Storage Coefficient

Abstract A coefficient is a mathematical quantity that relates two or more variables in an equation describing some physical process. In equations describing the flow of fluids in a porous medium (e.g., groundwater in an aquifer), the storage coefficient is the quantity that relates the change in fluid potential (i.e., hydraulic head) in the aquifer to the change in the amount of water stored in the aquifer at a given point. In practical terms, the storage coefficient tells us the volume of water that can be removed from an aquifer for a given drop in hydraulic head. There are three storage coefficients used in groundwater flow equations (the units of each are given in [brackets] following each definition): Specific Storage ( S s ) is the volume of water removed from a unit volume of a confined aquifer for a unit drop in hydraulic head [L −1 ]. Storativity ( S ) is the volume of water removed from a unit area of an aquifer for a unit drop in hydraulic head; in confined aquifers, it is equal to the specific storage times the thickness of the aquifer, in unconfined aquifers, it is equal to the specific storage times the thickness of the aquifer plus the specific yield [−]. Specific Yield ( S y ) is the volume of water removed from a unit volume of an unconfined aquifer for a unit drop in water level; this quantity is also defined as the volume of water that will drain by gravity from a saturated unit volume of an aquifer and is equivalent to the effective porosity [−].