May 2010, Vol. 22, No.5


Dry Soil Holds First Precipitation Separate From the Rest of the Water Cycle

How water moves through soil after a dry spell is fundamentally different than originally thought, according to a new study by researchers at Oregon State University (OSU; Corvallis) and the U.S. Environmental Protection Agency (EPA).

Before this study was conducted, water entering the soil as precipitation was thought to displace water previously there, pushing the existing water deeper into the soil and eventually into local waterways. But this new study suggests otherwise. It shows that soil clings tenaciously to the first precipitation after dry weather and almost never allows it to mix with other water, according to an OSU news release.

According to a report on the study published in Nature Geoscience, “Ecohydrologic Separation of Water Between Trees and Streams in a Mediterranean Climate,” water that falls from the first precipitation event on dry soil is locked into small pores, with little potential to mix with other water until transpiration empties the pores.

The watershed studied, located in the H.J. Andrews Experimental Forest in Oregon’s Cascade Mountains, contains mobile water, which eventually enters the stream, and bound water used by plants.

The study found that soil in the watershed is distributed into two groups with different pore sizes. Approximately 40% of the soil’s pores are greater than 0.3 mm, and 45% are smaller than 0.03 mm in the upper soil. Below a depth of 1 m, this division changes to 70% of pores smaller than 0.03 mm. The smaller pores contain water that is relatively immobile, compared with water in larger pores.

In the rainy season, water flows vertically through larger pores and seems to not mix with water in the smaller pores. During the dry summers, plants use the bound water for transpiration after the mobile water has drained to groundwater and streams.

“The ability of water to move through a pore increases with pore size,” said J. Renée Brooks, a research plant physiologist with EPA and courtesy faculty in the OSU Department of Forest Ecosystems and Society.

“Some of that water [that falls after a dry period] will go into larger pores and that water will move through the system, but the proportion of water from those first rains that reach the stream is much, much smaller than the proportion from rains that fall later in the year,” Brooks said.

One example cited in the report says that, following the first large storm after a dry spell, only 4% of the precipitation entering the soil entered the stream. But after soil moisture had been recharged, 55% of the precipitation made its way into the streams. Throughout the rains, almost all of the water that originally recharged the soil around plant roots remains in the soil.

The study’s findings were made possible by advances in the speed and efficiency of stable isotope analyses of water, the news release says. Scientists were able to identify water as unique, as well as tell where it came from and where it moved to. The natural isotopes of precipitation are stable and vary naturally in the environment, Brooks said.

“For precipitation, the heavier isotope prefers the liquid compared to the vapor phase; thus, heavy isotopes accumulate in the first rains, and a vapor cloud will produce lighter and lighter precipitation as it continues to rain,” Brooks said.

The study shows that scientists don’t have a complete understanding of how water moves through soils, the news release says. This also changes the understandings of how streams function and how nutrients and pollutants move through soils.


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