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The vadose zone, also termed the unsaturated zone, is the part of Earth between the land surface and the top of the phreatic zone, the position at which the groundwater (the water in the soil's pores) is at atmospheric pressure ("vadose" is from the Latin word for "shallow").
It can also be simply explained as the depth below which the ground is saturated. The water table is the surface where the water pressure head is equal to the atmospheric pressure (where gauge pressure = 0). It may be visualized as the "surface" of the subsurface materials that are saturated with groundwater in a given vicinity. [2]
The part above the water table is the vadose zone (also called unsaturated zone). The phreatic zone size, color, and depth may fluctuate with changes of season, and during wet and dry periods. [ 1 ] [ 2 ] Depending on the characteristics of soil particles, their packing and porosity, the boundary of a saturated zone can be stable or instable ...
are very dependent (in a non-linear way) on the water content, which makes solving the unsaturated flow equation difficult. In fact, the variably saturated K for a single material varies over a wider range than the saturated K values for all types of materials (see chart below for an illustrative range of the latter).
Some workers restrict their definition of the capillary fringe only to the tension-saturated base portion and exclude it wholly from the vadose zone. [1] [2] This is more common among workers addressing solute transport and water flow. Others define the capillary fringe as including both the tension-saturated and unsaturated portions.
A horizontal water balance is applied to a long vertical column with area extending from the aquifer base to the unsaturated surface. This distance is referred to as the saturated thickness , b . In a confined aquifer , the saturated thickness is determined by the height of the aquifer, H , and the pressure head is non-zero everywhere.
A newer method that allows 1-D groundwater and surface water coupling in homogeneous soil layers and that is related to the Richards equation is the Finite water-content vadose zone flow method solution of the Soil Moisture Velocity Equation. In the case of uniform initial soil water content and deep, well-drained soil, some excellent ...
The Richards equation represents the movement of water in unsaturated soils, and is attributed to Lorenzo A. Richards who published the equation in 1931. [1] It is a quasilinear partial differential equation; its analytical solution is often limited to specific initial and boundary conditions. [2]