Most basic assumptions about soil hydrology may have to be reconsidered

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Washington, January 22 (ANI): In a new study, scientists have discovered that some of the most fundamental assumptions about how water moves through soil in a seasonally dry climate are incorrect - and that a century of research based on those assumptions will have to be reconsidered.

The study, by scientists from Oregon State University (OSU) and the Environmental Protection Agency, showed that soil clings tenaciously to the first precipitation after a dry summer, and holds it so tightly that it almost never mixes with other water.

According to the researchers, the finding could affect our understanding of how pollutants move through soils, how nutrients get transported from soils to streams, how streams function and even how vegetation might respond to climate change.

"We used to believe that when new precipitation entered the soil, it mixed well with other water and eventually moved to streams. We just found out that isn't true," said Jeff McDonnell, an OSU distinguished professor and holder of the Richardson Chair in Watershed Science in the OSU College of Forestry.

"This could have enormous implications for our understanding of watershed function. It challenges about 100 years of conventional thinking," he added.

What actually happens, the study showed, is that the small pores around plant roots fill with water that gets held there until it's eventually used up in plant transpiration back to the atmosphere.

Then, new water becomes available with the return of fall rains, replenishes these small localized reservoirs near the plants and repeats the process.

But, all the other water moving through larger pores is essentially separate and almost never intermingles with that used by plants during the dry summer.

The study found in one test, for instance, that after the first large rainstorm in October, only 4 percent of the precipitation entering the soil ended up in the stream. 6 percent was taken up and held tightly by soil around plants to recharge soil moisture.

A month later when soil moisture was fully recharged, 55 percent of precipitation went directly into streams.

As winter rains continue to pour moisture into the ground, almost all of the water that originally recharged the soil around plants remains held tightly in the soil - it never moves or mixes.

"This tells us that we have a less complete understanding of how water moves through soils, and is affected by them, than we thought we did," said Renee Brooks, a research plant physiologist with the EPA and courtesy faculty in the OSU Department of Forest Ecosystems and Society. (ANI)

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