The unsaturated zone is the portion of the subsurface above the ground water table. It contains, at least some of the time, air as well as water in the pores. Its thickness can range from 0 meters, as when a lake or marsh is at the surface, to hundreds of meters, as is common in arid regions.
First, there is storage -- of water, plant nutrients, and other substances. The unsaturated zone is not always considered a major storage component of the hydrologic cycle because it holds only a tiny fraction of the earth's fresh water and this water is usually difficult to extract. But it is of great importance for storing water and nutrients in ways that are vital to the biosphere.
Second, the unsaturated zone is a zone of transmission of water and other substances. This is how it has been seen from some hydrologic viewpoints, as a zone that to a large degree controls the transmission of water to aquifers, as well as to the land surface, to water on the surface, and to the atmosphere. It may be a controlling factor in the amount of water that replenishes an aquifer, or it may yield information that permits this replenishment to be quantified. It is often regarded as a filter, removing undesirable substances before they affect aquifers. This idea contains some truth, but is sometimes presented as an oversimplification of highly complex processes.
Third, the unsaturated zone is a zone of natural and human-induced activity. Its constituents do not passively reside in place or pass through to the water table. The unsaturated zone experiences physical phenomena such as thermodynamic interactions, transport processes of various kinds, and chemical reactions. There also are chemical reactions involving both natural and artificial substances. There is the biological activity of plant roots, rodents, worms, microbiota, and other organisms. As a zone of human activity, it is used all over the earth for the cultivation of plants, construction of buildings, and disposal of waste.
The most basic measure of the water in an unsaturated medium is water content or wetness (commonly symbolized q), defined as the volume of water per bulk volume of the medium. Water is held in an unsaturated medium by forces whose effect is expressed in terms of the energy state or pressure of the water. Various types of pressure may be relevant in unsaturated hydrology, but one called the matric pressure or matric potential (arising from the interaction of water with the rigid matrix) is of unique interest, as it substantially influences the chief transport processes. Matric pressure (commonly symbolized y) is the pressure of the water in a pore of the medium relative to the pressure of the air. When a medium is unsaturated, the water generally is at lower pressure than the air, so the matric pressure is negative.
Greater water content goes with greater matric pressure. Zero matric pressure is associated with high (saturated or nearly saturated) water content. As matric pressure decreases the water content decreases, but in a way that is nonlinear and hysteretic. The relation between matric pressure and water content, called a retention curve (right), is a characteristic of a porous medium that depends on the nature of its pores. This relation strongly influences the movement of water and other substances in unsaturated media
The hydraulic conductivity, a measure of how easily water moves through the medium for a given driving force, is a second characteristic that is critical to water movement. The hydraulic conductivity (commonly symbolized K) has a highly sensitive and nonlinear dependence on the water content.
Usually we assume that the flow rate of water is equal to the hydraulic conductivity times the driving force (typically gravity and pressure differences). This relation is known as Darcy's law. When the flow is steady, Darcy's law may suffice in itself for quantifying the flow. The more general case of unsteady or transient flow in unsaturated porous media is a highly dynamic phenomenon. This may be represented quantitatively by a combination of Darcy's law and the continuity or conservation law for water. Richards' (1931) equation combines both of these laws in one formula. Use of this formula requires knowledge of two properties of the medium: the unsaturated hydraulic conductivity, and the differential water capacity, which can be directly calculated from the water retention curve.
The use of measured or estimated hydraulic properties with formulations such as Darcy's law and Richards' equation can quantify the movement of water in the unsaturated zone. The flow rate of water is often very directly of interest, for example in estimating how fast water moves down to the water table, defined as the aquifer recharge rate. It also is critical in the transport of contaminants-- whether they are dissolved in the water or moving in a nonaqueous liquid or solid form. The usual first step in assessing the rate of spreading of contaminants in the subsurface is to assess the flow rate of water that to some degree moves the contaminant along with it.
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