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<br />CHAPTER 2. ANALYSIS OF PRECIPITATION <br /> <br />Section 2.01. Introduction <br /> <br />Precipitation is the general term for all forms of moisture emanating <br /> <br /> <br />from clouds and falling to the ground. The principal forms are dew, <br /> <br /> <br />rain, hail, sleet and snow. Estimates of intensity, depth and areal <br /> <br />distribution of precipitation will be discussed in this chapter. <br /> <br />Limitations in the application of precipitation data are largely <br /> <br /> <br />related to statistical sampling inadequacies. The existing precipitation <br /> <br /> <br />network in the United States and most other countries is ordinarily not <br /> <br /> <br />sufficiently dense to define storm precipitation for average depth over <br /> <br /> <br />areas of the size suited for hydrograph analysis. In fact, it is known <br /> <br /> <br />that many "cloudbursts" occur that are not recorded at any observation <br /> <br /> <br />station . <br /> <br /> <br />The nature of storm precipitation and the uses for which precipita- <br /> <br /> <br />tion data are intended should determine network density, since the <br /> <br /> <br />probability that a storm center will be defined adequately varies with <br /> <br /> <br />network density. A relatively sparse network of stations will often suf- <br /> <br /> <br />fice for studies of large general storms or for determining long-term <br /> <br /> <br />averages over large areas of level terrain, but a dense network is required <br /> <br /> <br />to delineate the rainfall pattern in small-area storms. <br /> <br /> <br />In this regard, it should be noted, however, that no feasible means <br /> <br />of accounting for all the effects of random variations in the areal <br /> <br />distribution of rainfall have been devised, nor is information on precip- <br /> <br /> <br />itation (and other hydrologic variables) sufficient to define areal varia- <br /> <br /> <br />tions accurately. Accordingly, some assumptions of uniformity in precipi- <br /> <br />tation patterns in subareas are necessary, and the maximum subarea size <br /> <br /> <br />feasible for hydro graph analysis is limited by the degree of random pre- <br /> <br /> <br />cipitation variation. This limitation can be relatively minor in areas <br /> <br /> <br />of general storms where precipitation is highly correlated from point to <br /> <br /> <br />point. On the other hand, it can be highly restrictive where precipitation <br /> <br /> <br />is erratic, as in areas that experience cloudbursts. <br /> <br />2-01 <br />