Well Drawdown Model by Daniel Gallagher
Click on image to see sample screen.
The purpose of this program is to graphically illustrate the drawdown of the piezomentric surface over time in the vicinity of a pumping
well. The model assumes a homogenous, confined aquifer, with radial flow to the pumping well. The drawdown can be estimated from the Theis equation:
where
h0 |
the initial elevation of the piezometric surface (m) |
r |
distance from the pumping well (m) |
t |
time since pumping began (d) |
h(r,t) |
the elevation of the piezometric surface at distance r and time t (m) |
Q |
the pumping rate (m3/day) |
T |
aquifer transmissivity |
u |
|
S |
aquifer storativity |
W(u) |
well function, defined as the integral in the first equation. This function is tabulated in most ground water hydrology textbooks. It can also be
predicted based on a polynomial equation. |
h0-h(r,t) |
drawdown (m). Shown as - z on the graph. |
The program solves the Theis equation and plots the drawdown as a function of either distance or time. Radio
selection buttons are used to control the x-axis. The values of the input parameters (time, distance, flow
(pumping rate), transmissivity, and storativity) are all set using sliders. Simply move the slider bar up and down
to change the parameter value. The graph is automatically redraw as you move the slider. This approach
allows for the sensitivity of the drawdown to be easily assessed. Boxes at the top and bottom of the sliders can be used to set the range.
Only one of the time and distance sliders is available depending on the x-axis. For example, if you are plotting drawdown with distance, the distance slider is automatically turned off.
You also control the scale of the drawdown displayed on the graph with the "Max Drawdown" input box. This
does not affect the calculations in any way. It simply scales the graph so that you can zoom in or out.
INSTRUCTIONS
- Web Based Version
The model will run within your browser if you are using MS Internet Explorer 4 or higher. This version uses
several ActiveX controls and is not compatible with Netscape or other browsers. The first time you run the
model, the download times may be rather long. After that, the model should download very quickly, since only
changes are checked. You will need to set your browser's security setting to low the first time you run the model. I urge you to reset it back to medium afterwards.
NOTE: ActiveX Document technology is somewhat temperamental at this point and may not be able to run on
your particular machine. If a 'File Download' dialog box appears, press 'Cancel' and press the button below
again. If a 'Security Warning' dialog box appears, press 'Yes'. You may need to try this sequence a couple
times. If still unsuccessful, download the Setup version, by pressing the 'Download Version' button below.
- Download Version
A downloadable version is available for Windows 9x or Windows NT.
Comments:
Daniel Gallagher
Department of Civil and Environmental Engineering
Virginia Tech
Blacksburg, VA 24061-0246
email: dang@vt.edu
Copyright © 1998 Daniel Gallagher
Last Modified: 10-1-1999 |
