## Theis Equation Calculator

#### Calculation

#### Instructions:

The form at the top of this page may be used to calculate the drawdown associated with a specific well if the following parameters are known: Storage Coefficient, Transmissivity, and the Discharge Rate. The user will also supply arbitrary values for the radius and time. The resulting drawdown will be calculated using the Theis equation. The user may select specific units where available and the conversion happens automatically.

The user may also utilize this form to calculate other variables in the equation, however due to the complexity of the Theis equation, this process must be done iteratively by the user. The user must know the drawdown as well as four out of the five remaining variables. The user shall input the known variables into the respective fields and adjust the remaining variable until it corresponds with the known drawdown.

A chart is automatically generated for the input data. The chart is plotted through the use of Google Chart API. The X-axis range is determined at twice the entered radius value. Note that radii entered that are too small will not generated a proper chart. Take into account the well casing size and use this as a minimum. To get a decent chart, enter a value that is at least 20 times the diameter of the well casing.

#### Information:

This page was developed with information from the *Civil Engineering Handbook: Second Edition* edited by *W.F. Chen &
J.Y. Richard Liew*.

Pumping a well causes a cone of depression, or drawdown, of the water table of an unconfined aquifer or
of the piezometric surface for a confined aquifer. The drawdown s(r,t) at a distance r from a fully

penetrating well at time t after the beginning of pumping at a constant rate Q from a confined aquifer

with transmissivity T and storage constant S is given by the Theis equation:

This is a tool that solves the Theis equation. The Theis equation was developed to determine the groundwater drawdown at any given readius from the well. The equation may be used alone or to model an aquifer's operating characteristics. The system presented here may be used as an alternative to more expensive software such as modflow. The Theis equation utilizes the flow yield which is usually a result of pumping the well. The equation takes into account the transmissivity of the outlying formation. An assumption the model makes is that the well resides in a confined aquifer. The Theis equation is most often applied to water wells.

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#### Disclaimer:

The data, information, and utility presented in this page is to be used at own risk. Results should be verified before relying on the result.