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<br />storms are transposed in mountainous areas. account must be taken of the <br />intricate effects of irregular top09raphy, barrier orientation, prevail- <br />ing wind direction and relative storm occurrence. Usually a base pattern <br />of normal orographic precipitation can be constructed from average sea- <br />sonal precipitation amounts computed from all available data. in con- <br />junction with correlation studies relating such average precipitation to <br />orographic features. Then lines of storm precipitation expressed as a <br />ratio to the base-pattern amount can be transposed over a much wider <br />areal range than if expressed as direct rainfall depth. Such normali- <br />zation does not entirely remove orographic related patterns but is <br />generally acceptable when combined with other adjustment techniques. <br />It is not ordinarily possible to express in terms of probability or <br />frequency the flood that would result from a transposed storm. It does <br />not even necessarily follow that floods in different areas resulting <br />froln transposition of the same storm to those areas would be equally <br />severe (equally probable). since an actual storm is not necessarily <br />balanced (equally severe) with respect to different areas and time <br />patterns that different river basins respond to most strongly. In the <br />chapters below, transposition of a balanced storm will be used in order <br />to approach equal flood severity as nearly as possible, but actual <br />probability evaluations will be based on observed flood frequencies <br />wherever adequate runoff data are available. In the case of very rare <br />events, some judgment or inference of extreme probabilities might be <br />made for standard project and probable maximum floods independent of <br />recorded runoff at the location. <br /> <br />Section 2.06. Rainfall frequency <br /> <br />The frequency or probability of occurrence of maximum rainfall amounts <br />for a specified location and duration can be estimated from recorded rain- <br />fall at that location using methods described in VoluMe 2. Point rainfall <br />estimates thus obtained can be used to construct maps of precipitation <br /> <br />2-08 <br />