The effects of electrolyte concentration and soil sodicity on the infiltration rate and extent of crust formation of a calcareous and a noncalcareous soil were studied using a rain simulator. The infiltration rate was more sensitive to the sodicity of the soil and to the electrolyte concentration of the applied water than was the permeability of the underlying soil. The mechanical impact of the raindrops and the relative freedom for particle movement at the soil surface may account for the greater sensitivity of the infiltration rate. These observations suggest that crust formation is due to two mechanisms: (i) a physical dispersion of soil aggregates caused by the impact action of the raindrops, and (ii) a chemical dispersion which depends on the soil exchangeable sodium percentage (ESP) and the electrolyte concentration of the applied water.

Raindrop impact destroys the surface aggregates of soils and gradually forms a continuous crust. Infiltration rates of bare soils are determined by this process. A theoretical model presented previously was tested under field conditions with a sandy loam soil. The effect of the crust's structure was tested under three rainfall intensities and in wetting and drying cycles. The results of the tests confirmed the theoretical model. According to the model potential infiltration rates, in bare soils, can be predicted by the equation It = (Ii ‐ If) ·e−γptl + If. The study demonstrated quantitatively that the major factor determining the reduction of infiltration rates is crust formation and not moisture regime.

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The effect of water drop impact energy and water salinity on the infiltration rate (IR) of two sodic soils: Calcic Haploxeralf (loess) and Typic Chromoxerert was studied using a rainfall simulation. Rain was applied at two energy levels: high energy rain with kinetic energy of 22.9 J/mm m2 and a low energy rain, with kinetic energy less than 0.01 J/mm m2. In the low energy rain, the initial IR of the soils (44 mm/h) was maintained during a distilled water rain on soils with ESP 2.5, and during saline water rain on soils with ESP values of 6.5 and 17–21.0. In Haploxeralfs with ESP values of 6.5 and 21.0 the final IR (FIR) under low energy rain dropped to 28.8 and 11.5 mm/h respectively. When the same soil and ESP levels were subjected to high energy rain, the FIR during saline water storms dropped to 6–9 mm/h and during distilled water simulated storms the FIR dropped to 0.9–1.4 mm/h. It was concluded, that both the water drop impact energy and the electrolyte concentration in the applied rain have a decisive effect on the IR drop of soils. When the chemical dispersion processes are minimal (low ESP and high water salinity) the mechanical impact of the drops predominate and the crust formation is mainly due to the distintegration of the soil aggregates and their compaction to a thin skin seal. When the soil ESP is high (> 2.5) and the water salinity is low (rain water), chemical dispersion processes have an increasing role in determining the IR of the soils.

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