Societal & Science goals

Planetary changes affect both the physical system (including soils and climate) and life (vegetation cover, biodiversity). They add up to heavy social and economic evolutions (demographic rates and inequalities) to modify the vulnerability of our societies and life in general. The intertropical band includes numerous regions undergoing fast evolutions, but with often limited institutional abilities for adaptation, for instance.

West-African population may double by the start of 2040, and access to safe and drinkable water is still a major goal for sustainable development, while climate models still show strong uncertainties in the area. Current system modification already show tipping points in the water cycle, impacting population and suggesting drastic adaptation.

To better anticipate on the continental hydrological cycle is therefore a fundamental goal to, on the one hand, support adapted water resources (groundwater, agronomic, surface water) management solutions and hydro-climate hazards (floods, drought) mitigation and, on the other hand, detect possible tipping situations.

natural and anthropized (pumping, forest-to-crop conversions, irrigation, urbanization...) critical zones at different scales (plot, elementary catchment, large catchment, regional), and showing major states (DS: storage variations) and fluxes: P: precipitation, ET: evapotranspiration, D: Drainage

Anticipation is required. This calls for the prediction of long-term evolution of the critical zone (CZ) compartments, as well as fluxes between them. Such evolutions result both from external forcings (climate, demography...) and from the highly non-linear, scale-dependent, CZ functionning.

The water storage is a key state variable, both to understand the CZ functioning, but also as a fundamental resource. For a given system input (i.e. precipitation), water storage integrates the competitions between output fluxes (evapotranspiration, streamflow, pumping...). To put it in other words: space-time distribution of storage controls/is controled by the fluxes.

Studying water cycle in the CZ calls for integrative modeling and observation strategies to describe processes  and estimate water budgets at the target scales.

To get there, we must caracterize the interactions within the CZ, that is, from the top of the atmospheric boundary layer down to the bottom of the weathered soil profile, which is a real, and big, challenge. Particularly, space-time distribution of water storage are challenging to observe, and has, so far, limited our ability to integrate below-ground water storage when studying hydrological processes and feedbacks on the climate system.

The CZ, as most of physical systems, is highly under-sampled, and observed by various methods which produce highly heterogeneous data. This limits our ability to combine data and models at the target scales. Recent developments in hyperresolution critical zone models allow to adress integrated CZ issues at large (i.e. regional to global) scale by keeping a high resolution (< 1km) and with state-of-the-art physics, but raise new questions on the communication between data and models at such scales (what parameters? How realistic?), together with new, Big Data-like questions on how to analyse the huge model output data to reveal scale-dependent interactions in the CZ?