Water

There are no clear guidelines for water consumption, therefore it tends to be a far more subjective concept to list, here.

While the NRC book, and others point out that cats can concentrate urine, and that individual cats have individual needs based upon canned or dry diets, and activity levels, there are some clear guidelines such as the following, as quoted from the UCDavis site, "Dogs and cats should always have fresh water available. Bowls should be emptied and cleaned daily. Milk should not be a substitute for water and can cause diarrhea in many animals. Lack of water or only offering stale water can lead to dehydration and diseases associated with the kidneys or urinary system. "

In the latest book, Nutrient Requirements of Dogs and Cats, on Page 249, it is stated that, "As fundamentally desert animals, cats have developed adaptations to accommodate periods of water unavailability". Later the book clarifies that, depending upon activity levels and environmental temperature, cats need 0.6 to 0.7 in relation to one gram of dry matter consumed, thereby excluding the water provided by the food, dry or canned. For cats living outdoors, the ratio was increased to 0.9:1. There are studies relating to these differences in requirements, listed at PubMed

Personally, our recommendation is to leave water out near travel paths taken by your cat in the house, as cats like to 'graze' on water as much as they do catching bugs, lizards, and whatever else happens across their path. If your cat changes habits and starts drinking from a toilet, bathtub, or sink, then possibly it is looking for fresher sources of water and needs the water being provided to be changed more frequently. We use honey buckets, (2 pound bucket) (see in the background of first photo), to keep the water at a higher level for more comfortable consumption, and the larger volume remains cool longer. Moving water from a cat fountain may also be useful, but as you can see from the 'honey bucket' photo linked above, we had one out at that point in time and it was ignored over the ability to sit and drink, rather than have to bend low to the ground, so the honey bucket won.  The last link here shows Pepper halfway down the page having a gay old time splashing the water all over the kitchen floor, so you may find you need to use those draining mats for dirty boots as protection, or something similar.

The exception to this would be if you were trying to use fluids to flush out kidneys, or reduce crystals, which is all too common a concern these days. Here, the recommendation by the NRC is that possibly using canned food will cause natural diuresis, with alternate suggestions of high protein and increased mineral content, "hence the total solute load, and subsequent need for water may be reduced", (Thrall, B.E. and L. G. Miller, 1976. Water turnover in cats fed dry rations. Fel Pract. 6:10-17)