Intensity Duration Frequency Analysis
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NetSTORM ranks precipitation depths for any duration selected by the user. It also computes and ranks storm volumes and durations based on a user-specified inter-event time, and tabulates summary statistics. The summary statistics include hourly, monthly, and annual mean precipitation, event counts, and mean event analysis. The ranked precipitation depths are assigned return periods based upon a user-specified plotting position, and fits the empirical data to the generalized extreme value (GEV) distribution using the method of L-moments as described by Hosking (1990). These functions give the user the ability to produce specialized intensity  duration  frequency distributions for any locale. While regional precipitation atlases such as the U.S. Midwest atlas (Huff and Angel, 1992) or the older TP-40 (Hershfield, 1961) present regionalized distribution maps, the NetSTORM user can produce station-specific results, albeit without adjusting station results for regional skew considerations. The results are thus less appropriate than a multi-station analysis for determining very low frequency extrema (e.g. storm depths with 1% annual probability). However, the program can be used to identify IDF statistics for censored datasets such as a particular month or season of interest. The program also reports design statistics for sub-annual return periods, so that it can be used to identify 3- or 6-month storms, which are of interest for combined sewer system overflow studies. Since the program both lists events and compiles summary statistics, it can be used, for example, to identify all historic events in a long-term record where the n-hour precipitation exceeds a specified probability threshold.