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Does Water Flow Become Unstable in All Soils?


Does Water Flow Become Unstable in All Soils?

UC Riverside research has serious implications for agricultural water management

(February 14, 2003)

Three dimensional illustration of a finger formed during redistribution in a 10 cm diameter column with transparent walls. The column was frozen after the experiment ended  to preserve the shape of the finger.  The<br />
picture was taken after the column was removed from the freezer and the loose soil that had no water in it fell out.  The brown object is, therefore, an ice sculpture with soil sticking to it.  (Photo credit: Bill Jury.)Enlarge

Three dimensional illustration of a finger formed during redistribution in a 10 cm diameter column with transparent walls. The column was frozen after the experiment ended to preserve the shape of the finger. The
picture was taken after the column was removed from the freezer and the loose soil that had no water in it fell out. The brown object is, therefore, an ice sculpture with soil sticking to it. (Photo credit: Bill Jury.)

RIVERSIDE, Calif. -- In the February issue of Vadose Zone Journal, scientists from the University of California, Riverside show that one of the most common occurrences in soil--the redistribution of water in the soil profile after irrigation or rainfall stops--will cause the infiltrating water to form narrow channels called fingers that can move much deeper than the rest of the water in the soil profile.

The researchers used observations of water movement in the laboratory and in a field experiment to help create a model that describes the final position of the fingers as a function of measurable soil retention and hydraulic functions, plus relationships describing finger size and spatial frequency.

The model assumes that all soils are unstable during redistribution, but shows that only coarse-textured soils containing a lot of sand will form fingers capable of moving more than a few inches deeper than the rest of the infiltrating water.

Laboratory experiments in a sandy soil showed that as little as 5 cm (2 inches) of water added to a dry soil would create fingers that could move more than 1 m (3.2 ft) during redistribution. In addition, the wetted pathways formed by the fingers persisted in soil for long periods of time, and were able to channel subsequent water applications as long as a month later.

"These findings help explain field observations of deep chemical movement in soils without cracks or holes that have baffled other scientists and myself for over 20 years" said William Jury, Distinguished Professor of Soil Physics in the department of environmental sciences at UC Riverside and Principal Investigator on the project.

The research discovery has serious implications for agricultural water management in coarse-textured soils. Fingering can move water and agricultural chemicals below the crop root zone, which is costly and inefficient, and can increase the possibility of ground water contamination. The researchers suggest that longer and less frequent water applications might decrease the possibility of fingering near the surface.

The research, conducted in 2000-2002, was sponsored by the US-Israel Binational Agricultural Research and Development Fund.

Vadose Zone Journal is published by the Soil Science Society of America and is an outlet for interdisciplinary research and assessment of the vadose zone, the mostly unsaturated zone between the soil surface and the permanent groundwater table.

The UC Riverside Department of Environmental Sciences offers B.S. and B.A. degrees in Environmental Sciences, and M.S. and Ph.D. degrees in Soil and Water Sciences. The department is part of the College of Natural and Agricultural Sciences. The forerunner of the department was an agricultural chemistry research unit in the world-renowned California Citrus Research Center and Agricultural Experiment Station established in Riverside in 1907.

The University of California, Riverside (www.ucr.edu) is a doctoral research university, a living laboratory for groundbreaking exploration of issues critical to Inland Southern California, the state and communities around the world. Reflecting California's diverse culture, UCR's enrollment has exceeded 21,000 students. The campus opened a medical school in 2013 and has reached the heart of the Coachella Valley by way of the UCR Palm Desert Center. The campus has an annual statewide economic impact of more than $1 billion.

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