Storage coefficient or storativity in an unconfined aquifer is equal to:

• A. Porosity
• B. Specific yield
• C. Hydraulic conductivity
• D. Permeability

Answer: (B)

In an unconfined aquifer, the amount of water that storage releases when the water level falls is governed by gravity-driven drainage from the pore spaces. This drained portion of water is quantified by specific yield—the fraction of the porosity that can drain per unit decline in head. Since storage changes in an unconfined aquifer are dominated by this drainable water, the storage coefficient (storativity) is essentially equal to the specific yield. Porosity is the total void fraction, not how much water can drain; hydraulic conductivity and permeability describe how easily water moves, not how much water can be stored. So the best fit is specific yield.