Homeowners are used to a simple equation: When the value of their homes increase, their taxes rise.

But now, in a declining real estate market, homeowners are learning a sad truth: When the value of their homes falls, their taxes still increase.

That's because no matter how lean times get for ordinary citizens, local government continues to live off the fat of the land.

There are contracted raises to hand out — usually couched as "fixed costs" — to town employees even as the taxpayers who must support those increases may not have seen raises themselves in years. There are "vital services" to maintain, even as residents pare back their own living expenses to the bare minimum.

And as long as municipal government retains the power to levy taxes, you can be sure there will always be enough money to keep the gravy train running.

So as residents of Methuen have seen the value of the average home fall from $295,720 to $283,070 in the latest revaluation, they can expect their tax bill to increase. Taxes due on a home are computed by multiplying the tax rate — currently $10.25 per $1,000 of valuation — times the value of the home. So as values fall, expect Methuen's City Council to adjust the tax rate so that the city's revenue take does not decline.

The same thing is happening in communities across the Merrimack Valley. Tax bills everywhere are going up, even as property values fall. In Andover, the average homeowner can expect to pay $255 more as the tax bill on the average home will be $7,054. In North Andover, the average homeowner will see a $48 increase, a figure the selectmen kept low by shifting more of the tax burden to businesses.

Yes, times are tough all over — except in the fantasy world of municipal government where, come good times or bad, the tax take always goes up.