Human population growth has contributed to a 6 percent global drop in surface water sources such as lakes, rivers and marshes. (Credit: Flickr user bcfoto70)

When populations expand, the demand for fresh water rises. And over the past two decades, population growth has contributed to a 6 percent decline in worldwide surface water, according to a new study.

Only about one-third of one percent of all the water on Earth is usable fresh surface water, which includes lakes, rivers, and wetlands. Growing populations use these surface water sources. People drain marshes for urban development and for agriculture to support the ever-increasing population, for example.

Subtropical and tropical equatorial areas like South America and South Asia – particularly China and India – contributed to more than half of the overall decrease in land surface water over the last two decades, said Catherine Prigent of the Observatoire de Paris, in France, and lead author of the study to be published in Geophysical Research Letters, a journal of the American Geophysical Union.

Taking water coverage data from multiple satellites, including several NOAA satellites, researchers noted the change in surface water over a 15-year period from 1993 to 2007.

Prigent and her fellow researchers found that important declines are often located in regions where the population increased significantly, likely due to the draining and destruction of wetlands for development and construction. Wetlands are known to filter hazardous chemicals and contain large of amounts of greenhouse gases like methane and carbon dioxide, gasses which are released when the wetlands are drained.

Prigent mentioned there may be additional causes of the decline, but other potential causes she investigated – including precipitation, evaporation and temperature – did not show as strong a link as population density.

A lack of other studies analyzing the global change of surface water means that it is difficult to tell if this decline is normal or greater than usual, Prigent said. Nevertheless, the past two decades of data show the decline is there. Recognizing the drop is only part of the process, she said, noting that more environmentally sustainable land-use planning is necessary.

“It’s important to be aware that the decline is there,” Prigent said. “The problem is how do you avoid that decline?”

Prigent, C., Papa, F., Aires, F., Jiménez, C., Rossow, W., & Matthews, E. (2012). Changes in land surface water dynamics since the 1990s and relation to population pressure Geophysical Research Letters DOI: 10.1029/2012GL051276

-Eric Villard, AGU science writing intern