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DIVISION S-6-SOIL & WATER MANAGEMENT & CONSERVATION

Mineralogy and Water Quality Parameters in Rill Erosion of Clay–Sand Mixtures

^a Usda-Ars, Weslaco, TX 78596 USA
^b Dep. of Soil and Atmospheric Sciences, Univ. of Missouri-Columbia, Columbia, MO 65211 USA

bradford{at}pop.tamu.edu

Accuracy of the prediction of rill erodibility can be improved, in part, through an increased understanding of soil variables controlling particle detachment. To test the hypothesis that rill erodibility of clay–sand mixtures increases (i) as the dominant clay mineral changes from montmorillonite to illite to kaolinite, (ii) as total salt in the eroding fluid decreases, and (iii) when Ca is replaced by Na, concentrated flow erosion tests were conducted by mini-flume test procedures. Chelsea fine sand was mixed with 2 to 20% (w/w) Ca or Na forms of either kaolinite, illite, or montmorillonite, and water to a moisture potential of -490 Pa. Rill erodibility of Ca–montmorillonite–sand mixtures was 4 to 40 times less than for Ca–illite–sand and Ca–kaolinite–sand mixtures. Na–montmorillonite–sand mixtures were one-half as erodible as Ca–montmorillonite, while Na–kaolinite and Na–illite were 3 to 25 times as erodible as their Ca forms. Decreasing salt concentration of the eroding fluid from 0.01 to 0.001 M in all cases decreased the critical shear stress value on average 0.8 Pa. Decreasing the salt concentration from 0.1 to 0.001 M had no effect on the erodibility of Ca–kaolinite and Na–montmorillonite, but increased the erodibility of Ca–illite and Ca–montmorillonite. The erodibility of clay–soil mixtures decreased from 9.6 to 0.04 ms m^-1 as the water holding capacity of the clay in the mixture increased from 0.2 to 6.9 g g^-1 and appears to be related to the effective surface area of the clay.

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