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~ection 2.03. Estimatin2 Areal Depth of Precipitation The average depth of precipitation over a specified area is required in many hydrologic problems. The three most commonly used methods for computing mean precipitation over an area are: (1) the station-average method, (2) the Thiessen method and (3) the isohyetal method. These methods are illustrated in fig. 2.02. In each of the methods, the accu- racy with which rainfall depth over an area can be estimated depends on the number and spacing of precipitation stations. In general, the larger the area, the greater the number of sampling points included within the area, and the greater the resulting accuracy of average depth determina- tions. Station-Avera~e Method The simplest method to obtain the mean areal depth is to compute the arithmetic mean by dividing the sum of the depths at all stations by the number of stations, as illustrated in fig. 2.02. This often is as accu- rate as is justified for the purpose or by the basic data. If stations are spaced with reasonable uniformity and the individual gage catches do not vary widely from the mean, the arithmetic average will usually suffice. In most cases, however, the gages are not uniformly spaced, and topographic and other influences produce a large variation in the areal distribution of precipitation. In such cases, more precise methods are required. Thiessen Method In the Thiessen method, it is assumed that the amount of precipita- tion at any station can be applied halfway to the next station in any direction. A weighting process is used by determining the area of influ- ence associated with each station and assuming the occurrence of uniform precipitation over each of these areas is equal to the measured station 2-04