Runoff and Groundwater

I'm tinkering with the groundwater routines in SWMM, and have run a question or two:

The general form of the groundwater equation is:

GWFLW=A1(D1-BC)^B1 - A2(TW-BC)^B2 + A3(D1*TW)

where this represents lateral flow from the saturated zone to the receiving water (selected portions copied directly from text). I think I understand all of the components except for the last one. Can someone explain a little about where this term comes from, or what it physically represents? I'm sure that I'm making the assumption that the groundwater routine is a little simpler than it actually is, and this is throwing me off, however i have an instance where my initial parameter sets chosen allow this third term to control my outflow depth in the model.

thanks,
Wes Byne
fbyne@engr.uga.edu

Wes,

We developed a small model for a green roof and simulated water moving through the substrate and then filling up and lifting the groundwater level till it reached an outflow pipe. We found it was a very "delicate" operation and in some instances the model didn't close the water balance very well. We found it was very easy for more water to leave the system than enter it.

Scott

ps I'm sorry don't have a good answer for your question.

Ok,
After actually READING the SWMM Q&A:

http://www.chi.on.ca/swmmqa293.html

I think I've got it. Sorry for posting before searching.

-Wes