My brother-in-law lives in Lawrenceville, so I'm quite aware that the drought there is over! Not knowing just how your system is designed, it's be bit hard to speculate on just where or what the problem is. Typically, there should be a master shutoff valve going from your water supply line to the back flow preventer. This valve should be off in the wintertime. If that valve does not shut off completely, then there will be water in your irrigation side. Did you have the lines blown out for winter? If not, you really should before it freezes hard. When that is done, the shutoff valve should be checked and replaced if necessary. Bottom line is, there should be zero water in the system in the winter time at least where freezing is a possibility.