PROGRAM AND PLANS - Branch Policy on Selected Subjects May 1, 1973 SURFACE WATER BRANCH TECHNICAL MEMORANDUM NO. 73.16 Subject: PROGRAM AND PLANS - Branch Policy on Selected Subjects Our recommendations on certain technical aspects of surface-water hydrology change with time in response to the better understanding provided by additional data and further analytical developments. Major changes are generally incorporated in TWRI's, but some changes evolve gradually and are not formally disseminated. This memorandum: describes Branch policy on several subjects. 1. Extension of frequency curves To discourage indiscriminate extensions, Surface Water Branch Technical Memorandums 69.11 and 70.05 state that we should not publish frequency characteristics for recurrence intervals greater than twice the period of record (2N). At recent Branch meetings, it was agreed that the 2N rule should be modified. The following paragraph describes a revised policy on limits of frequency curve definition and supersedes previous directives on this subject. Listed below are the recommended minimum years of record needed to define events of selected recurrence interval. Recurrence interval - 10 25 50 100 Minimum years of record - 10 15 20 25 Magnitudes of floods for these recurrence intervals will have about equal reliability, according to Hardison's paper, "Accuracy of Streamflow Characteristics" (Prof. Paper 650-D, p. 210-212). These extension limits are intended as a restriction for publication of general information on flow characteristics at gaged sites. Greater extensions than those shown are permitted for certain specified uses. For example, it is permissible to estimate a 50- year flood discharge from 10 or 15 years of record for inclusion in a regional regression on basin characteristics, but the value should not be published. Also, greater extensions are permissible when a value is needed for a hydrologic investigation; for example, a 500-year peak may be estimated and used for a flood insurance study. However, a discharge characteristic which is based upon a longer extension than described above and which is to become public information, should be coordinated with other concerned Federal agencies to identify and resolve potential conflicts. Minutes of the 72-3 meeting (June 6, 1972) of the Hydrology Committee, Water Resources Council, noted that this coordination was the responsibility of the investigating agency and should be accomplished early in the investigation. As described in Water Resources Council Bulletin 15, the log- Pearson Type III analysis is the standard method for interpreting a flood peak record (but not a low-flow or mean-flow record). However, a graphical curve should be used if the LP Type III curve does not provide a reasonable fit to the observed record or if any significant historical data are available. 2. Flood characteristics at gaged sites Flood characteristics at a gaged site could be defined either by probability analysis of the gage record or by a regional relation. The characteristic to use and to publish should be the more reliable of the two or perhaps a weighted average with the weights based on an equivalent years of record concept. This decision may include some subjective judgment as well as the indices of reliability, and no specific guidelines are provided. The purpose in discussing this problem without furnishing decision guidelines is to rescind a policy of 10 or more years ago that recommended use of characteristics estimated from regional relations at gage sites. 3. Regression analyses The statement that 20 stations is a minimum sample that should be used in a regression study (SWB Tech. Memo 70.05, p. Ei) was not meant to apply to all hydrologic studies by regression methods. Obviously, a regression using a lesser sample with only one or two independent variables might provide a very useful relation. The adequacy of a sample of data probably should be judged as much upon its representativeness of the population as upon the number in the sample. The standard error of a regression is only an index of the reliability of estimates formed from the relation. Standard errors for linear log-log relations should be reported to no more than two significant figures in percent. For relatively small standard errors (40% or less), the positive and negative deviations may be averaged and reported as a single value. For larger standard errors the positive and negative errors should be reported as two separate values. Standard errors should be reported for defined relations, but in a designer's manual there is no need for detailed explanations of the concepts and uses of standard errors. The most practical relation from a series of step-forward or step-backward regression relations defined to estimate a flow characteristic, should be selected on the basis of reliability and simplicity of use. A small reduction in the standard error of estimate by inclusion of an additional independent variable in a regression relation may not indicate a comparable increase in reliability of estimates. The decrease in standard error of estimate must be weighed against the difficulty in defining an additional independent variable. 4. Adherence to guidelines The Preface to the earlier TWRI's contains an excellent statement on adherence to guidelines. "Judgment must be used in deciding how closely to adhere to instructions. Instructions which include information on preparation of data for use in a computer program must be closely followed. Instructions on methods of analysis are generally less binding and are not to be utilized to the extent of inhibiting initiative or stifling progressive development. However, before using a technique which differs substantially from one which has been recommended, it should be discussed with the office of the appropriate Branch Chief." Walter Hofmann Chief, Surface Water Branch WRD Distribution: A, B, S, FO