Robert Reinecke , Sebastian Gnann, Lina Stein, Marc Bierkens, Inge de Graaf, Tom Gleeson, Gualbert OudeEssink, Edwin Sutanudjaja, Claudia Ruz-Vargas, Jarno Verkaik, Thorsten Wagener 

Groundwater is essential for maintaining healthy ecosystems and securing human access to freshwater. Here we show that current estimates of global groundwater accessibility by ecosystems and humans are highly uncertain. To quantify this uncertainty, we define three categories of accessibility and investigate four global groundwater models. Averaged across these models, we estimate that 23% [most deviating model: 71%] of the land area contains groundwater accessible to ecosystems and humans, 57% [29%] is accessible to humans only, and 20% [0.01%] is costly to access or inaccessible. We find that the uncertainty in estimating water table depth severely affects our ability to assess groundwater's crucial role in ecosystem health, global water supplies associated to food security, and human health, with possible implications for achieving multiple Sustainable Development Goals. To reduce this uncertainty, we outline three pathways towards (1) better global datasets, (2) alternative strategies for model evaluation, and (3) greater cooperation with regional experts.