Image copyright Fernando Jaramillo Image caption The Mekong is one of the rivers showing the highest increases in evapotranspiration

New calculations show that our already sizeable water footprint is 18% bigger than we thought.

The study is based on a century's worth of observational data drawn from 100 river basins across the world.

It reveals a significant increase in the water being "lost" to the atmosphere as a direct result of human activity.

This occurs through evaporation from land and water surfaces, and from plants as they transpire.

Based on data

The old estimates of human water consumption add up to a staggering number: 9,100 cubic km per year. This volume is about twice the size of Lake Michigan in the US.

Fernando Jaramillo and Georgia Destouni from Stockholm University in Sweden have now used freely available data to update these assessments, to reveal a more precise picture of how much humans are impacting the natural water cycle at the global scale.

Their research suggests human water management strategies are responsible for an additional quantity of water the size of Lake Ontario being transferred to the atmosphere - a significant element of consumption that the earlier approaches had underestimated. It means the human water footprint is actually closer to 10,700 cu km a year.

"Previous assessments of human water consumption were mainly based on national statistics and global scale models. The novelty of this study is that it's based on data," Dr Jaramillo emphasized.

The team has published its work in Science magazine. A presentation was also made this week to the Fall Meeting of the American Geophysical Union - the world's largest annual gathering of Earth scientists.

Evaporation and transpiration

Image copyright Fernando Jaramillo Image caption The irrigation techniques used in agriculture drive more water into the atmosphere

The authors explained that water consumption by humans goes beyond drinking water, industrial water, and water for municipalities.

Water that would normally be in rivers but ends up being in the atmosphere as vapour due to human activity has been largely overlooked, and this is what the authors focused on.

"The loss is from the landscape through the atmosphere and that leaves less water for both humans and other ecosystems in the landscape", Prof Destouni explained.

This displacement of water into the atmosphere occurs through water evaporation or through transpiration by plants. So the total flow of water to the atmosphere is given by evapotranspiration.

"We used a simple water budget: water lost by evapotranspiration is precipitation minus water runoff," Dr Jaramillo answered when asked how they used the data.

The footprint

Looking at their data, the scientists could see an increase in evapotranspiration when comparing the first and second halves of the last century.

The question they then asked was how much of this change was being driven by human activity.

They found that areas previously identified as having experienced the ecological consequences of dams and irrigation also showed the biggest increase in evapotranspiration.

"Dam and irrigation developments - even though local - have a big global impact on human water consumption. That's what has not been calculated before and what we've estimated in this paper," Prof Destouni said.

"The water footprint could be up to 20% larger than previously estimated," Dr Jaramillo revealed.

"In dry areas, reducing the water in the environment can have an enormous impact on humans and ecosystems. In a wet landscape, it is in relative terms not as big in the direr areas. Central Asia (Aral Sea), Middle East, areas around the Mediterranean - these are examples of most vulnerable."

Irrigation, dams, and ground water

Image copyright Fernando Jaramillo Image caption The large water surface area created by a dam causes more evaporation

Dr Jaramillo explained how human water management leads to more evapotranspiration.

"With irrigation, we are moving water from one place to another - and part of this water will simply evaporate from the ground surface. These crops in order to grow, they take this water in and by respiration they send this water to the atmosphere.

"When you build a dam and make a reservoir, you're also consuming a lot of water. The water surface you create causes more evaporation; otherwise that water would be running on a river.

"Also if you increase the level of the water by say 90m, the groundwater on the hillsides will also increase by 90m. A lot of plants now have access to water that previously wasn't there. Those plants will also transpire more water."

Prof Destouni explained how evapotranspiration induced by human activity is linked to climate change and weather: "The atmosphere is warmer due to global warming and warmer air can hold more water vapour."

Transport of moisture in the atmosphere is also changed: "There is more vapour coming from the land rather than the sea. You get the effect that it rains more in other places.

"If we don't understand how transfer of water and water vapour happens, we will misinterpret things and then our measures for mitigating climate change or adapting to climate change will not be effective."

Uncertainty lingers

"In some regions, we just open the tap and water comes out. We don't think about it. How much water are we consuming?" Dr Jaramillo said, highlighting the worldwide uncertainty regarding the numbers on human water consumption.

"Our lack of knowledge is gigantic. We're lagging so much on studies on the water cycle and water consumption.

"It is serious that at this point in time people did not use all the data that was available to actually calculate this change. People continued to make estimates using old data," Prof Destouni commented.

The Stockholm University scientist emphasized the importance of using this openly accessible data to "continue to measure these kinds of things so that we can actually see these changes happening".