@noffle points out that toilets take an amount of water which is an order of magnitude more than the amount used for hydration Source. For example, five daily flushes is 42 days of drinking water.
This is an interesting measurement if you are looking at personal water consumption, but it is not a useful metric for curtailing water usage. It is not useful because it lacks context. I could accurately state that Lake Baikal has 5,670 cubic miles of water, or 22% of the surface fresh water on Earth, and therefore it should be drained and Russia should stop hogging water, but it wouldn't make sense to do so. Likewise, stating that 5 gallons of water is a lot of water doesn't say anything about the system in which that variable may be meaningful.
Tangentially, putting a number on something outside of its context activates internal biases humans have, which result in the impression that large numbers are impressive. You can easily see this by suggesting that I drink 2% of all of the water I actually use, which is small, and therefore I don't need to worry about drinking water, because it's so little. In reality, this could mean I should actually waste less water elsewhere in my life. Framing numbers is important.
A better way of understanding personal water usage is to ask these questions:
- What is the total amount of water in my direct environment?
- How much water is wasted, and how much is recycled?
- What is the cost for the water, and is this cost appropriate given the longevity of the water system?
- What is the source of the water, and is it sustainable?
- How much water do other activities I partake in take? For instance: food, washing, electronics, restaurants, civil services, and so on?
- Is the dollar the most useful metric for the total amount of water used and in the system, and does it reflect my load on the system?
- Is my water subsidized?
- Is personal, consumer curtailment the best way to ensure sustainable water usage?
- Are other activities more relevant: such as eating food which does not take the same amount of irrigation (aka, not almonds)?
- Am I better off dedicating my personal thought-hours elsewhere than my personal consumption: is there a charity, NGO, or co-operative where I can most effectively partition my time to help fight water exploitation? (Phrased another way: am I better off digging an outhouse to stop water consumption, or would I be better off coding a map of water usage that goes viral, promoting knowledgeable water usage?)
The US is experiencing, and will continue to experience, a water shortage that seems to correlate more with human interference than a simple drought can explain. In some areas, the aquifer is being emptied at an unsustainable rate, and there are long term fears about the long term supply of fresh water.
In other areas, this may be less meaningful. For instance, Montréal appears to have a constant influx of water from the Great Lakes, which it manages at two main water treatment plants in the city. Their website is, unfortunately for my research, in French. The main inlet for the Atwater plant can bring in excess of 2,160,000m3 a day 1. 3% of the world's fresh water is in Quebec, most of it in the St. Lawrence 2. Treating this water to make it safe to drink is costly. I don't know how this cost is apportioned, or whether my water bill covers all of it.
In California, things are obviously more dire. California depends on the snow-pack 1, and drills into the aquifer when it is too small. Unfortunately, the aquifer is a finite resource. The Central Valley used to be almost entirely marshland - now, most of it is farms, which take up most of the water.
Ideally, water usage is kept minimal, both personally and by businesses, farming, and other large scale uses. However, it is hard to correctly advise exactly how much water ought to be used, and to appropriately apply a price-tag to water usage. Most water humans use needs to be filtered, pumped, and otherwise shipped around in various states: this is expensive. But the tag doesn't reflect the sustainability of the system, but rather current demand and expected short-term trends. This can conflict with advice to consumers - don't flush unless necessary, don't keep the tap on while brushing your teeth, etc - which, in general, suggests cultural microchanges which may be ineffectual overall compared to curtailing business usage and eating sustainable foods.
It is very hard to go beyond this summary, due to conflicting measurements of water, and difficulty finding relevant details regarding how much water is available in the system. However, in order to have a reasoned discussion about water usage, we may need to do so.