groundwater drawdown

hello,
feflow is new to me. How can i calculate or visualize the groundwater drawdown? i know somehow i have to use the expression editor? i have a transient model that runs a year. from april to september i have irrigation wells running. i want to see how much the wells have an impact on the system.

thanks
andi

Add a nodal user data set.  Export the head from one of the times you want to use in the drawdown calculation (to a dat file), add that dat file as a map and "link to parameter" -> the new user data you just added.  set the link up so that the NODE field of the dat file points to node number.  activate the new user data and select all nodes in a 3d view (I'm assuming a 3d model).  set the data assign toolbar to the linked map and click the green check.  now select the other time you want to use in the ddn calc, and then use the expression editor to recalculate the new user data using "current = current - head".  next select the expression in the data assignment toolbar and then click the green checkmark.  Your new nodal user data should contain the difference between the head at the two times you selected.

Hi Pete,

I have the same problem like Andi. Feflow is not new for me, but I still couldn't follow your steps how you described above.
For example: - after you export the head as a head.dat and you bring in as a map you link FINIT values to head, is that correct?
                    - im also unable to make the expression how you descibed

Can you please write down step by step how you manage to solve this issue. Print screens would help us a lot.

Thank you very much,
Tibor from Golder

I link to maps a lot, but I can never get the node assignment to work. If you're having trouble with it, try slice instead of node. Not as helpful for point drawdowns, as it averages several nodes, but better than nothing at all.

1) Run a model with no pumping for the same period. This will control for the other boundary conditions and fluxes in your model.
2) Export nodal head quantities for the time period you are interested in from both the model with wells and the model without wells
3) Use Surfer or ArcMap to create a raster map of groundwater head for each data set
4) Subtract one raster from the other
5) You now have a raster showing drawdown due to the wells

FINIT is the field you link to he user data set you created.  When you select "link to parameter" the window will have a line that reads "node/slice selection".  In the box to the right of that, you use the dropdown selector to select "filed containing node number". 

when you set the data assign toolbar to "current expression" then double click the words "current expression" the expression editor will appear.  Unless you have a stored expression it will be a default of "current new = current"  and the second current will be green.  You replace the green "current" with an expression.  to the right there is a data tree that lists all the variables (model input and outputs) that you can use to build expression including the nodal data set you added (it will have an abbreviation like "ND.1").  To compute a difference, you simply type something like "Head - ND.1".  I think there are training videos at feflow.info

sorry, not much time for creating screen captures

cheers

Even easier way, thanks to my coworker (from an e-mail from Julia Mayer):

Go to "User Data" in the data window and add a "Nodal Expression". Next, right click and go to "Edit Parameter Expression". Assuming you want the drawdown from the initial time, type "Current = Head@(0) - Head" for the drawdown.

Adam

oh, don't bother exporting data and using Surfer or ArcMap - absolutely no need for that!

If you're trying to copy heads from one time step to a reference distribution: you don't need to export and re-import the data. I usually go to a 3D window, select all nodes, then right click on head and use 'copy', go to the nodal reference distribution and click 'paste' (see attached paste_to_refDist.png). Depending on your selections, you can also paste to one slice/layer only.

The easier way is indeed to reference a head at a specific simulation time as Adam has shown: Head - Head@(time). The drawdown gets calculated "on the fly" which means you can step through the results and see the current drawdown at any selected time.

Personally, I prefer to show the zero-pressure contour in 2D cross-section view: Establish your "initial condition" (steady state) and copy the pressure (instead of the head) to a nodal reference distribution ('Steady State Pressure'). Add this reference distribution to your cross-section view as one contour of value 0.0. Then also add the zero isoline for pressure. Start your transient simulation including your pumping (stress) and watch how the current watertable (zero isloline of pressure) relates to the steady state condition. See crosssectionview.png for an example.