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The universal soil loss equation, developed by Wischmeier and Smith, has been used for many years in the United States for predicting soil loss from cultivated areas. The equation consists of a number of terms, one of which is the rainfall factor. This factor is determined from the product of the kinetic energy of rainfall and the maximum 30-min rainfall intensity. The studies by Wischmeier et al. showed that this factor was the one of many rainfall-related variables tested that related best to the soil loss quantities. The kinetic energy of rainfalls and the rainfall factor are of interest, therefore, in evaluating the potential soil erosion rate for a given location. The rainfall rate data base for the prairie provinces now is sufficient both in number of stations and length of record for the statistics of kinetic energies and rainfall factors to be determined. A study was initiated to determine the two quantities and their frequency of occurrence, for each recorded storm during summer periods and for all stations in Manitoba, Saskatchewan and Alberta. The seasonal rainfall factors were determined for all stations for which sufficient rain gauge data were available. The log-normal distribution, fitted by the method of moments, produced the most consistent fit to the seasonal data. The standard deviations of the rainfall factor tended to be high at all stations because one or more high intensity storms usually greatly influenced the seasonal value. Contour plots of mean seasonal rainfall factor and coefficient of variation were produced. These plots, together with statistical relationships for the log-normal distribution, provide the information for calculation of seasonal rainfall factor for any location and for any return period. Estimates of soil erosion rates then can be made using the Universal Soil Loss Equation.
rainfall and runoff factor for erosion estimates
Wigham, J. M. and W. J. Stolte 1986. RAINFALL AND RUNOFF FACTOR FOR EROSION ESTIMATES
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