Three decades of depth-dependent groundwater response to climate variability and human regime in the transboundary Indus-Ganges-Brahmaputra-Meghna mega river basin aquifers

•We examined multi-depth groundwater level (GWL) response to climate (precipitation and global climate cycles) and human influence.•Distinct temporal phase lags between precipitation and GWL linked to precipitation amount, sub-surface depth, and abstraction.•Non-intuitive differential GWL response to global climate cycles in exploited parts, especially at greater depths.•Abstraction, importantly at the deeper depths is the dominant driver for GWL change.•Groundwater abstraction playing an important role in modifying the link between hydro-climate and groundwater. Groundwater plays a major role in human adaptation and ecological sustainability against climate variability by providing global water and food security. In the Indus-Ganges-Brahmaputra-Meghna aquifers (IGBM), groundwater abstraction has been reported to be one of the primary contributors to groundwater storage variability. However, there is still a lack of understanding on the relative influence of climate and abstraction on groundwater. Data-guided statistical studies are reported to be crucial in understanding the human-natural complex system. Here, we attributed the long-term (1985–2015) impact of local-precipitation, global-climate cycles, and human influence on multi-depth groundwater levels (n=6753) in the IGBM using lag correlation analysis, wavelet coherence analysis, and regression-based dominance analysis. Our findings highlight the variable patterns of phase lags observed between multi-depth groundwater levels and precipitation depending on the different nature of climatic and anthropogenic drivers in different parts of the basin. We observed intuitive responses, i.e., rapid response in shallow groundwater and relatively delayed responses to the global climate patterns with increasing depth. However, in the most exploited areas, the hydrological processes governing the groundwater recharge are overwhelmed by unsustainable groundwater abstraction, thus decoupling the hydro-climatic continuum. Our results also suggest groundwater abstraction to be the dominant influence in most of the basin, particularly at the greater depth of the aquifer, thus highlighting the importance of understanding multi-depth groundwater dynamics for future groundwater management and policy interventions.