Water Consumption

Perhaps I am being an over-fretting mother, but I've noticed that my cats are consuming large quantities of water lately...to the point that I'm re-filling their bowl on a daily basis, not simply changing the water.  They all are behaving within the normal range of activity, all of them are still playful with their human house-mates and each other, though the oldest, Blue, we suspect has begun to have some minor joint pain.

Should I be worried about this?  (Probably not.)  But I thought I'd throw it out here anyway.

Factors:
1.)  I live in San Antonio, and we are in the heat of the summer.  Evaporation?
2.)  The three cats involved:
        - Blue, age 10, blue tabby/korat. (male neuter, healthy weight)
        - Killit, age 6, ginger tabby. (male neuter, chunky)
        - Inu, age 2, black/white tabby. (female neuter, healthy weight)

[Soror]

Photos:  http://pics.livejournal.com/soror/gallery/00004q7d

[Avas Mom]

i would say if its hot its not big deal, especially since all 3 are doing it and not just one i wouldnt worry