Science Questions with Surprising Answers
Answers provided by
Dr. Christopher S. Baird

Does wasting household water remove it from the water cycle?

Category: Earth Science      Published: September 23, 2014

washing hands
Public Domain Image, source: Christopher S. Baird.

No, wasting household water does not ultimately remove that water from the global water cycle, but it does remove it from the portion of the water cycle that is readily accessible and usable by humans. Also, "wasting" water wastes the energy and resources that were used to process and deliver the water.

Parents, city officials, and environmentalists seem to be often telling us, "Don't waste water!" and "Conserve water!". By these statements they typically mean that we shouldn't leave the faucet on needlessly, should fix all the plumbing leaks right away, and should only use water sparingly. At the same time, our science teachers tell us that all of the water on the earth is part of an ongoing global water cycle. They tell us that water that goes down the drain does not get destroyed. Rather, it just moves on to the ocean and on to the next part of the water cycle. There seems to be a contradiction between these two positions. How can we "waste" water, if it just continues on in the water cycle and inevitably makes its way back for us to use again?

The answer is that humans need accessible fresh liquid water to survive, which is only a small subsection of the overall water cycle. When city officials tell you to not waste water, they mean that you should not needlessly divert water from the human-usable part of the water cycle to the non-human-usable part of the water cycle. According to the book "Water in Crisis" edited by Peter H. Gleick, only 0.77% of the water on the earth is in the form of fresh, liquid water in and on the ground. The rest is in salty oceans and lakes, locked up as ice, or in the atmosphere. Letting water run down the sink while you grab a snack typically sends that water right out of the part of the water cycle that is accessible and usable by humans. With this in mind, wasting water does not directly hurt the global environment, since the water is not destroyed. Rather, wasting water  hurts humans, as it leaves them with less accessible, usable water. Additionally, wasting too much water can hurt the local environment as it drains too much usable water away from the natural ecosystem.

Wasting water does not always hurt humans or the environment. In areas where there are few humans but large amounts of usable water, you can't really "waste" water because new water arrives into the human-accessible portion of the water cycle faster than you can send it back out. For instance, consider a remote cabin that is situated in a temperate mountainside forest and that draws its water directly from the ground using a private well. People in such a cabin can't really waste water. They can leave their faucets on all day if they want and they won't be affecting the environment or other humans much. They may not be wasting the water, but they are wasting the electricity that is used to pump the water up from the ground.

In contrast, in areas that have a lot of humans (e.g. cities and suburbs) or very little usable water (e.g. deserts), wasting water can impact the environment and the humans. In regions of the world that are both densely populated and arid, wise use of available water resources is critical to human survival. The identification of what exactly constitutes the wise use of available water resources is really more of a political issue and a personal moral issue than a scientific one, and therefore is best left to the policy makers and to personal opinion. Let us instead focus here on the science.

What is the human-usable portion of the water cycle? The answer is: fresh liquid water on or near the surface of the earth. Humans cannot survive drinking salt water, so pumping untreated water directly in from the oceans and from salt lakes is not an option. It's true that salty ocean water can be converted to fresh water through distillation or reverse osmosis, but these processes are relatively costly so that they are not economically feasible in most parts of the world. For most regions, the ocean is simply not a viable source of human-usable water. Similarly, water in the atmosphere is not a reasonable source of water for human consumption. While the water in the air and in the clouds is fresh water, it is widely dispersed and is hard to collect. Humans must therefore wait for the water in the oceans to evaporate into the atmosphere and then rain or snow back down to the ground. The places near or on the surface where rainwater and snowmelt collect are the places that constitute the human-usable portion of the water cycle. These places include freshwater rivers, lakes, ponds, reservoirs, and groundwater.

Note that there are two parts that affect how much usable water can reach human households: 1. the amount of human-usable water available in the local environment, and 2. the capacity of the human-built infrastructure to process and deliver the water. Even though fresh water may be plentiful in a certain region, it may still need to be cleaned of dirt, microbes, and minerals; and then pumped through pipes to the buildings. For this reason, an area may have abundant water resources but may still have to be careful about conserving water if it has inadequate infrastructure. For instance, a small town on a large river may provide water to all the residents of the town by treating and pumping water from the river. Although such a small town may never be able to deplete the river's water, no matter how much water the townspeople waste, the water use rate of the residents may exceed the rate at which the town's system can clean and deliver the water. In such a case, the residents will still have to be careful not to waste water, even though there is plenty of water in the river. In this way, natural human-usable water resources and human-built infrastructure determine whether it is possible for residents to "waste" water (i.e. whether it is possible to divert water too quickly to the non-human-usable portion of the water cycle). Even if there is plenty of water in the local system so that "wasting" water itself is not really possible, over-use of water can still be a waste of the energy and resources that was used to process and deliver the water.

Topics: environment, freshwater, groundwater, rain, water, water cycle