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One of the main sources of natural variability in soil erosion rate from a given soil surface for similar size storm events is the within-storm temporal distribution of rainfall intensity. This paper investigates the effect of rainfall temporal distribution during a storm event on soil erosion rate and the possibility of using a rainfall energy flux density (rain power) type model for more accurate estimation of rainfall erosivity. This soil erosion study was performed using the Guelph Rainfall Simulator II, which was designed and thoroughly tested at the University of Guelph (Tossell et al., 1987; Tossell et al., 1990a; Tossell et al., 1990b), and found to meet many of the criteria for rainfall simulators. Simulated rainfall was applied in various temporal distribution patterns and intensities to a 1.0 m2 plot. Five different rainfall distributions were tested on two soil types (sand and loam) with a minimum of three replications per test. Figures 1 and 2 show some of the results from the experiments for the two soils. This study clearly indicates the importance of the lingering transient effects of within-storm variations of rainfall intensity on soil erosion rates. The soil erosion rates spiked following every sharp increase in rainfall intensity followed by a gradual decline to a steady erosion rate (Figures 1 and 2). These transient effects resulted in the soil erosion rates for an oscillatory rainfall distribution to be more than two folds higher than the soil erosion rate for a steady-state rainfall intensity event with the same duration and the same average rainfall intensity for both soil types. The 3-parameter and 4-parameter rainfall energy flux density (rain power) models investigated in this study provided a process-based measure of rainfall erosivity. The 4-parameter model better matched the trends in the observed data and predicted more accurately the soil erosion rates under low intensity rainfall. However, further work is necessary to improve the accuracy of soil erosion prediction models for a wider range of rainfall distributions.