Coupled Heat and Water Transfer in Soil.

Movement of heat and water across the soil surface largely drives both the atmosphere and the terrestrial hydrosphere. In the near-surface soil, coupled transfer of heat and water is the rule rather than the exception, but the coupling process is poorly understood. The principal objectives of this project were to collect a comprehensive set of transient coupled heat and water transfer measurements, and to analyze the observations with a fully coupled soil heat and water transfer model.  Measurements were made possible by developing an innovative soil-insulated column set-up, instrumented with multiple thermo-time domain reflectometry (TDR) sensors.  The sensors were extensively evaluated to determine their effectiveness in fine-scale measurement of thermal properties, water content, and water flux.  The cell design provided one-dimensional heat flow conditions by reducing ambient temperature influences, which had been a major limitation to previous experimental work. The thermo-TDR probes effectively measured transient soil temperature, thermal conductivity, and water content, which permitted novel dynamic experiments.  Transient coupled heat and water transfer experiments were conducted using natural and hydrophobized soils over a range of initial water contents. Calibrated coupled heat and water transfer theory was able to predict transient soil temperature and water content when changes in boundary conditions were moderate, but deviated from observation under extreme shifts in boundary conditions. Measurement techniques were also extended to consider heat and water transfer with soil water evaporation under field conditions. Results show the time and depth evolution of the transient evaporation zone in the upper millimeters of the soil as it develops after rainfall events. Overall, these experiments represent the most extensive focused coupled heat and water transfer measurements to date.