The new study shows that dams and irrigation considerably raise the global human consumption of freshwater by increasing evapotranspiration. This effect increases the loss of freshwater to the atmosphere and thereby reduces the water available for humans, societies and ecosystems on land.
The researchers have compiled and analyzed data from 1901 to 2008 for climate, hydrology and water use in one hundred large hydrological basins spread over the world. Their results raise the previous estimate of the global human freshwater footprint by almost 20 percent. The increase in total freshwater loss from the landscape to the atmosphere from human activities is calculated to be around 4,370 km3 per year. This corresponds to two thirds of the annual flow of the Amazon River, the world's largest river by discharge.