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United States Department of Agriculture

Agricultural Research Service

Title: Salt Transport in Cracking Soils: Bromide Tracer Study

Authors
item Shouse, Peter
item Letey, J. - UNIV. CA. RIVERSIDE, CA
item Jobes, Jack
item Fargerlund, Joan
item Burch, S. - IMPL. WTR. DIST. IMPL.,CA
item Oster, J. - UNIV. CA. RIVERSIDE, CA
item Rhoades, James
item O'Halloran, T. - IMPL. WTR. DIST. IMPL.,CA

Submitted to: American Society of Civil Engineers
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 2, 1997
Publication Date: N/A

Interpretive Summary: Cracking clay field soils pose unique and complex water and salt management problems. Cracks can be extremely important conduits for water and salt movement in large agricultural fields, but little is known about the effect water flow in cracks has on the water quality of surface runoff water during irrigation. We measured the changes in surface water quality during girrigation using non-toxic tracer chemical salts and food coloring. We found that significant amounts of salt movement occurred in the surface runoff from irrigated cracking soils. This salt movement may be an important mechanism for leaching in soils which produce large cracks between irrigations. Traditional concepts of water and salt movement do not apply to cracking soils. Other theories must be investigated.

Technical Abstract: Cracking clay soils pose unique and complex water and salt management problems. Crack volumes can be extremely important in determining water and salt movement in these soils. The purpose of our study was to determine the effect of crack flow on the water quality of the tail water, and to determine if crack flow contributed to the leaching process. Our experiment was conducted on two farmers fields near Brawley, California, i the Imperial Valley. One field contained a non-swelling sandy loam soil and the other field contained a high shrink-swell clay soil. Both fields were planted to alfalfa on shallow beds. Soil samples were taken prior to bromide treatment, after treatment, and after several subsequent irrigations. Three experiments were conducted on each soil: 1) initial bromide spike injection, 2) mid-stream bromide spike injection, and 3) infiltration box bromide injection. Surface water was sampled during bromide treatment and during subsequent irrigations. On the sandy loam, non-cracking soil the water and salt moved to a depth of 60 to 75 cm. During subsequent irrigations salt was leached to deeper depths and surface waters had only small amounts of bromide. In the clay soil, water and salt rapidly filled and flowed in the soil cracks. Significant quantities of bromide were found at the 90 to 120 cm depth. Also, surface waters were laden with bromide during subsequent irrigations, and bromide was transported laterally down furrow by irrigation water. Traditional water and salt transport concepts for porous media are not applicable to cracking clay soils.

Last Modified: 7/30/2014
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