The local environmental club keeps urging me to stop “wasting water”—to take shorter showers, for instance, and to water my lawn less often. But how is it possible to waste water when it’s constantly being recycled through evaporation and rain?

It’s true: Thanks to the hydrologic cycle, we drink and bathe in the same H 2 O that rained on the dinosaurs. And, theoretically, at least, the Earth has more than enough for all of us: According to Brian Richter, co-director of the Nature Conservancy’s Global Freshwater Program, human activities—agriculture, manufacturing, bathing, drinking, and so on—consume only about 10 percent of the planet’s available freshwater supply.

Water shortages are really a problem of distribution. We may have enough freshwater on Earth to meet the global population’s current needs, but we can’t always make it available where it’s needed, when it’s needed, and in the quality in which it’s needed.

You can think of a community’s water supply as a bank balance: If the community takes out more than can be returned in a timely fashion, it may reach a point at which it doesn’t have enough water to grow crops, wash clothes, or flush toilets. Communities withdraw water from local surface waters (such as rivers, lakes, or reservoirs), groundwater aquifers, or both. Those sources do eventually get replenished by precipitation, but that can be a very long, slow process—with groundwater, for example, it can take hundreds or even thousands of years. Storing the water is a challenge as well: According to a 2003 report from the Government Accountability Office, the holding capacity of the United States’ existing reservoirs may be declining due to aging dams and increased sedimentation. Constructing new dams is not only expensive but can also damage aquatic ecosystems.

When considering a region’s water balance, remember that not all water uses are created equal. When the Lantern was growing up in California in the late ‘80s, the brochures she brought home from school assemblies listed all kinds of no-no’s—running the tap while you brushed your teeth, watering your lawn at high noon, not letting the yellow mellow in your toilet. But depending on where your community sends its wastewater, some of those uses may be more damaging than others.

Let’s say your city takes water from a nearby river and then returns its treated wastewater to the same source. (This is usually how it works in cities that withdraw from surface waters.) In that case, the water that goes down your sinks, toilets, and tubs stays in the local system; it quickly gets recycled, becoming available for reuse in the same watershed. *

On the other hand, water sprayed on a lawn will ultimately evaporate or transpire—it’s essentially lost to the community, making it what’s known as a consumptive use. (That category also includes most water consumed by humans, animals, and plants, or incorporated into products.) That water may return from the atmosphere as rain, but if you live in an area that doesn’t get a lot of precipitation, then you can’t exactly count on receiving a timely, balance-restoring deposit. So when it comes to conserving water, your first priority should be cutting down on excess consumptive use, like lawn care or car washing.

If you live in a city that pumps most of its water out of the ground, however, the distinction between consumptive and nonconsumptive uses may be moot. Though some utilities make an effort to pump treated wastewater back into the source aquifer, most discharge it into a stream or river that eventually flows out to the ocean—meaning water that spirals down your drain doesn’t get returned to the city’s account. So in those areas, epic showers are just as sinful as profligate lawn spraying.

Energy costs further distort the image of water as a renewable resource. For every gallon of tap water you use, your utility company has to extract it, clean it, pump it to your house, pump it back out, reclean it, and eventually discharge it.

National numbers haven’t yet been compiled, but supplying a Northern Californian with potable tap water and then treating that water after it spirals down the drain requires about 0.4 kilowatt-hours of electricity per day. (The Lantern based this back-of-the-envelope calculation on a 2006 report from California estimating the lifecycle energy costs of water and a 1999 study suggesting that an individual uses about 70 gallons of water indoors daily.) That’s almost as much energy as it takes to run a 60-watt lightbulb for 7 hours, but it doesn’t include heating the water, which takes even more energy than the treatment processes combined. So to conserve energy along with your water, keep it cool.

Is there an environmental quandary that’s been keeping you up at night? Send it to ask.the.lantern@gmail.com, and check this space every Tuesday.

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Correction, Oct. 19, 2010: This sentence originally implied that when water sourced from a river flows down household drains, it is reused in the same water supply system. The water does stay in the same watershed, but is usually reused downstream by other cities and farms. (Return to the corrected sentence.)