Endorsement of Sediment Records Calculation Program (SEDCALC) In Reply Refer To: August 1, 1995 Mail Stop 415 OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM NO. 95.09 Subject: Endorsement of Sediment Records Calculation Program (SEDCALC) Techniques used by the U.S. Geological Survey to compute daily records of suspended-sediment discharge changed little from the 1940's to the 1970's. In the 1980's, several programs for computing sediment dis charge were developed independently in district offices of Water Resources Division (WRD). Under the direction of John Gray, several of these programs were combined to form SEDCALC, an interactive, menu- driven set of programs developed to facilitate computations of daily suspended-sediment discharge records. The purposes of this memorandum are to announce the publication of the documentation for SEDCALC, to endorse its use within WRD to calculate daily records of suspended-sediment discharges, and to provide guidelines for its use. The documentation of SEDCALC has been published as: "User's Manual for SEDCALC, A Computer Program For Computation of Suspended-Sediment Discharge" by G.F. Koltun, John R. Gray, and T.J. McElhone, USGS Open-File Report 94-459, Columbus, Ohio, 46 pages. SEDCALC enables one to use an entire streamflow time series to compute instantaneous and daily suspended-sediment discharges, thereby eliminating computational errors that may result from use of a subset of the streamflow time series in manual subdivision for periods of rapidly varying conditions. Other advantages of SEDCALC over traditional techniques are that it (1) speeds record computation by automating some of the labor-intensive tasks associated with manual sediment-record computations, (2) provides a uniform set of tools for sediment-record computations, (3) facilitates exploratory data analyses, and (4) facilitates the archiving and restoration of data for later analysis and review. SEDCALC provides four methods for developing a time series of suspended-sediment concentrations on identical intervals as the associated discharge time series to calculate suspended-sediment discharges: (1) By using a digitilized curve of sediment concentration versus time. (2) By log-linear interpolation in time between measured and estimated suspended-sediment concentration values, (3) As in (2) above, but using a non-linear cubic-spline interpolation scheme that approximates the shape of a hand-drawn curve, and (4) By log-log transport relation with potential adjustment for retransformation bias. Any of the first three options are acceptable for calculating loads so long as data used for input have been analyzed using methods described by Porterfield, TWRI, Book 3, Chapter C3 , (1972) and the appropriate checking procedures have been performed. It is recommended to have at least a full year of record, and a large number (>75) of observations of instantaneous suspended-sediment concentration covering the range of flows that are to be estimated before using the transport-curve option to compute primary record. It seamiest appropriately used for estimating the shape of the suspended-sediment trace for periods not exceeding several days at a time, and even then only for periods where land-use and hydrologic conditions are consistent with those existing when data used to define the transport curve were collected. Proper use of either log-linear or non-linear interpolation requires that a sufficient number of instantaneous suspended-sediment concentrations--either measured or estimated--are available to adequately approximate the sediment trace. Particular caution should be exercised when using the non-linear interpolation scheme, as it may yield erroneously large or small suspended-sediment concentrations if the sediment trace is inadequately defined. The log-log transport-relation option should be used for computing discrete periods of primary record only when there are insufficient data for application of one of the first three options, or until a sufficient set of comparative record has been calculated to evaluate the performance of this option. A bias-correction factor (BCF) must be applied to sediment- discharges calculated by means of the log-log transport relation option if the equation for the transport relation was determined by means of regression on log-transformed variates. Either of two methods -- Minimum Variance Unbiased Estimator (MVUE), or Duan's Smearing Estimator -- may be appropriate (OSW Memorandum 93.08). They are used when the distribution of errors are normal and non-normal, respectively. A statistical routine based on the statistics package, Statithas, has been developed to assist in determining the parameters required to perform the bias correction. It is critical that results of BCF calculations be reviewed by a surface-water specialist or other person knowledgable in hydrology and statistics before using the results to calculate suspended-sediment discharges. Greg Koltun, Ohio District, is responsible for supporting SEDCALC. Please direct any questions about obtaining or using SEDCALC to him by E-mail to gfkoltun@usgs.gov, or by telephone to 614/469-5553. A limited number of users manuals are available fromthe Ohio District. Ernest F. Hubbard Acting Chief, Office of Surface Water WRD Distribution: A, B, FO, PO