Depth of Surface Soil-runoff Interaction as Affected by Rainfall, Soil Slope, and Management

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Sharpley A.N
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The effective depth of interaction (EDI) between surface soil and runoff (the thickness of surface soil in which the degree of interaction is equal to that at the soil surface) was determined for five soils of varying physical and chemical properties under simulated rainfall, in order to quantify the effect of rainfall and soil characteristics on EDI. For all soils EDI increased (1.30-37.43 mm) with an increase in rainfall intensity (50-160 mm h-I ) and soil slope (2- 20%), although the magnitude differed between soils. The effect of rainfall intensity was attributed to increased runoff energy enhancing mixing in the surface soil and was also a function of soil aggregation. The magnitude of the EDI increase with increasing soil slope was independent of soil type being a function of runoff energy alone. An avg 73% reduction in EDI following the incorporation of 100 kg wheat straw (Triticum autivum L. sp.) ha-I and 80% reduction with a 0.5-mml mesh screen, simulating crop cover, was obtained compared to the control (3.36 mm). For all soils the logarithm (In) of soil loss was linearly related to the In EDI. This is to be expected since factors affecting EDI (rainfall intensity, runoff energy, and soil aggregation) also influence soil loss. Regression slope of the logarithmic relationship was similar (at the 5.0% level) for all soils, and regression intercept was related to soil aggregation. Thus, EDI and the effect of rainfall and soil management can be estimated from soil loss. This relationship will improve the prediction of adsorbed chemical (P and pesticides) transport in solution, since chemical transport models presently use a fixed EDI value.