I have two 12 or 13 year old female litermates.  They are strictly indoor
cats and have only escaped once several years ago.

For about the last two months, these cats have increased their water intake
substantially, and subsequently their urine output has increased
accordingly.  The mystery is that the volume of their feces has declined
substantially and is very dry.  We feed them wet food, and used to
supliment that with dry treats daily.  We have cut out the cry treats, but
their turds are still dry.

I service their boxes daily to remove the urine soaked liter and the few
dry turds that are in the boxes.

Whatever the problem is has affected both cats.

Other than this one problem, they appear to be in perfect health.

Any suggestions.

Thanks