storativity

Storativity, also spelled storativity, is a dimensionless parameter in hydrogeology that describes how much water will be released from an aquifer or stored when the hydraulic head changes. It is defined by S = ΔVw / (A Δh), where ΔVw is the change in the volume of water stored, A is the area of interest, and Δh is the corresponding change in hydraulic head. In practice, storativity relates the decline or rise of the water table or piezometric head to the amount of water gained or lost from storage.

For confined aquifers, storativity is related to specific storage Ss and aquifer thickness b by S = Ss × b. Specific storage is the volume of water released per unit volume of aquifer per unit decline in head, and multiplying by the thickness yields a dimensionless storativity.

In unconfined aquifers, storativity is typically controlled by the drained water and is commonly approximated by the specific yield Sy, the amount of water that drains from the aquifer per unit surface area per unit decline in the water table. Unlike confined storativity, S for unconfined systems is not constant and can vary with degree of saturation and effective stress.

Storativity is a key parameter in aquifer testing and groundwater flow models. It influences the rate at which drawdown propagates away from pumping wells and, together with transmissivity T, determines the hydraulic diffusivity D = T / S.