Spatial and Temporal Variability of Soil Thermal Properties in Tilled Layers.

Many factors, including soil texture, water content, bulk density, and structure, affect soil thermal properties. However, existing experimental and modeling efforts tend to focus on the relationship between soil thermal properties and a single soil or environmental factor. Few reports consider changes in soil thermal properties as related to soil structural variation. The objective of this research is to investigate the spatial and temporal characteristics of soil thermal properties in a tilled soil layer, where soil water content and bulk density vary continuously as a result of age hardening and wetting/drying cycles. Field experiments are performed on loam and clay loam soils to examine the dynamic thermal characteristics of the plow layer following conventional tillage and subsequent soil settling. Soil temperature, water content, bulk density, and thermal properties at 3, 7, and 12-cm depths of the tilled layer are monitored continuously. A new model is developed to describe the time dynamics of soil bulk density. The new model is combined with existing models that relate soil thermal conductivity and volumetric heat capacity with water content. Thus, soil bulk density in addition to water content, is used to describe the changes of soil thermal conductivity and heat capacity under field conditions. The measurements and models are useful in understanding the spatial and temporal characteristics of soil structure and thermal properties, and can improve current models for coupled heat and water transfer near the soil surface.