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CORVALLIS, Ore. - A global analysis of how water moves through the ground and is taken up by plants may overturn the way scientists understand a key part of the hydrologic cycle.

It has been assumed for more than a century that once water enters the ground, it becomes part of a well-mixed pool. From there, the theory goes, water flows into groundwater below, remains trapped in soil particles, or is withdrawn from the soil and sent back into the air by plants.

However, by analyzing the chemical signatures of water at 47 sites on six continents, researchers have discovered that the notion of a well-mixed pool in the ground is wrong. In fact, they report in a letter in this week's edition of the journal Nature, water in plants comes from a compartment in the soil that is separate and disconnected from water that flows elsewhere.

"This is a new interpretation of the hydrologic cycle," said Jeff McDonnell, co-author and a courtesy professor in the College of Forestry at Oregon State University. McDonnell is the former Richardson Chair in Watershed Science at Oregon State and a professor and associate director of the Global Institute for Water Security at the University of Saskatchewan.

The findings are based on analyses of chemical isotopes -- different versions of an element -- of hydrogen and oxygen at locations representing tropical and temperate environments including forests, grasslands and deserts. The work builds on previous research that McDonnell published with colleagues at Oregon State and at the U.S. Environmental Protection Agency (led by Renee Brooks) in Corvallis.

If they are confirmed, the findings would require revision of computer models that are used for irrigation, in-stream flows and climate analysis as well as for other purposes, said lead author Jaivime Evaristo, a Ph.D. student working with McDonnell at Saskatchewan. "All existing models of water flows -- many, if not most of which, are being used in a wide range of water resource management purposes -- are predicated on the assumption that the waters underneath our feet are well mixed, as though they are in one, huge tank," said Evaristo. 

It is not yet known what the implications will be for water management practices in forested watersheds or on farms. The work suggests that trees do not use water that would otherwise make it to streams that serve towns and cities. In addition, knowing that plants have a preference for taking water from some parts of the hydrologic cycle and not others may affect the way fertilizers are applied to farmland.

"Fast flowing water and all that is dissolved in it will eventually recharge the ground and make its way into the streams. Nutrients (from fertilizers) will only be useful for plants if they are retained by the soil," said Evaristo. "Down the line, this new knowledge will translate into redefining how we view and model water flows for practical purposes."