The ISIMIP Groundwater Sector: A Framework for Ensemble Modeling of Global Change Impacts on Groundwater

Abstract. Groundwater serves as a crucial freshwater resource for people and ecosystems, vital in adapting to climate change. Yet, its availability and dynamics are affected by climate variations, changes in land use, and excessive extraction. Despite its importance, our understanding of how global change will influence groundwater in the future remains limited. Multi-model ensembles are powerful tools for impact assessments; compared to single-model studies, they provide a more comprehensive understanding of uncertainties and enhance the robustness of projections by capturing a range of possible outcomes. However, to this point no ensemble of groundwater models was available. Here, we present the new groundwater sector within ISIMIP which combines multiple global, continental, and regional-scale groundwater models. We describe the rationale for the sector, present the sectoral output variables, show first results of a model comparison, and outline the synergies with other existing ISIMIP sectors such as the global water sector and the water quality sector. Currently, eight models are participating in this sector, ranging from gradient-based groundwater models to specialized karst recharge models, each producing up to 19 out of 23 modeling protocol-defined output variables. Utilizing available model outputs for a subset of participating models, we find that the arithmetic mean global water table depth varies substantially between models (6–127 m) and shows a shallower water table compared to other recent studies. Groundwater recharge also differs greatly in the global mean (78–228 mm/y), which is consistent with recent studies on the uncertainty of groundwater recharge but with different spatial patterns. Groundwater recharge changes between 2001 and 2006 show plausible patterns that align with droughts in Spain and Portugal during this period. The simplified comparison highlights the value of a structured model intercomparison project which will help to better understand the impacts of climate change on the world’s largest accessible freshwater store – groundwater.