Glossary - S - specific yield
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Specific yield (Sy) is a ratio between 0 and 1 indicating the volumetric fraction of the bulk aquifer volume that a given an aquifer will yield when all the water is allowed to drain out of it under the forces of gravity. Roughly, this is the effective porosity, but there are several subtle things which make this value more complicated than it seems.
Some water always remains in the formation, even after drainage; it clings to the grains of sand and clay in the formation. Also, the value of specific yield may not be fully realized until very large times, due to complications caused by unsaturated flow. Tables can be found in the references which give typical ranges for Sy, Ss and S. Specific storage is the primary mechanism for releasing water in confined aquifers, while specific yield is the primary mechanism for releasing water in unconfined (water table) aquifers.