The unsaturated zone is the part of the subsurface between the land surface and the groundwater table. The definition of an unsaturated zone is that the water content is below saturation (for the specific soil). Hence, ‘unsaturated’ means that the pore spaces between the soil grain particles or the pore space in cracks and fissures are partially filled with water, partially with air. The unsaturated zone can be from meters to hundred of meters deep.

If an unsaturated zone exists below the ground surface the water infiltrating through the top soil will flow vertically through the unsaturated zone before the water recharges the saturated zone. From the unsaturated zone, the water is lost by i) plant uptake (transpiration), ii) direct soil evaporation and iii) recharge. In the unsaturated zone, the driving force for the flow of water is the vertical gradient of the hydraulic head (consisting of gravity and capillary forces), and the soil characteristics (unsaturated hydraulic conductivity).

The vertical flow through an unsaturated soil is solved numerically using the Richards Equation. This equation is developed by combining the Darcy’s law with the law of conservation of mass and the result is a partial differential equation for one-dimensional vertical flow in unsaturated soil.

Source: http://iwmi.dhigroup.com/hydrological_cycle/unsaturatedflow.html