Theis Recovery Method
Theis Recovery Method
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This is the Theis recovery analysis method. It is developed for confined aquifers but has also been used successfully for leaky and unconfined aquifers.
Using the method:
- Ensure that your data set has recovery data. From Edit | Aquifer Parameters, make sure the box Test Includes Recovery Data is checked. Make sure the time at which pumping stopped has been entered.
- Select Theis Recovery from the View menu. A graph will appear with arithmetic drawdown on the vertical axis and log of time ratio t/(t - tp) on the horizontal axis. Only the recovery data will appear on the graph.
- Transmissivity is determined by drawing a best fit line through the data points. The slope of the line is used to determine transmissivity. Buttons at the bottom of the screen are used to control which points are used in the best fit determination. The calculated transmissivity is displayed directly on the screen.
Controlling the Best Fit Line:
Buttons on the bottom of the screen control how the line is fit to the data. Data points are added or subtracted from either end of the data graph. Points not included in the best fit procedure are displayed with the symbol selected for excluded data from Options | Graph Symbols.
In the Theis recovery graph, points on the left side of the graph are from late time and points on the right side are from early time. In other words, time increases moving from right to left. All other graphs increase time from left to right.
The buttons on the bottom left corner of the screen add or subtract data points from the end (late time) of the analysis. The left arrow adds points to the analysis. The right arrow subtracts points from the analysis.
The buttons on the bottom right corner of the screen add or subtract data points from the beginning (early time) of the analysis. The right arrow adds points to the analysis. The left arrow subtracts points from the analysis.
Determining Transmissivity:
The slope of the best fit line is used to determine transmissivity from the following equation:
where:
T is transmissivity
Q is the well discharge rate
s is the change in residual drawdown per log cycle of time ratio.
Time Ratio:
The horizontal axis is the log of time ratio. Time ratio is t/(t - tp), where t is time since beginning of pumping, and tp is the time at which pumping ceased.