An Experimental Study on Sodicity Effects on Rill Detachment Capacity

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ABSTRACT Particle detachment is a key sub‐process in soil erosion, and its rate can increase in croplands irrigated with brackish or saline water. Despite this adverse effect on soil, no studies have explored the impact of Sodium Adsorption Ratio (SAR, a key parameter measuring sodium excess among the exchangeable cations in soil) on rill erosion, the most common form in steep and long hillslopes. To fill this gap, this study has evaluated the changes in rill detachment capacity (hereafter ‘D c ’), rill erodibility (K r ) and critical shear stress (τ c ) of soils sampled in Northern Iran at five values of SAR compared to the soil with a very low sodium concentration. The simulations of the rill detachment process in flume experiments have shown that: (i) a SAR of 7.13 mmol/L 1/2 is the threshold resulting in a significant increase in D c ; (ii) over this limit, D c increases by percentages between 150% (SAR = 14.3 mmol/L 1/2 ) and 300% (SAR = 27.2 mmol/L 1/2 ); (iii) D c is not correlated to soil aggregate stability, and its increase may be explained by the decrease in van der Waals' forces binding together the soil particles; (iv) also K r noticeably increases with SAR over 7.13 mmol/L 1/2 until a maximum value measured at the highest SAR values of this study; (v) in contrast, the variability in τ c with SAR is not monotonic, this parameter showing a minimum value in soils with SAR of 7.13 mmol/L 1/2 and increases at the extreme values of SAR (0 and 27.2 mmol/L 1/2 ). Overall, the study indicates that the risk of rill erosion noticeably increases in soils treated with brackish or saline water, and, therefore, the use of irrigation water with SAR lower than seven is suggested to land managers.

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