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Highlights
Hydropedology
Hangsheng Lin

The emerging field of hydropedology is an intertwined branch of soil science and hydrology that encompasses multiscale basic and applied research of interactive soil and water processes and their properties in the unsaturated zone. This synergistic integration suggests a renewed perspective and a more integrated approach to study landscape-soil-water dynamics across scales and their relations to climate, ecosystems, contaminant fate and land-use.

Given its links to a wide array of environmental, agricultural, geological and natural resource issues of societal importance, hydropedology is becoming a promising field that could contribute significantly to the study of Earth's critical zone. The critical zone encompasses the pedosphere — the thin skin of soils on the planet's surface that acts as a "geomembrane" across which water and solutes, as well as energy, gases, solids and organisms actively exchange with Earth's atmosphere, biosphere, hydrosphere and lithosphere to create a life-sustaining environment.

Soil and water are two critical components of the critical zone, which represent a key interface between the biotic and abiotic. Water controls a variety of soil physical, chemical, and biological processes that lead to the formation of diverse soils which support an array of land uses and biological communities. On the other hand, soils play a key role in the global hydrological and biogeochemical cycles, contribute to the maintenance of water quality and ecosystem functions, and act as a living filter and remediation medium for waste materials.

The interactions of soil and water are so intimate and complex that they cannot be studied in a piecemeal manner, but rather as a system across spatial and temporal scales. Seven working models or perceptions of soil could be used to evaluate the relevancy of hydropedology to the study of Earth's critical zone: 1) soil as a natural body, with water as a major driving force of soil dynamics; 2) soil as a water reservoir and transmitting mantle that impacts water quantity and quality; 3) soil as a gas and energy regulating geoderma (the skin of Earth) that links to greenhouse gas emission and global warming; 4) soil as a component in ecosystems, supporting and regulating the fluxes of air, water and nutrients for organisms to thrive; 5) Soil as a medium for plant growth, with water flow mediating nutrient cycling, crop yield, irrigation and drainage; 6) soil as a material for engineering, with soil mechanical properties as influenced by water content being crucial in hillslope stability, landslide and other structural protections; and 7) soil as an integral part of the environment, where soil-water interactions are significant.

Hydropedology calls for adequate attentions to soil morphology (including soil structure) in the field and soil patterns over the landscape to guide optimal hydrologic measurements, field monitoring site selections, experimental designs and flow and transport modeling in the critical zone. The essential role of soils in landscape hydrology has often been overlooked (Reuter and Bell, Soil Science Society of America, v. 67, p. 365), and yet is a key issue for advancing hillslope hydrology and watershed modeling (Ridolfi et al., Journal of Hydrology, v. 272, p. 264).

Translating soil and hydrologic properties and processes across scales has emerged as a major theme in contemporary soil science and hydrology. Hans-Jörg Vogel and Kurt Roth (Journal of Hydrology, v. 272, p. 95) have suggested a "scaleway" as a potentially promising tool for predictive modeling of flow and transport in the subsurface at any scale. Michael Sommer and colleagues (Geoderma, v. 112, p. 179) have presented an integrated method for soil-landscape analysis, in which they developed a hierarchical expert system for multi-data fusion of inquires, relief analysis, geophysical measurements and remote-sensing data. They further combined the soil-forming factorial model with the scaleway of Vogel and Roth to address soil variability across scales.

Pattern recognition across scales is becoming a forefront in soil science and hydrology (Grayson et al., Advanced Water Resources, v. 25, p. 1313), which offers rich and comprehensive insights regarding variability and the underlying processes. Murugesu Sivapalan (Hydrological Processes, v. 17, p. 1037), in addressing the connection between process complexity at hillslope scale and process simplicity at the watershed scale, emphasized the "patterns that have physical meanings that transcend the range of scales." Markus Deurer and co-workers (Journal of Hydrology, v. 272, p. 148) illustrated that a similarity might exist between stream networks and the network of water flow pathways in soil profiles.

Important to future work is that soil investigations no longer be limited to the top 2 meters of Earth's surface but extend well into the deeper vadose zone. It requires a concerted effort to study the soil and underlying material to whatever depth is needed to meet our scientific needs. Geologists are extending their studies to the surface and including the biosphere and surficical processes, so it is paramount that soil scientists redirect their efforts to interface with other geoscientists in making soil science an integral part of the earth and environmental sciences.

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Lin is an assistant professor of hydropedology and soil hydrology at the Pennsylvania State University in University Park. E-mail: henrylin@psu.edu.>

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