The volume of water that an unconfined aquifer releases from storage per unit surface area per unit decline in the water table. It represents the drainable porosity of the aquifer material.
Specific yield (Sy) is a dimensionless aquifer property defined as the volume of water that drains from a unit volume of saturated aquifer material under the force of gravity, divided by the total volume. It represents the portion of total porosity that can actually yield water to wells and is equivalent to the effective porosity for unconfined aquifers. Specific yield values typically range from 0.01-0.10 for fine-grained materials (clays, silts) to 0.10-0.30 for coarse-grained materials (sands, gravels). The difference between total porosity and specific yield is the specific retention, which represents the water held in pores by surface tension and molecular forces against the pull of gravity. Specific yield is a critical parameter for calculating groundwater storage changes from water table fluctuations, estimating the sustainable yield of unconfined aquifers, and calibrating groundwater flow models. It can be estimated from pumping test analysis, laboratory drainage experiments, or empirical relationships with grain size. The water table fluctuation method, which uses specific yield and water level changes to estimate recharge, is one of the most widely applied recharge estimation techniques.