Projected Water Table Depth Changes of the World's Major Groundwater Basins and Potential Human Impact

Groundwater represents the largest stock of freshwater on Earth. It amounts to nearly 33% of the freshwater withdrawals (given that all of it is not easily accessible). Hence, to improve future mitigation and adaptation plans in water management, it is essential to study water table depth (groundwater levels) changes induced by climate change.

Here, we analyze the water table depth changes of unconfined aquifers in the 218 world's major groundwater basins, using global climate simulations from 1850 to 2100 run with the CNRM climate models. These fully coupled ocean-atmosphere-land models are able to capture groundwater-climate feedbacks. The simulations we use were conducted for the Coupled Model Intercomparison Project phase 6 (CMIP6). They follow four of the latest IPCC scenarios. Results show that on global average, groundwater are expected to rise. However, this rising of groundwater level is not spatially uniform and presents strong regional disparities. We find that the main drivers in the long-term water table depth changes are precipitation and evapotranpsiration. To explore the uncertainties on the projected changes of these two variables, we conduct a multi-model analysis based on CMIP6 simulations run by 30 global climate models.

The projected water table depth changes could be modified by anthropogenic groundwater withdrawals, which are currently not represented in our models. Using projections of population in 2100, we establish a global map of the areas where groundwater withdrawals could have an impact on the projected groundwater changes. This analysis also provides a mapping of the future risks associated with water table depth changes (flooding or water stress issues). Then, we propose a simple method to account for groundwater withdrawals for irrigation. This analysis shows of how the direct effect of climate change on groundwater level is likely to be modulated by anthropogenic groundwater withdrawals.