Storage Coefficient Calculator
Calculate storage coefficient with our free science calculator. Uses standard scientific formulas with unit conversions and explanations.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
Ss = rho_w * g * (alpha + n * beta); S = Ss * b
Where Ss is specific storage, rho_w is water density, g is gravity, alpha is aquifer compressibility, n is porosity, beta is water compressibility, S is storativity, and b is aquifer thickness.
Worked Examples
Example 1: Confined Sandstone Aquifer
Problem:30 m thick, porosity 0.25, compressibility 1e-9/Pa, water compressibility 4.6e-10/Pa, drawdown 5 m.
Solution:Ss = 1000 x 9.81 x (1e-9 + 0.25 x 4.6e-10) = 1.094e-5/m\nS = 1.094e-5 x 30 = 3.281e-4\nVolume = 3.281e-4 x 5 = 0.00164 m3/m2
Result:S = 3.281e-4 | Ss = 1.094e-5/m | Volume = 0.00164 m3/m2
Example 2: Compressible Clay Aquifer
Problem:15 m thick clay, porosity 0.40, compressibility 5e-8/Pa.
Solution:Ss = 9810 x 5.018e-8 = 4.923e-4/m\nS = 4.923e-4 x 15 = 7.385e-3
Result:S = 7.385e-3 | Ss = 4.923e-4/m
Frequently Asked Questions
What is the storage coefficient of an aquifer?
The storage coefficient (storativity) describes water volume released per unit surface area per unit change in hydraulic head. For confined aquifers it is typically 0.00001 to 0.001 because water releases through compression and expansion. For unconfined aquifers it is 0.01 to 0.30 because water drains by gravity. This parameter is essential for predicting aquifer response to pumping and designing well fields.
How is specific storage different from storativity?
Specific storage (Ss) is water released from a unit volume per unit head decline in units of inverse length. Storativity (S) equals Ss times aquifer thickness b, making it dimensionless. Specific storage is an intrinsic material property independent of thickness. Typical values range from 1e-6 to 1e-4 per meter, and engineers use storativity in well hydraulics equations like the Theis equation.
What factors control the storage coefficient?
Storage coefficient is controlled by compressibility of aquifer skeleton and pore water, porosity, and thickness. Aquifer compressibility depends on material type with clay much more compressible than sandstone. Water compressibility is about 4.6e-10 per Pascal. Higher porosity increases the water contribution. Greater thickness directly increases storativity. Temperature and dissolved gas also have minor effects on water compressibility.
How do you measure storage coefficient from pumping tests?
The coefficient is determined from time-drawdown data at observation wells. The Theis curve-matching fits data to W(u) versus 1/u where u = r^2*S/(4*T*t), extracting transmissivity and storativity simultaneously. Cooper-Jacob simplifies for late-time data solving S from the drawdown versus log-time intercept. At least one observation well at known distance is required since pumping well data is affected by well losses.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy