Objectives 1
Department of Earth Science |Northeastern Illinois University

ESCI 337
Spring, 2009

Daily Objectives #21 (April 7, 2009)                                        Dr. Sanders

By the end of today's class, you should be able to do the following: 
Ground Water Modeling: Practice

Perform a sensitivity analysis of your model:

Try changing some variables and see what happens.  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; keep notes on your work, as you'll need it to write up your results later. 

  • 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:  What's a reasonable range of possible values?  How much difference does it make?
  • recharge to the aquifer through infiltration:  What if recharge is doubled?  Halved?
  • porosity:  What difference would it make if the porosity were 30%?  20%?  Some other value?
  • initial head:  How does changing initial heads change the model results?  (Consider the constant head cells and active cells separately.)
  • rate of pumping from wells:  What if your clients' industrial production increases, and they need to increase pumping of the wells by 20%?  50%?
Finally, now that you know the effect of changing each variable, rate the effect as "strong", "moderate", or "minimal". 
Ground Water Modeling: Evaluating a Model

How good is your model?  To evaluate it, do the following:

First, apply the "Is this reasonable?" test.  Ask yourself questions about whether or not various aspects of your model make sense.  For example, does it show water flowing in what appears to be a reasonable direction?  (How can you tell?)  Does it make sense to have the flow system configured the way the model says it is?  Where does water enter the flow system, and where does it leave?  Are there good reasons for the gradients and flow directions to be as indicated on the map?  Do the particle tracks for any pumping wells indicate a reasonable size and shape of an area providing water to the wells?  

Second, compare your model to others'.  How do they differ?  Why do they differ?  What assumptions did the others make that you did not, and vice-versa?  If you were in court defending your model against those of other geologists on the opposing side, could you defend your choices?

Department of Earth Science | Northeastern Illinois University

Copyright 2009 Laura L. Sanders.  Last updated April 7, 2009.