Investigating the Simultaneous Effects of Salinity, Moisture, and Soil Texture on Heat Transfer Dynamics in an Unsaturated Porous Medium

Document Type : Research Paper

Authors

• Mohadeseh Abdi ¹
• Masoomeh Delbari ¹
• Fariborz Abbasi ²
• Peyman Afrasiab ¹

¹ Department of Water Engineering, Faculty of Water and Soil, University of Zabol, Zabol, Iran

² Department of Irrigation and Drainage, Agriculture Engineering Research Institute (AERI), Agriculture Research, Education and Extension Organization (AREEO), Karaj, Iran

Abstract

Keywords

• Multiphase soil environment
• Particle size
• Moisture retention curve
• Matric suction
• Soil thermal conductivity

Main Subjects

• فیزیک خاک

Introduction

Heat transfer in unsaturated porous media is a complex, nonlinear process critical to numerous engineering and environmental applications, from geothermal systems to agricultural water management. Soil thermal conductivity (λ) is the key property governing this process. While the influence of factors like soil texture, moisture content, and density on λ is well-documented, the role of salinity—particularly under the combined stress of drought and salinization prevalent in arid regions like Iran—remains less understood and often contradictory in the literature. This study aims to comprehensively investigate the simultaneous effects of matric suction, moisture content, and salinity levels on the thermal conductivity of two dominant soil textures (loam and sandy-loam) from the drought-affected Sistan Plain. The research seeks to address a critical knowledge gap by analyzing how the interaction of these factors, alongside wetting and drying paths, influences heat dynamics, providing essential data for improving thermal models in saline, arid environments.

Materials and Methods

A comprehensive experimental study was conducted on soils from the Sistan Plain. Undisturbed and disturbed soil samples were collected from two dominant soil textures: loam and sandy-loam. Samples were prepared with three levels of salinity (approximately 2, 4, and 6 dS/m). Standard methods were employed to characterize the physical and chemical properties of the soils, including particle size distribution (hydrometer method), bulk density, organic matter content (Walkley-Black method), pH, and electrical conductivity (EC). The soil water retention curves (SWRCs) for both drying and wetting paths were determined using a combination of a sandbox apparatus (for low suction ranges from 0 to 100 cm) and pressure plate extractors (for higher suction ranges up to 15,000 cm). The van Genuchten model was fitted to the SWRC data. Thermal conductivity (λ) was measured directly using a KD2 Pro thermal properties analyzer (Decagon Devices, USA). Measurements were taken at various matric suction levels (equivalent to a water pressure range of 1 to 70 cm) during both the drying and wetting processes, allowing for the analysis of hysteresis effects. The experimental design facilitated a detailed examination of the interplay between soil texture, salinity, moisture path, and suction on thermal conductivity.

Results

The results revealed a complex, non-linear relationship between thermal conductivity and the studied parameters. Key findings include:

Interactive Effects: The behavior of λ could not be described by a simple function of moisture or suction alone. Instead, it was determined by the complex interactions between salinity, particle geometry, moisture level, and the moisture path (hysteresis).

Effect of Salinity: The influence of salinity was highly dependent on soil texture. In loam soil, increased salinity generally led to a decrease in λ, particularly during the wetting path. In contrast, sandy-loam soil showed a more variable response; medium salinity sometimes increased λ (potentially due to salt crystallization improving particle contact), while higher salinity levels could reduce it.

Hysteresis Effect: A clear hysteresis was observed in the λ-moisture-suction relationships. The values and patterns of λ differed significantly between the drying and wetting paths for both soil types, emphasizing that the hydraulic history of the soil is a crucial factor.

Texture Dependence: Sandy-loam soil exhibited greater stability and less pronounced changes in λ with varying salinity compared to the more sensitive loam soil. This is attributed to the coarser pore geometry and lower specific surface area of the sandy-loam, making its thermal properties less susceptible to changes induced by salt concentration.

Conclusion

This study demonstrates that soil thermal conductivity in saline, unsaturated soils is governed by a dynamic interplay between salinity, moisture content, matric suction, and soil texture, all mediated by the moisture path. The findings move beyond simplistic models and highlight that predictive models for heat transfer in arid, saline environments must incorporate these interactions to be accurate. The results have direct practical implications for improving the design and management of agricultural systems (e.g., irrigation and drainage), geothermal systems, and environmental modeling in regions facing water scarcity and soil salinization. Future work should employ advanced multi-scale modeling approaches to further elucidate the mechanisms behind these complex interactions.

Author contribution: 

M.A. (PhD Student) conducted the investigation, performed the experiments, curated the data, and wrote the original draft. M.D. (Supervisor) conceived the research idea, acquired funding, supervised the project, and reviewed & edited the manuscript. F.A. and P.A. (Advisors) contributed to the methodology, validation, and reviewing & editing of the manuscript

Data availability statement: 

Data are available on request from the authors.

Ethical considerations

The authors avoided data fabrication, falsification, plagiarism, and misconduct.

Conflict of interest

The authors declare no conflict of interest.