The unsaturated zone is the portion of the subsurface above the ground water table. The soil and rock in this zone contains air as well as water in its pores. In some places the unsaturated zone is absent, as is common where there are lakes and marshes, and in some places it is hundreds of meters thick, as is common in arid regions.

Unlike the aquifers of the saturated zone below, the unsaturated zone is not a source of readily available water for human consumption. But it is of great importance in providing water and nutrients that are vital to the biosphere, and is intensively used for the cultivation of plants, construction of buildings, and disposal of waste.

Hydrologically, the unsaturated zone is often the main factor controlling water movement from the land surface to the aquifer. Thus it strongly affects the rate of aquifer recharge, critical for the use and management of ground water. It is often regarded as a filter that removes undesirable substances. To some extent this is true, but a more general fact is that flow rates and chemical reactions in the unsaturated zone control whether, where, and how fast contaminants enter ground-water supplies. Understanding of unsaturated-zone processes is therefore crucial in determining the amount and quality of ground water that is available for human use.