So one of our readers asked us this question the other day: How does using water irresponsibly remove it from the water cycle? I keep hearing about how we are wasting water and that it is a limited recourse. How is it possible, given the water cycle will reuse any water we use?


Technically, it will not. What it might do, however, is limit the amount of water within the tiny subspace of the water cycle which we (and the rest of the wildlife within our part of the ecosystem) can use directly, which is surface fresh water.

The key thing to get here is that water transits from one reservoir to the next at limited rates, and that if your water consumption exceeds the rate at which fresh drinkable water can reach you, you are in trouble.

The easiest way to increase the general water supply might be to use the water we’ve taken from it more efficiently. For everyone watering their lawn with gray water (or better yet not having a lawn), there’s more water available for other uses. Lawns are I think America’s biggest crop by some measures, so that’s nontrivial. Also, rainwater harvesting at the personal level will help to alleviate the strain on the water table, but several states in the US have laws actually prohibiting rainwater harvesting for personal use. The idea being that the water was going to end up in a river and used by someone else, and you are diverting it for your own use, basically stealing water. There was one study that led to a change in the Colorado law that shows that most rainwater doesn’t make it to streams and rivers, but is lost to evaporation.

Also, keep in mind that aside from being costly, many solutions are also energy-intensive. In California, about 20% of California’s energy use is dedicated to water transportation and treatment. If arid states were to turn to increased desalination or shipping in more water, the energy consumption would only go up, raising concerns over energy sustainability.

Another issue to consider is that a lot of water has been stored in “fossil” aquifers for a very long period of time. These aquifers are not currently being recharged to any significant degree in modern-day climatic conditions. They are preserved relics of wetter times. If you draw water from these sources it’s effectively non-renewable on human timescales. There are a disturbing number of desert areas with these types of aquifers. Once they are depleted, that’s it for a few centuries at least, maybe longer.

So Strictly speaking, you’re wasting (lack of) entropy. There is tons of water around, but most of it is mixed with other things. This is almost exactly the same situation as recycling aluminum – aluminum is one of the most common elements on earth but it’s incredibly energy intensive to purify it, so recycling actually saves energy and not elemental aluminum.

So the answer is: “Using” water mixes it together with other things, turning “fresh” water into contaminated water. We can make fresh water out of impure, to supplement what we get from nature, but it takes a lot of energy to reduce the local entropy in this way.

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Last Update: October 6, 2016

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