Re: Soggy backyard
Do you have a septic system or is it city sewer? If the septic tank is full and you are using alot of water, it may not have time to recover.
But, since when you turn on the sprinkler system there is no loss of pressure I don't think it is that. Also, even if one of the sprinkler lines were broke you wouldn't see it until the sprinkler system came on.
Turn the system off for a week and see if the water goes away. Also, find where your water meter is located at and where it goes into you house. See if the meter is turning (using water) and maybe that could be it.
Process of elimination. Good luck.