Diffusive Soil Transport and Hillslope Evolution

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PIs will pursue a comprehensive theory of diffusive soil transport and hillslope evolution associated with biological activity and surface rainsplash, building on recent, key developments in hillslope geomorphology. Their objectives are: (i) to clarify the fundamental ingredients and forms of constitutive (transport) laws describing these processes, demonstrating the conditions under which transport is proportional to the land-surface gradient or to the product of the soil thickness and surface gradient; (ii) to provide the analogue of a kinetic theory for the diffusion-like coefficient used in hillslope evolution models, demonstrating how it is an explicit nonlinear function of soil-particle activity, particle size, soil porosity and soil thickness, and indirectly related to climatic and biological conditions; (iii) to experimentally obtain the form of the constitutive law for transport by rainsplash using high-speed video and particle tracking techniques; (iv) to illustrate the fundamental significance of particle-activity gradients in producing transport, and demonstrate this using sandbox experiments for the case of rainsplash, and using field measurements of soil properties and tracer particles for the case of subsurface transport; (v) to clarify the ingredients of surface and subsurface particle dispersal, mixing and sorting, demonstrating how these processes contribute to hillslope catena structure; and (vi) to develop an advanced computational model of diffusive transport and hillslope evolution for exploring the full coupling between land-surface geometry, soil transport, soil thickness and soil production, including nonlinear responses to external and boundary forcing. This four-year program will mesh theoretical, experimental, field-based and computational components. The PIs theoretical work will focus on clarifying: (i) the ingredients of particle dispersal, mixing and sorting soil-depth and catena scale; and (ii) the forms of constitutive transport laws. PIs experimental/field work will: (i) focus on the process of rainsplash, and on particle mixing and sorting within soils, and at the soil surface; and (ii) be designed to test/validate the theoretical and computational aspects of their work. They will also apply high-performance algorithms based on discontinuous spectral element methods in developing advanced computational codes for simulating particle behavior at the soil-depth scale, and for simulating hillslope evolution at geomorphic scales. Under the auspices of the Center for Earth Surface Processes Center (CESPR), this project will involve collaboration of scientists expert in geomorphology, fluid mechanics, applied mathematics and advanced computations. The results of this work will constitute a major, and critical, contribution to the growing theoretical foundation of hillslope geomorphology.