Abstract. The Critical Zone (CZ) is a holistic framework for integrated studies of water with soil, rock, air, and biotic resources in terrestrial environments. This is consistent with the recognition of water as a unifying theme for research on complex environmental systems. The CZ ranges from the top of the vegetation down to the bottom of the aquifer, with a highly variable thickness (from <0.001 to >10 km). The pedosphere is the foundation of the CZ, which represents a geomembrance across which water and solutes, as well as energy, gases, solids, and organisms are actively exchanged with the atmosphere, biosphere, hydrosphere, and lithosphere to create a life-sustaining environment. Hydropedology – the science of the behaviour and distribution of soil-water interactions in contact with mineral and biological materials in the CZ – is an important contributor to CZ research. This article reviews and discusses the basic ideas and fundamental features of the CZ and hydropedology, and suggests ways for their advances. An "outward" growth model, instead of an "inward" contraction, is suggested for propelling soil science forward. The CZ is the right platform for synergistic collaborations across disciplines. The reconciliation of the geological (or "big") cycle and the biological (or "small") cycle that are orders of magnitude different in space and time is a key to understanding and predicting complex CZ processes. Because of the layered nature of the CZ and the general trend of increasing density with depth, response and feedback to climate change take longer from the above-ground zone down to the soil zone and further to the groundwater zone. Interfaces between layers and cycles are critical controls of the landscape-soil-water-ecosystem dynamics, which present fertile grounds for interdisciplinary research. Ubiquitous heterogeneity in the CZ can be addressed by environmental gradients and landscape patterns, where hierarchical structures control the landscape complex of flow networks embedded in mosaics of matrices. Fundamental issues of hydropedology are linked to the general characteristics of the CZ, including (1) soil structure and horizonation as the foundation of flow and transport characteristics in field soils; (2) soil catena and distribution pattern as a first control of water movement over the landscape; (3) soil morphology and pedogenesis as signatures of soil hydrology and soil change; and (4) soil functional classification and mapping as carriers of soil hydrologic properties and soil-landscape heterogeneity. Monitoring changes in the crucible of terrestrial life (soil) is an excellent (albeit complex) environmental assessment, as every soil is a "block of memory" of past and present biosphere-geosphere dynamics. Our capability to predict the behaviour and evolution of the CZ in response to changing environment can be improved significantly if a global alliance for monitoring, mapping, and modeling of the CZ can be fostered.

tegrated soil and water studies in the context of agriculture (NRC, 1993a), groundwater vulnerability (NRC, There is a growing recognition that synergy could be generated 1993b), watershed management (NRC, 1999), earth sciby bridging traditional pedology with soil physics and hydrology to enhance integrated studies of soil-water relationships across spatial ences (NRC, 2001a), water resources (NRC, 2001b), and temporal scales. Hydropedology is suggested as such a bridge to and environmental sciences (NRC, 2001c). address: (i) knowledge gaps between pedology, soil physics, and hy-To address diverse soil and water issues at various drology; (ii) multiscale bridging from microscopic to mesoscopic and spatial and temporal scales, it becomes clear that bridgmacroscopic levels; and (iii) data translations from soil survey dataing traditional pedology with soil physics, hydrology, bases into soil hydraulic information. Knowledge gaps include flow and other related disciplines is necessary as well as synand transport in the structured unsaturated zone, soil structure quantiergistic. This bridging is justified not only by the interrefication, preferential flow modeling, landscape hydrology, soil spatial lationship among the disciplines but also by the complex and temporal variability, quantitative use of field soil morphology for nature of the problems. This review attempts to examine inferring soil hydrology, mechanisms controlling individual and interthe differences and connections between pedology, soil active soil-water processes at multiple scales, pedotransfer functions (PTFs), and others. Hydropedology integrates the pedon and land-physics, and hydrology and suggests that hydropedology scape paradigms to link phenomena occurring at microscopic (e.g., pores is a natural linkage among them. The bridging of disciand aggregates), mesoscopic (e.g., pedons and catenas), and macroplines, scales, and data is discussed here as an important scopic (e.g., watersheds, regional, and global) scales. Through approaches component for developing synergistic and integrative such as PTFs, hydropedology also facilitates the bridging of data behydropedology. tween soil survey databases and soil hydraulic information needed in simulation models. The bridging of disciplines, scales, and data PEDOLOGY, SOIL PHYSICS, represents potentially unique contributions of hydropedology to integrated soil and water sciences. It is hoped that hydropedology would AND HYDROLOGY contribute to our enhanced understanding of a variety of environmen-Traditionally, pedologists have focused on field soil tal, ecological, agricultural, and natural resource issues of societal profiles (pedons) as observed in the landscape, soil physimportance. These include water quality, soil quality, landscape proicists have emphasized theoretical studies and laboracesses, watershed management, nutrient cycling, contaminant fate, waste tory investigations using small soil samples (only in redisposal, precision agriculture, climate change, and ecosystem functions.cent decades ...

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Abstract. Ecohydrology and hydropedology are two emerging fields that are interconnected. In this study, we demonstrate stemflow hydrology and preferential water flow along roots in two desert shrubs (H. scoparium and S. psammophila) in the south fringe of Mu Us sandy land in North China. Stemflow generation and subsequent movement within soil-root system were investigated during the growing seasons from 2006 to 2008. The results indicated that the amount of stemflow in H. scoparium averaged 3.4% of incident gross rainfall with a range of 2.3–7.0%, and in S. psammophila stemflow averaged 6.3% with a range of 0.2–14.2%. Stemflow was produced from rainfall events more than 1 mm for both shrubs. The average funneling ratio (the ratio of rainfall amount delivered to the base of the tree to the rainfall that would have reached the ground should the tree were not present) was 77.8 and 48.7 for H. scoparium and S. psammophila, respectively, indicating that branches and stems were fully contributing to stemflow generation and thereby provided considerable amount of water to deep soil layer. Analysis of rhodamine-B dye distribution under the shrubs showed that stemflow entered the soil preferentially along root channels contributing to deep storage and that the depth of stemflow infiltrated increased with increasing incident rainfall amount. Distribution of soil water content under the shrubs with and without stemflow ascertained that stemflow was conducive to concentrate and store water in deep layers in the soil profiles, creating favorable soil water conditions for plant growth under arid conditions. Accordingly there is a clear linkage between aboveground ecohydrology and belowground hydropedology in the desert shrubs, whereby an increase in stemflow would result in an increase in soil hydrological heterogeneity.