1. For surface water, the relation of storage capacity in a reservoir to the mean annual flow of a stream above the dam forming the reservoir. b. For groundwater, primarily in a confined aquifer, it is a measure of the volume of water an aquifer releases from, or takes into storage per unit surface area of the aquifer per unit change in head. This definition is similar to the specific yield for a water table aquifer. However, confined aquifers remain saturated at all times and therefore water release is not met by drainage of the void spaces as is the case of an unconfined aquifer, but due to changes in pore pressure.