluate impacts of climate change and anthropogenic activities on aquifer storage. The findings from transient simulations indicate that the aquifer is experiencing a worrisome depletion rate of 2m/year. Numerous sections within the basin are currently in a critical or over-exploited state. Previous research indicates that the western section of the basin has experienced higher levels of exploitation compared to the eastern portion, and investigations are underway to understand the reasons behind this disparity. It is explored whether the aquifer can naturally achieve a sustainable balance over a longer time frame despite the mismatch between year-to-year recharge and draft, leading to overexploitation. Primary driver of the aquifer depletion: climate, anthropogenic factors, or their complex interplay are also examined with an aim to discover sustainable development pathways for the basin.