Specific Capacity Analysis Theis Equation

Specific Capacity Analysis Theis Equation

Specific Capacity Data

Specific Capacity

Users may directly input specific capacity by selecing the 'Specific Capacity' option. Please note the expected unit format (GPM/ft).

Pumping Rate and drawdown at well

Alternatively, users may instead enter a known pumping rate and observed drawdown at the well to compute specific capcity.

Solver / Algorithm

Theis (1963)

For drawdown at very small radius (e.g., at the edge of the well), the Theis equation (1963) can be arranged to yield:

where Q/s is the specific capacity of the well, t is the period of pumping, r is the radius of the pumping well, T is the aquifer transmissivity, and S is the aquifer storativity.

To solve the equation for T when Q/s is known, an initial guess of T must be made to solve the equation. Then T is adjusted until the calculated values of Q/s is reasonably close to the measured value. An estimate of S is also required.

NOTE: this approach assumes that the well is 100% efficient. When specific capacity is measured at a real-world site, there are turbulent friction losses as water passes into the well (well efficiency < 100%), so the well drawdown will be greater than the drawdown in the aquifer immediately outside of the well. If an estimate of well efficieny is available, one may convert from observed well drawdown, s_well, to aquifer drawdown, s_AQ, using: s_AQ=s_well*E, where E is the well efficieny (expressed as a percentage).

Driscoll (1986)

Driscoll (1986) proposed a simple way to esimate transmissivity for a confined aquifer data if the specific capacity, Q/s, is known:

where Transmissivity, T, is in units of gpd/ft (gpd=gallons per day), the pumping rate, Q, is in units of gpm (gallons per minute), and drawdown, s, is in units of ft.

Razack and Huntley (1991)

This approach relies on an empirical relationship between the transmissivity and specific capacity measured at 214 wells in an alluvial groudnwater basin:

where Transmissivity, T, is in units of m²/day, the pumping rate, Q, is in units of m³/day, and drawdown, s, is in units of m; or:

where Transmissivity, T, is in units of ft²/day, the pumping rate, Q, is in units of ft³/day, and drawdown, s, is in units of ft.

Mace (1997)

A similar approach was developed in the analysis of transmissivity data and specific capacity data from 71 wells in a karstic aquifer:

where Transmissivity, T, is in units of m²/day, the pumping rate, Q, is in units of m³/day, and drawdown, s, is in units of m; or: