Laboratory methods for measurement of soil erodibilities (K factors) for the universal soil loss equation

This paper reports a comparison of several methods for estimating the K (erodibility) factor for the Universal Soil Loss Equation (USLE) on the basis of laboratory measurements of soil properties. All methods used the nomograph of Wischmeier et al. (J. Soil Water Cons., 1971, 26, 189-93) to calculate K on the basis of laboratory data, but the data inputs ranged from: dispersed particle sizes as originally used in the nomograph; non-dispersed particle size, measured after shaking in water; and equivalent sand size distributions, based on settling velocity distributions of particles (aggregates and sand grains) at the soil surface under rain. A further method tested with the use of aggregated particle sizes resulting from rainfall wetting was a correction of the calculated K based on average density of wet sediment >0.100 mm diameter. Estimated K factors were compared with K factors derived from field measurements of soil loss for five soils. Use of dispersed particle sizes gave poor prediction of field K values for the three clay soils, and size distributions measured after rainfall wetting gave poor predictions of field K values for the four soils that had sediment of low density. Predictions of K from the use of non-dispersed particle sizes, and from the use of particle size at the soil surface under rain combined with a correction for sediment density were good for three soils, reasonable for one, and little different to that from the nomograph for the remaining soil. These latter two methods gave similar results, and were successful in predicting field values of K. As well, both methods appear to be sensitive to effects of cropping management on soil structure and erodibility. Either method could be used, depending on laboratory resources available and possible other uses of the data obtained. Simpler methods for measuring size distributions after rainfall wetting and for estimating sediment density are suggested.

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