Sciences Earth’s Critical Zone and hydropedology: concepts, characteristics,

Abstract. The Critical Zone (CZ) is a holistic framework for integrated studies of water with soil, rock, air, and biotic re-sources in the near-surface terrestrial environment. This most heterogeneous and complex region of the Earth ranges from the vegetation top to the aquifer bottom, with a highly vari-able thickness globally and a yet-to-be clearly defined lower boundary of active water cycle. Interfaces among different compartments in the CZ are critical, which provide fertile ground for interdisciplinary research. The reconciliation of coupled geological and biological cycles (vastly different in space and time scales) is essential to understanding the com-plexity and evolution of the CZ. Irreversible evolution, cou-pled cycling, interactive layers, and hierarchical heterogene-ity are the characteristics of the CZ, suggesting that forc-ing, coupling, interfacing, and scaling are grand challenge