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. N01lcoincident Flood events or tributary inflows that have peaking times that are ~ synchronized. Normal Distribution A normal distribution is a mathematical distribution that is com- lIlOnly referred to as the "bell shaped" curve. A log-Pearson type III curve has a normal distribution when, the skew coefficient is equal to zero. Overland Flow Length An idea1ized estimate of the ave1;'age length of the overland flow distance that runoff water will travel to get to ,a stream or channel in a hydrologic model subarea. Partial Duration Denotes the type of discharge-frequency analysis. An annual series curve is based on using a single momentary maximum event each year. A partial duration series considers all pesks above a given base. Peak Discharge The maximum instantaneous discharge of a flood at a given loca- tion. It usually occurs at or near the time of the flood crest. In the graphical representation of flow versus time, which is known as a flood hydrograph, the peak discharge occurs between the ascension limb and the recession limb. Peaking Time The lag time from the start of a runoff-producing rainfall to the time the flood reaches its maximum discharge. Probability The annual chance of occurrence of specific hydrologic events, such as rainfall, over a specified area or peak discharge at a speci- fied location expressed in percent, e.g., 5 percent representing one chance in 20 of the event occurring in any year. The 10-, 50-, 100-, and 500-yesr floods are floods having a 10-, 2-, 1-, or .2-percent probability, respectively, of occurrence in any year or an average reCurrence interval in the order of once in 10, 50, 100, or 500 years, respectively. It may be based on statistical analyses of streamflow records and/or the analyses of rainfall and runoff characteristics in the general region of the watershed. Rainfall Distribution (See "Hyetograph") A-4 .