In my personal experience, blue does not thin due to low water, but it
just goes dormant. As soon as it cools off and moisture returns, it greens
up and is again thick. I my experience other factors cause it to thin,
like too little sun, bad drainage and other soil problems.

My lawn only has a thin layer of top soil. This may explain the reason
why some area of my lawn is thinning out. I am under the impression
that the thin top soil doesn't hold water well, and the bluegrass
consequently doesn't get enough water. Therefore, I regard my
grass-is-thinning problem as a lack-of-water problem instead of
lack-of-good-top-soil problem. I don't know...

I doubt this has to do with lack-of-sun-light. The areas that are
thinning tend to correlate with having a lot of sun exposure, instead
of lack of sun exposure. I am working on the assumption that
plenty-of-sun-exposure combining with lack of watering may be the
cause of thinning-lawn. But I can be wrong; the reason is that there
are also other factors that may be causing the problem (such as soil
compaction caused by heavy snow piled up along driveway and walkway).

If what you said is correct, I should overseed with bluegrass for now.
If the local government impose watering-ban, I will just have to let
the bluegrass to go dormant, and wait for it to return when they lift
the watering-ban. This sounds like a plan to me. Thanks.

Jay Chan