Department of Earth Science |Northeastern Illinois University
Daily Objectives #21 (November 8, 2007)                                        Dr. Sanders

 By the end of today's class, you should be able to do the following: v Ground Water Modeling: Practice Explain what the results of your model are (i.e. tell what the direction of flow is, and how pumping affects the flow system).  Review Explain/defend the results of your model, telling if/why it is a reasonable representation of the flow system.  Review Explore what happens to the modeled flow systems when you change values for variables.  Review Perform a "sensitivity analysis" for your model.

Today's Agenda:

v Ground Water Modeling: Evaluating the Model

(Continued from last time) How good is your model?  First, apply the "Is this reasonable?" test.  Does it show water flowing in what appears to be a reasonable direction?  Are there good reasons for the gradients to be as indicated on the map?  Does the five-year recharge area look reasonable?

Second, try changing some variables and see what happens to the five-year recharge area.
Consider how good your estimates are.  How reliable is your estimate of, for example, hydraulic conductivity?  Could the actual value reasonably be higher?  Lower?  By how much?  Change the values and re-run the model; see how the results differ.  Here are some suggested approaches:
• top elevation of the aquifer: how close to reality do you think your input data are?  what happens if you change them?
• bottom elevation of the aquifer: what if these data were changed?  what would the results look like?
• hydraulic conductivity (horizontal): what's a reasonable range of possible values?  How much difference does it make?
• vertical hydraulic conductivity: how does it change the results if the Kh/Kv ratio is 5:1, or 20:1, instead of 10:1?
• recharge to the aquifer through infiltration: what if recharge is doubled?  Halved?
• porosity: what difference would it make if the porosity were 30%?  20%?
• initial head: how does changing your initial heads change the model results?
• rate of pumping from wells: what if two of the wells were shut down for maintenance, and the third well had to do all the work?  What if the population of the village increases by 10%, and as a result the rate of pumping increases by the same percentage?
Finally, do a "sensitivity analysis": Now that you know the effect of changing each variable, rate its effect as "strong", "moderate", or "minimal".   You may wish to use the document attached here as an aid to organizing your results

© 2007 Laura L. Sanders.  Last updated November 8, 2007.