COMPUTATIONS--Automatic Entry of Streamflow Statistics into the Streamflow/Basic Characteristics (SBC) File In Reply Refer To: February 6, 1986 WGS-Mail Stop 415 OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM NO. 86.03 Subject: COMPUTATIONS--Automatic Entry of Streamflow Statistics into the Streamflow/Basic Characteristics (SBC) File The Daily Values (DV) and SBC Files are two of the data files contained in the National Water Data Storage and Retrieval system (WATSTORE). Procedures are available for retrieving data from the DV File, performing certain statistical analyses, and then storing these statistics in the SBC File. One series of programs for doing this is known as Daily Value Statistics (programs A969 and A193). In this procedure, the daily flows are retrieved from the DV File using the standard retrieval program, annual D-day high- or low-flow values are complated by program A969 and these annual values are passed to program A193, and the logarithms of these annual values are fit to a Pearson Type III distribution (WATSTORE User's Guide, Volume 1, Chaper IV, Section G). Program A193 creates an output file of flow statistics (e.g., 1-, 3-, 7-day, 2-, 5-, 10-year low flows, etc.) that can be used in a subse- quent job step to update the SBC File. Program L156 reads the output from program A193 and formats these flow statistics for entry into the SBC File using program E772 (WATSTORE User's Guide, Volume 4, Chapter II, Section A). The retrieval of data, computation of flow statistics, and entry of these statistics into the SBC File can be accomplished in one job execution. Prior to January 10, 1986, program A193 produced an incorrect output file of flow statistics for entry into the SBC File when there were zero values in the sample. This problem was identified and documented in Surface Water Branch Technical Memorandum No. 82.11. Program A193 has now been modified (effective January 10, 1986) to produce a correct output file of flow statistics for entry into the SBC File. The program is applicable to all sites including those that have zero values in the data series. The determination of a log-Pearson Type III frequency curve for a sample with zero values requires special computational procedures because it is not possible to take the logarithm of zero. Conditional probability is used to adjust for the number of zero values in the sample and to define the adjusted (unconditional) frequency curve. The computational procedures used in program A193 are described in Surface Water Branch Memorandum No. 70.07 and in a paper by M. E. Jennings and M. A. Benson in Water Resources Research, volume 5, number 1, 1969, pages 276-280. The flow statistics and computed frequency curve should be carefully reviewed before entry into the SBC File. The analyst should determine if the logarithms of the data fit a Pearson Type III distribution and if the conditonal proba- bility adjustment for zero flow values is appropriate. Program A193 will OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM NO. 86.03 2 produce line printer plots of the data and the frequency curves if the analyst requests them. If the computed frequency curve does not adequately fit the data, as described by the Weibull plotting positions, then the frequency curve should be drawn graphically using the plotting position. In this case, the flow statistics must be entered manually into the SBC File. The Districts are responsible for updating the contents of the SBC File (Water Resources Division Memorandum No. 74.127, Surface Water Branch Technical Memorandum No. 76.03). This file should be updated on a regular basis because it is a valuable asset in regional studies of streamflow characteristics. Any questions about the modifications to programs A969/A193 should be directed to George Dempster in the Branch of Computer Technology, Scientific Information Management. Questions concerning the computation of a frequency curve may be directed to your appropriate Regional Surface Water Specialist. Verne R. Schneider Acting Chief, Office of Surface Water WRD Distribution: A, B, S, FO, PO This memorandum supersedes Surface Water Branch Memorandum 82.11.