Coupled Heat and Water Transfer in Soil

Iowa State University, Ames IA

Investigators

Abstract

0337553 Horton Movement of heat and water across the soil surface largely drives both the atmosphere and the terrestrial hydrosphere. In near-surface soil, coupled transfer of heat and water is the rule rather than the exception, yet the coupling process is poorly understood. In fact, predictions made using the de facto standard Philip and de Vries type coupled transfer model can be substantially off. Current models, based as they are on data sets covering only a narrow range of conditions, ignore at least two important considerations: thermally-driven advection ('soil breathing'), in which daily heating at the soil surface cyclically alters the density of the gas phase and so drives advection of soil air, and wettability, which controls both the height of capillary rise and the area of the air-water interface. We propose to conduct detailed experiments measuring heat and water in both sealed and open soil columns. Our experiments will use the new thermo-TDR probe, capable of measuring temperature, thermal properties, and water content, allowing us to conduct several months of experiments on each individual soil column. This permits us to subject each soil column to a wide range of boundary conditions, and so to collect data for both calibration and validation from each soil column. We will examine effects of soil texture and wettability, water content, mean temperature, direction of thermal gradient, and influence of fluctuating boundary conditions on coupled heat and water movement in sealed soil columns. We will then study effects of soil texture and wettability, mean temperature, upper boundary condition, and advective vapor barrier on evaporation from open soil columns. In parallel with the laboratory experiments, we will develop a network model of coupled heat and water transfer in soils. A network model operates from first principles, and (in contrast with conventional continuum models) properties such as unsaturated hydraulic conductivity, air/water interfacial area, and soil breathing emerge from the network rather than being explicitly programmed in. This fundamental approach will allow us to examine which properties and processes are important under which conditions, furthering our understanding and focusing the development of improved continuum models in the future. This project will develop and disseminate new models and approaches in both experiment and modeling. These ideas, data, and models will be made available in the peer-ed literature, at conferences, and on a group website. Additionally, the research will be integrated into broader programs and activities of national interest, such as natural resource management and climate change.