Using NASA's fleet of Earth-observing satellites, scientists have provided estimates for the global water cycle budget for the first decade of the 21st century, taking the pulse of the planet and setting a baseline for future comparisons.
The water cycle describes how water evaporates from the Earth's surface, rises into the atmosphere, cools, condenses to form clouds and falls again to the surface as precipitation.
About 75 percent of the energy (or heat) in the global atmosphere is transferred through the evaporation of water from the Earth's surface.
"To document change, you need to understand what the current state is. If we have a good, sound understanding and quantification of the current state, then we can tease out changes in the future," said lead study author Matt Rodell, hydrologist at NASA's Goddard Space Flight Centre in Greenbelt, Maryland.
The results, published in the Journal of Climate, show that each year heat from the Sun evaporates 449,500 cubic km of water from the world's oceans.
On land, 70,600 cubic km of water evaporates from soil and plants.
The moisture collects as water vapour in the atmosphere, and winds blow it to other parts of the world where it condenses into clouds, rainfall and snowfall.
"The water cycle includes freshwater used in households and for agriculture, so any changes to the patterns of where rain and snow occur due to the changing climate may have huge impacts for communities worldwide," the authors noted.
The scientists also calculated that 403,500 cubic km of precipitation fall over the ocean each year, an estimate about five percent higher than the previous standard estimate.
Nearly 116,500 cubic km of precipitation fall over land. Of the precipitation over land, 45,900 cubic km runs through streams and rivers into the oceans and 70,600 cubic km evaporates into the atmosphere.
Each year, all humans on Earth collectively consume for agriculture, industry and water supply 9,100 cubic km, less than 8 percent of total land precipitation.
In addition to the global and annual numbers, the researchers calculated water cycle estimates for each of seven land masses and nine ocean basins, as well as provided monthly estimates for the globe and each region.
"These data for the current state of the water cycle will be used to improve how climate models predict the distribution and intensity of rain and snowfall events," said co-author Tristan L'Ecuyer, atmospheric science professor at the University of Wisconsin at Madison.
The study is a rigorous accounting of the movements of Earth's water from 2000 to 2010 and the first to rely solely on satellite observations and data-integrating models.
