The diversity of climate, geology, land cover and land use present at the instrumented sites of the French critical zone (CZ) initiative (OZCAR, [1]) is a unique opportunity to use a transversal and multi-site approach to tackle "big" CZ questions such as: Where do plants get their water? How does plant functioning affect water partitioning between catchment pathways? To which extent do the type and structure of the weathered profile (i.e., the below-ground part of the critical zone) control the hydrologic functioning of vegetation? As climate grows drier, are water partitioning and solute export increasingly controlled by green water use (soil evaporation and plant transpiration)? We propose to select a handful (less than 5) of small OZCAR catchments (1-10 km2) spanning a gradient of climatic seasonality, regolith type and land use, while overlaying a similar type of granitic/gneissic bedrock (focusing on local to intermediate groundwater flow paths). The current shortlist includes the intensively cultivated Kerbernez site (France - temperate oceanic climate), a managed forest at Strengbach site (France - temperate oceanic mountainous), crop/fallow/grass cover at Ara catchment (Benin - humid Sudanian), and a pristine deciduous forest at Mule Hole (India - sub-humid / monsoon). At the selected OZCAR sites, we use the diversity and temporal extent of observations, and local expertise, to constrain a suite of hydrological and geochemical models. The same two tools are used at the selected sites: 1) a physically-based, spatially-distributed ecohydrological model (EcH2O-iso, [2]), accounting for the coupling between energy balance, critical zone hydrology and vegetation dynamics coupled to a modular geochemical model (WITCH, [3]), and 2) a parsimonious physically-based, particle-tracking and reactive transport model (Hs1D, [4]) conceptualizing catchments as 1-dimensional equivalent hillslopes. Both tools are able to provide water travel times, which, along with storage, are taken as central variables to characterize critical zone functioning, at hillslope to catchment scales. Furthermore, this multi-site effort seeks to initiate discussion within the French critical zone community around current practices for data use in models and model set up and how they could be shared.