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<br />This defines only the part of the distribution above the flood base; the part below the flood base is not <br />defined, but is of no practical importance. <br />These conditional probability adjustments are used not only to construct the final Bulletin 17B <br />frequency curve but also to construct a systematic-record frequency curve that takes into account any zero <br />flows or below-the-gage-base peaks but does not reflect any historic information or outlier tests. <br /> <br />Estimation of Generalized Skew Coefficient <br /> <br />The skew of a frequency distribution has a tremendous effect on the resulting shape and thus the <br />values of the distribution. The discussion in this section concerns the development of appropriate <br />generalized skew coefficients for the program's flood frequency analysis. The following discussion is <br />modified from Bulletin 17B (p. 10-14). <br />The skew coefficient of the station record (station skew coefficient, 0) is sensitive to extreme events; <br />thus it is difficult to obtain an estimate of an accurate skew coefficient from a small sample. The accuracy <br />of the estimated skew coefficient can be improved by weighting the station skew coefficient with a <br />generalized skew coefficient estimated by pooling information from nearby sites. The following guidelines <br />are recommended for estimating generalized skew. <br />The recommended procedure for developing generalized skew coefficients requires the use of at least <br />40 stations, or all stations within a 100-mile radius. The stations used should have 25 or more years of <br />record. It is recognized that in some locations a relaxation of these criteria may be necessary. The actual <br />procedure includes analysis by three methods: (I) skew isolines drawn on a map; (2) skew prediction <br />equation; and (3) the mean skew coefficient from selected stations. Each of the methods are discussed <br />separately. <br />To develop the isoline map, plot each station skew coefficient at the centroid of its drainage basin and <br />examine the plotted data for any geographic or topographic trends. If a pattern is evident, then isolines are <br />drawn and the average of the sum of the squared differences between observed and isoline values, mean- <br />square elTor (MSE), is computed. The MSE will be used in appraising the accuracy of the isoline map. If no <br />pattern is evident, then an isoline map cannot be drawn and is, therefore, not further considered. <br />A prediction equation should be developed that would relate either the station skew coefficients or the <br />differences from the isoline map to predictor variables that affect the skew coefficient of the station record. <br />These would include watershed and climatologic variables such as drainage area, channel slope, and <br />precipitation characteristics. The prediction equation should preferably be used for estimating the skew <br />coefficient at stations with variables that are within the range of data used to calibrate the equation. The MSE <br />will be used to evaluate the accuracy of the prediction equation. <br />Determine the arithmetic mean and variance of the skew coefficients for all stations. In some cases, <br />the variability of the runoff regime may be so large as to preclude obtaining 40 stations with reasonably <br />homogeneous hydrology. In these situations, the arithmetic means and variance of about 20 stations may be <br />used to estimate the generalized skew coefficient. The drainage areas and meteorologic, topographic, and <br />geologic characteristics should be representative of the region around the station of interest. <br /> <br />PEAKFQ <br /> <br />10 <br /> <br />DRAFT -1/30/98 <br />