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This study investigates the effect of rainfall temporal distribution pattern within a storm event on soil erosion rate and the possibility of using rain power type model for rainfall erosivity. Various rainfall distribution patterns, simulated by rainfall simulator, were used on 1.0 m 2 plot of silica sand and loam soil with a minimum of three replications. The results show that the soil erosion rates spiked following every sharp increase in rainfall intensity followed by a gradual decline to a steady erosion rate. Transient effects resulted in the soil erosion rates for an oscillatory rainfall distribution to be more than two fold higher than those obtained for a steady-state rainfall intensity event with same duration and same average rainfall intensity. The 3-parameter and 4-parameter rain power models were developed for a process-based measure of rainfall erosivity. The 4 parameter model yielded better match with the observed data and predicted soil erosion rates more accurately for silica sand under all rainfall distributions, and good results for loam soil under low intensity rainfall. More research is necessary to improve the accuracy of soil erosion prediction models for a wider range of rainfall distributions.

This paper summarizes the methodology and procedure developed for determining the Water Quality Capture Volume (WQCV) for stormwater BMP and LID facility designs. WQCV is directly related to the local rainfall pattern, watershed imperviousness, and drain time applied to the BMP/LID facility. The performance of a BMP facility is evaluated using the rainfall-runoff continuous simulation that computes the long term runoff volume-based and event-based capture ratios using the principle of water volume balance among rainfall amount, hydrologic losses, runoff volume captured in the BMP facility, and bypass flow. For a regional study, the procedure associated with the continuous simulation can produce optimized design values for WQCV. Typically, but not always, the optimal runoff volume and event capture ratios vary between 80 and 90%. This WQCV design and analysis procedure is more robust than the one used to estimate the WQCV's using the regression equations outlined in ASCE Manual of Practice No. 87 and WEF Manual of Practice No. 23. The computer model, WQ-COSM, was also developed as a freeware for evaluating the performance of a BMP facility or producing regional design charts. The model accepts the standard hourly and 15minute rainfall data format provided by the National Climatic Data Center. Hourly data are typically available for major metro areas in the United States for a period of 20 to 60 years.

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