BLUEGEM — Biosphere and Land Use Exchanges with Groundwater and soils in Earth system Models

The interactions between groundwater (GW) and soil moisture (SM) exert a key role to shape the critical zone (CZ), including soils, water resources, ecosystems, near-surface climate, and social systems. In this project, we focus on several interlinked CZ processes: SM increase by capillary rise from GW that is recharged when SM is abundant; GW depletion or exhaustion by irrigation, 38% of which is GW-fed globally; SM, GW, and irrigation response to but also impact on mean climate and extreme events (droughts, heatwaves, floods), ecosystem productivity (wetlands, croplands) and soil carbon. The relative influence of these coupled processes is difficult to apprehend based on observations, as they exhibit contrasting manifestations in space and time, and are subject to increasing anthropogenic pressures. Thus, we will combine advanced numerical modelling and participatory methods to explore their long-term evolution in the Anthropocene (1900-2100), at two spatial scales: a) Global, with factorial simulations by two Earth system models to compare the influence of anthropogenic warming, land-use and irrigation management, and GW-related feedbacks, on past and future CZ pathways; b) Regional in two "focal areas", metropolitan France and the Mekong River basin, to provide contrasting examples of the studied processes and enable anchored transdisciplinary work with stakeholders, social and natural scientists. The goal is to propose tailored narratives for sustainable management, using participatory approaches to iteratively combine expert knowledge, local surveys, high-resolution Earth system simulations, and integrated assessment modelling of land-use and water management. At both scales, we will consider uncertainties to identify robust responses (likely evolutions and trade-offs, e.g. linked to GW failure to sustain irrigation and ecosystems, or climate change attenuation/amplification). Ultimately, the project will help assess the usefulness of global scenarios and projections at regional scales and frame sustainable CZ management strategies at the global scale.