Submitted to: Water Resources Research
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/24/2001
Publication Date: 1/29/2002
Citation: Interpretive Summary: During the past decade, soil thermal research has motivated development of the heat pulse technique (HPP) for estimation of soil thermal properties. Apart from needing to characterize the soil's physical properties, knowledge of the soil thermal properties is required for accurate prediction of soil temperature and its influence on seed emergence and crop growth, soil water retention and the unsaturated hydraulic conductivity, and soil water vapor flow in coupled water and heat transport. It is expected that interest in the soil thermal regime will become increasingly important as both fundamental and applied research questions are posed regarding water movement in radioactive waste repositories, the fate and transport of high vapor pressure liquids (e.g. solvents) and microorganisms (e.g. bacteria and viruses). In addition, many chemical processes in soils are temperature dependent and their functioning in contaminant transport requires better prediction of soil temperature regimes. The soil thermal properties can be quickly and conveniently measured using heat pulse probes. The objective of this paper was to use inverse modeling to more accurately estimate soil water content, water flux density, and the water content dependence of soil thermal properties from HPP measurements.
Technical Abstract: Traditionally, analytical solutions for heat transport in soils have been used in combination with heat pulse probe (HPP) measurements to estimate soil thermal properties. Although the analytical method has resulted in accurate estimation of soil thermal properties, we suggest that parameter estimation using inverse modeling (IM) provides new and unique opportunities for soil thermal characterization. Moreover, we show that the IM approach provides accurate estimation of soil water flux density in both unsaturated and saturated soil conditions, for a wider range of water velocities than originally thought possible. Specifically, we show that accurate soil water velocity is obtained, simultaneously with soil thermal properties, if heat dispersion is included in the heat transport equation. The requirement for including heat dispersivity depends on the value of the newly defined dimensionless KJJ number, which is equal to the ratio of thermal dispersion to thermal conductivity. For example, when KJJ > 1, ignoring thermal dispersivity leads to errors in the water flux density which can exceed 10 percent. By including thermal dispersivity, water flow velocities were accurately determined for water flux densities ranging from 1.0 to larger than 10 m per day or m/day. We also demonstrate the general application of inverse modeling to estimate soil thermal properties and their functional dependence on volumetric water content, in a separate numerical experiment. We suggest that inverse modeling of HPP temperature data may allow simultaneous estimation of soil water retention (when combined with matric potential measurements) and unsaturated hydraulic conductivity (through water flux estimation) from simple laboratory experiments.