Transmissivity from Specific Capacity

The following equation, based on the Cooper and Jacob (1946) solution for flow to a well in a confined aquifer, computes the specific capacity, Q/S_w, of a well:

$$\frac{Q}{s_w}=\frac{T}{0.183\text{log}\left(\frac{2.25Tt}{r_w^{2}S}\right)}\text{(1)}$$

where Q is constant discharge rate [L³/T], r_w is pumped well radius [L], S is storativity [dimensionless], s_w is drawdown in the well [L], T is transmissivity [L²/T] and t is time [T].

Approximate Formulas for Estimating Transmissivity from Specific Capacity

Using equation (1), Driscoll (1986) assumed the following values to develop approximate formulas for estimating transmissivity from specific capacity in confined and unconfined aquifers:

T = 30,000 US gal/day/ft

S = 0.001 (confined) or 0.075 (unconfined)

r_w = 0.5 ft

t = 1 day

Given these assumed values, Driscoll (1986) estimates transmissivity given the specific capacity of a well as follows:

T = 2000(Q/s_w)    confined aquifer

T = 1500(Q/s_w)    unconfined aquifer

where T is transmissivity [US gal/day/ft], Q is constant discharge rate [US gal/min] and s_w is drawdown in the pumped well after 1 day [ft].

If we express Q in m³/day and s_w in m, the equations for computing T in m²/day are (Batu 1998):

T = 1.385(Q/s_w)    confined aquifer

T = 1.042(Q/s_w)    unconfined aquifer

You also may convert specific capacity to transmissivity using equation (1).

Transmissivity from Specific Capacity Calculator
Q/sw (t = 1 day) =		US gal/min/ft m3/day/m