Is Static Soil Density a Viable Assumption for Studying Surface Hydrologic Processes?

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A soil surface is an interface between the ground and the atmosphere. Soil surface layer conditions influence the Earth's hydrologic cycle by impacting rainfall infiltration and runoff, soil water evaporation, and storage of plant available water. Properties of soil surface layers are dynamic in time. Tillage loosens soil surface layers, while vehicular traffic and raindrop impacts cause soil surface layers to compact. The dynamic density and porosity natures of soil surface layers are not well known. Furthermore, the dynamic hydraulic properties of soil surface layers are almost unknown. This study is designed to measure dynamic soil surface layer properties under a range of field conditions and to evaluate the impacts of these properties on the hydrologic cycle. Advancing knowledge of these processes is critical for understanding important land surface-atmosphere exchanges (e.g. permafrost release of greenhouse gases and evapotranspiration) and could lead to better agricultural water management. The proposed study will use newly developed thermo-TDR sensors to provide co-located, in situ measurements of surface soil water content and thermal properties. Thermo-TDR sensors are also able to measure dynamic surface soil density and porosity. Proposed field and numerical experiments will address three fundamental questions: i. Over what temporal scales and to what extent does loosened soil density change? ii. Can patterns be identified to facilitate development of predictive relationships, incorporating transient density, with other fundamental soil hydraulic and thermal properties? iii. How does shifting soil density directly impact water and energy exchange at the land surface-atmosphere interface and mass and heat transfer in subsurface soil? Addressing these questions provides opportunity to transform the hydrologic sciences, strengthen analytical frameworks and improve predictive theory. Proposed work, guided by specific expertise on soil properties and processes, and supported with newly developed tools, provides the opportunity to measure dynamic soil surface layer properties under a range of field conditions. Because knowledge of soil density and porosity is fundamental to investigation of soil hydraulic, thermal, and gas transfer properties and processes, new discoveries achieved in this study offer transformative potential for wide ranging investigations of land surface, hydrologic cycle, and ecosystem dynamics.