The critical zone is the thin dynamic skin of the Earth, extending from the top of the vegetation canopy through the soil, down to the bottom of the groundwater. It is the place where “rock meets life”, and where humans and most of the animals live, and it is therefore ‘‘critical’’ to our survival. The critical zone is the domain where water cycle dynamics connect the subsurface to vegetation, atmosphere and climate, controlling water quantity and quality.

Understanding water storage and transfer within the critical zone is vital to address key environmental and social problems linked to ecosystem services in natural and human-impacted environments: maintaining soil productivity in intensively managed systems, ensuring forest vitality, and improving landscape resilience to natural hazards. Such an understanding is pivotal to develop sustainable management and use strategies that can ensure a reliable and consistent supply of clean surface water and groundwater, including providing water for human consumption, industry and agriculture, which are all themes of growing concern in Europe.

The ongoing modifications in climate and land cover are altering the structure of the critical zone and affect the partitioning of water in the hydrological cycle. Knowledge of when and where groundwater resides in the subsurface, and the conditions under which plants access diverse water sources is necessary for comprehending vegetation and groundwater dependent ecosystems resilience to environmental changes, and remains a key challenge in critical zone studies.

Improving understanding and prediction of the effects of changing environmental conditions on water availability in agricultural and forested landscapes, climate, hydrological, and land surface models urgently require detailed information on water partitioning in the critical zone. This includes how much precipitation and/or irrigation water is stored in soils, recharges the groundwater, or is transpired by vegetation, and the temporal dynamics of these processes across different climates. However, the spatial and temporal scales of water moving through and mixing in the critical zone can be highly variable limiting the comprehension of the feedbacks between groundwater, soil water and vegetation.

Quantifying water fluxes and knowing time scales of transport is essential to understand transfer and retention of water and solutes in soil, which in turn control biogeochemical cycling and contamination persistence, offering crucial information to assess the vulnerability of water resources.