Specific Storage

Specific Storage $(S_S)$

\[S_S = \rho_w g(\alpha+n\beta)\]

• $\alpha$     compressibility of aquifer matrix $\approx 10^{-8}$ m²/N
• $\beta$     compressibility of water $\approx 4.6\times10^{-10}$ m²/N
• $n$     porosity
• $S_S < 0.0001 \text{ ft}^{-1}$

Storativity $(S)$

Confined aquifer

\[S = bS_S\]

• $b$     aquifer thickness
• $S<0.005$

Unconfined aquifer

\[S = S_y + bS_S\]

• $b$       saturated thickness
• $S_y$     specific yield
• $0.02 < S < 0.3$
• $S_y \gg bS_S$ $\qquad\therefore S\approx S_y$

In MODFLOW, $bS_S$ is referred to as the “storage coefficient”.