Steady State Calculator

 

After aquifer transmissivity and storativity have been determined, the steady state calculator can be used to estimate the steady state drawdown for different pumping rates.

 

 

Implementation

 

The user enters transmissivity, storativity, maximum allowable drawdown, well radius, and pumping rate. The calculator solves the Theis (1935) solution equation consecutively for increasing times until the drawdown stabilizes. The Theis equation is given as:

 

image\THEIS.gif

where:

image\WU.gif

and:

image\U.gif

 

 

Five Minute Steady-State Tolerance

 

The Theis equation describes drawdown that approaches steady-state asymptotically, but never actually reaches a single steady-state value. Therefore some amount of change in drawdown per time must be specified to indicate the well has reached steady state. The value entered here is the amount of drawdown that occurs in five minutes of pumping to define steady-state. By default, this value is 0.001 meters. In other words, when the change in drawdown as described by the Theis equation is less than 0.001 meters in five minutes, it is assumed that steady-state has been reached. A smaller value may be justified in high transmissivity aquifers. Entering a very small value may result in the inability of the algorithm to converge, prompting an error message.

 

 

Applicability

 

Since the calculator uses the Theis equation, it is accurate to the extent that the Theis solution is applicable or appropriate.

 

The steady-state calculator is intended only to estimate actual aquifer conditions. Results should be verified by hand calculations.