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Distribution and properties of soft weathered bedrock at ≤ 1 m depth in the contiguous United States

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Correspondence to: Robert C. Graham, Soil and Water Sciences Program, Department of Environmental Sciences, University of California, Riverside, CA 92521-0424, USA. E-mail: robert.graham@ucr.edu

ABSTRACT

The weathered bedrock zone is increasingly recognized as an important part of ecological and hydrologic systems, but its distribution is poorly known in the contiguous United States. We used spatial and laboratory characterization data from the US Department of Agriculture (USDA)-Natural Resources Conservation Service to assess the distribution and soil-like properties of soft weathered bedrock (saprock and saprolite) within the 48 contiguous United States. Because USDA soil inventories generally do not extend below 2 m, and because the upper 1 m is clearly involved in ecosystem function and vadose zone hydrology, we restricted our inquiry to soft weathered bedrock within 1 m of the land surface. Soft weathered bedrock within 1 m of the land surface is widespread throughout the contiguous United States, underlying at least 6% of the land area. In-depth analysis of three states showed that soft weathered bedrock at the ≤ 1-m depth underlies 22% of the total land area in California, 33% in Wyoming, and 18% in North Carolina. Soft weathered bedrock hosts pedogenic activity, as indicated by morphological features such as roots, clay films, and iron (Fe)-/manganese (Mn)-oxide concretions recorded in pedon descriptions in the database. The physical and chemical properties of soft weathered bedrock are often similar to those of the overlying soil, suggesting that in many respects soft weathered bedrock behaves like soil. It supplies water and nutrients to plants whose roots penetrate into it and it modulates throughflow runoff to streams. For a more complete understanding of soft weathered bedrock, systematic data are needed on its thickness across landscapes and a consistent terminology for its various forms needs to be adopted and widely used. Copyright © 2012 John Wiley & Sons, Ltd.

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