E3 Physics of Soil as a Porous Medium

Topic

Soil is a porous medium composed of three simultaneous phases—solid, liquid, and gaseous—and it is precisely the hydraulic properties of this medium that determine how water, heat, and dissolved solutes move through it.

Soil texture—that is, the proportions of sand, silt, and clay—governs the distribution of soil pore sizes; this size distribution determines both the soil's saturated hydraulic conductivity and its water retention curve. Soil structure, formed by aggregates and macropores, can profoundly alter these same properties, particularly by creating preferential pathways where water flows much more rapidly than through the rest of the soil matrix.

The water retention curve describes the relationship between the soil's volumetric water content and its matric potential: when water content is high (in moist soil), matric potential approaches zero and water drains freely; when water content is low (in dry soil), matric potential can drop to minus 1,500 kilopascals—the permanent wilting point—beyond which roots can no longer extract water from the soil.

The van Genuchten model is the most widely used empirical fit for describing this retention curve, and its specific parameters depend—as does everything else discussed here—on the particular texture of the soil in question.

Unsaturated hydraulic conductivity, meanwhile, drops dramatically as the soil dries, following the Mualem relationship; it is precisely this steep decline that makes water movement increasingly difficult within soil that has already lost a significant portion of its moisture. In the uppermost soil layer—between twenty and fifty centimeters deep—heat and water vapor are transported in a coupled manner: heat moves via conduction, following Fourier's law (described earlier), while soil temperature itself drives the movement of water vapor toward cooler zones within the soil.

The expected behavior of this entire system involves a heat wave penetrating the soil; this wave loses amplitude exponentially and experiences increasing phase lag as it moves deeper—a pattern characterized by the specific thermal diffusivity of the soil in question. Thus, from the three phases introduced at the beginning of this document to the damped heat wave at the end—and encompassing factors such as texture, the retention curve, and unsaturated hydraulic conductivity—the physics of soil as a porous medium ensures that the movement of subsurface water, heat, and solutes is ultimately governed by the very porous structure that defines each soil.

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