Water within, moving through, and shaping the Earth's surface: Introducing a special issue on water in the critical zone

As the sole habitat for terrestrial life, the Earth's critical zone (CZ) refers to the upper, porous layer of the continental crust that is interacting with the circulating meteoric waters. Conceptually, water is central to the intriguing co‐evolution of both the hydrologic behaviour and the structure of the CZ. On one hand, the path and rate of water circulating through the CZ are shaped by the CZ physical structure itself, such as the distribution of material porosity and permeability. On the other hand, and at longer time scales, the circulating water shapes the CZ structure through physical and chemical alterations of porosity and permeability and through being an agent of denudation, thereby reshaping the surface of the CZ and shifting its hydraulic boundaries. It is possible to alternate between thinking of the CZ as a storage container for water or thinking of water as an agent shaping CZ architecture, in much the same way that it is possible either to see a vase or to see profiles of two faces in the classic Rubin's vase illusion (Figure 1). This perspective shifting duality of the water–CZ relationship is further modulated by biological forces such as plants and soil organisms, as well as anthropogenic actions that both alter the flow and chemistry of the water in the CZ. How the modern‐day CZ structure shapes current hydrologic processes, how vegetation and humans alter the hydrology and biogeochemistry of the CZ, and how the hydrology and CZ structure co‐evolve via feedbacks arising from this duality of the water–CZ relationship are the three broad categories of questions that motivated this collection of 20 papers comprising the Special Issue of Hydrological Processes on Water in the Critical Zone. A brief introduction to the contributions organized by these overarching questions follows.