Basic MODFLOW question

[bjacoby]

What is the difference/advantage/disadvantage to using the River Package verses a Changing Head Boundary Condition? I am having trouble understanding the function of each.

Any comments are appreciated!

Thanks!

[Alan Lemon]

Changing head boundary condition fixes the head in the cell where it is applied (to the value that you specify).

The river boundary condition is a head dependent boundary condition meaning that there can be a difference in head in the river bc and the head in the aquifer. Additionally exchange of water between the river boundary condition and the aquifer is impeded by the value of conductance. If the conductance value is really large then the river can act like a specified head boundary because there is nothing to impede the flow between the aquifer and the boundary condition.

Look at the MODFLOW 88 documentation on the RIV package and read through it for more explanation.

http://gmsdocs.aquaveo.com/modflow88I.pdf

[bjacoby]

Thanks Alan.

Here: http://water.usgs.gov/nrp/gwsoftware/modflow2000/MFDOC/index.html?chd.htm It talks about the CHB being time-variant. How can that be if the head is specified?

[Alan Lemon]

It is specified for each stress period. So the head in the cell can change but any change will be specified by the modeler.

[bjacoby]

thanks!