E4 Unsaturated Zone and Infiltration

Topic

The unsaturated zone—also known as the vadose zone—is the volume of soil extending between the surface and the water table; within this volume, soil pores contain both water and air simultaneously, rather than being completely filled by just one of the two.

Water movement within this unsaturated zone follows Richards' equation—previously mentioned in earlier documents—a non-linear diffusion equation that combines gravitational flow (always directed downward) with matric flow (directed toward the area of ​​lower matric potential, regardless of direction). It is precisely this equation that reveals extreme non-linearity, as soil hydraulic conductivity can vary by several orders of magnitude when shifting from saturated to dry conditions.

Infiltration is the process by which rainwater or irrigation water penetrates the surface of this unsaturated zone; the intensity of the matric suction gradient drives a high initial infiltration rate while the soil is still dry, with that rate subsequently declining asymptotically toward the soil's saturated hydraulic conductivity value as the soil becomes wetter.

There are three classic models describing this infiltration: the Green-Ampt model, which assumes an abrupt wetting front and allows for an analytical solution; the Philip model, which expresses infiltration as a power series of the square root of time; and the Horton model, which describes infiltration as an empirically fitted exponential decay.

When rainfall intensity exceeds the soil's infiltration capacity, the excess water generates surface runoff—the same type of runoff described in the context of flooding—which, in this specific context, is known as Horton runoff, named after that same exponential decay model. Preferential flow, on the other hand, occurs when water moves rapidly through macropores—cracks, root channels, or earthworm burrows—bypassing the soil matrix described by the Richards equation entirely; it is precisely this ability to bypass the soil matrix that allows preferential flow to transport contaminants to aquifers much faster than the Richards equation would predict.

Evaporation directly from the soil surface proceeds in three successive stages: in the first stage, evaporation is controlled by atmospheric demand, and the rate equals potential evapotranspiration; in the second stage, the soil's hydraulic conductivity controls a steadily declining evaporation rate; and in the third stage, evaporation drops to very low rates, limited solely by water vapor diffusion through the nearly dry soil. Thus, from the unsaturated zone where this document begins to this final stage of evaporation where it concludes—encompassing the Richards equation, infiltration models, and the preferential flow that bypasses the matrix—the unsaturated zone and infiltration ensure that the fate of each raindrop (whether it infiltrates, runs off the surface, or evaporates back into the atmosphere) is determined by the very soil through which it passes.

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