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REGIONAL ANALYSES OF STREAMFLOW CHARACTERISTICS 7 . Use of short records on small streams . The usual regional analysis is based on some long records and some short ones. Records of floods from small drainage areas are usually short; consequently even the 10-year flood may be poorly defined. In this case a regional analysis by one of the methods previously de- scribed will tend to produce results of low reliability. On the other hand, there may be more independence among the records for small streams than among those for large streams; if so, this should lead to increased reliability. The conterminous United States is covered by regional flood-frequency analyses, general- ly based on data for the larger streams. Since those analyses were made, 10 or more years of record have become available at many small- basin crest-stage gage sites, and the demand for flood frequency characteristics of small streams has greatly increased. The short rec- ords on small streams could be used with the records from larger areas to produce another regional analysis, one that would encompass the whole range of drainage area sizes. Such a procedure probably would give the best an- swer, but one which would more or less dupli- cate the available results for the larger drain- age areas. Furthermore, 5 years from now one might be justified in reanalyzing the records from the small areas, and this would call for another general analysis, resulting in more duplication (or confusion). Therefore, it is sometimes desirable to pro- duce a regional analysis limited to small drain- i age areas, one that will not duplicate or con- flict to any substantial extent with recently published analyses. This can be done in several ways. Given a regional analysis by the index-flood method, the defined relations can be extrapo- lated to small drainage areas. If the mean annual floods for the crest-stage stations check the extrapolated curves, the existing regional analysis may be considered applica- ble to small drainage areas. If not, the curves should be modified as indicated. Likewise, regression equations from an existing regional analysis may be checked . against data from small drainage areas. If this check indicates that those equations are not applicable, and if time and money are limited, a regression analysis applicable only to small drainage areas could be made. Be- cause of the short records available, such a regression usually will have a large standard error. Graphical regression may be adequate. An example, given in figure 4, is based on data used by Boner and Omang (1967). Note that some of the small10-year floods based on short records have been given little weight in de- fining the relation of figure 4. Some of the 10-year floods at the larger drainage areas are defined by many years of record and are used to tie this relation into one based on records for large streams. At many sites on small streams, both flood hydrographs and the causative rainfalls at short time intervals (15 minutes or so) are being collected. After a few years, these data should be adequate to calibrate a hydrologic basin model such as the one described by Dawdy, Lichty, and Bergmann (1972). Then a long record of precipitation can be used to synthesize additional flood peaks. These syn- thetic peaks can be combined with those of record to define the frequency curve to recur- rence intervals of 50 years or more. Using frequency curves defined to that length, a standard regionalization process should give good results. The practicability of the method depends on obtaining a good relation between floods and precipitation and on the availabil- ity of an applicable long precipitation record. Defining the flow characteristic The better the frequency curves which form the basis for the regionalized relation, the better that relation will be. Therefore, some effort should be made to improve the frequency curves. Where data are available, the rainfall-runoff approach described above can be used. Another method, utilizing his- torical data is described by Dalrymple (1960). Sometimes the definition of a frequency curve can be improved by correlation with a longer record, but this procedure generally results in improvement only if the correlation co- efficient between the two records is greater