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South Dakota StreamStats incorporates statewide regression equations for estimating instantaneous peak flows with annual exceedance probabilities of 50, 20, 10, 4, 2, 1, and 0.2 percent. These peak flows have recurrence intervals of 2, 5, 10, 25, 50, 100, and 500 years, respectively. The report below documents the regression equations, the methods used to develop them and to measure the basin characteristics used in the equations, and the errors associated with estimates obtained from the equations. Users should familiarize themselves with this report before using StreamStats to obtain estimates of flows for ungaged sites in rural drainage basins.
- Sando, Steven K., 1998, Techniques for estimating peak-flow magnitude and frequency relations for South Dakota streams: U.S. Geological Survey Water-Resources Investigations Report 98-4055, 48 p.
Users are advised that updated at-site peak-flow estimates were developed by Sando and others (2008), and the regression equations developed by Sando (1998) do not incorporate these updated estimates. A project is underway to update the statewide regression equations based on results from Sando and others (2008). It is expected that the StreamStats application will be updated with the equations once they become available. Users are further advised that especially large statistical uncertainties can occur when estimating peak flows for the Black Hills area of western South Dakota. Additional insights regarding peak-flow characterization for especially large floods are available from Harden and others (2011), who conducted paleoflood investigations in the Black Hills area.
South Dakota’s StreamStats application utilized the Watershed Boundary Dataset to enforce drainage boundaries. Drainage area values produced by StreamStats may be different than values published in the above report, a recent paleoflood investigation report (SIR 2011-5131), annual data reports, and in databases that pre-date the Watershed Boundary Dataset. Drainage area values produced by StreamStats are considered more accurate than previously published values, and should be used where differences occur.
For South Dakota, the drainage area values that are presented in the header information at the top of the outputs from the Estimate Flows using Regression Equations tool are actually the contributing areas, not the total drainage areas. This condition is unique to South Dakota, and was necessary because of programming limitations. Users can obtain total drainage areas by use of the Basin Characteristics tool.
South Dakota has varying and sometimes complex drainage patterns. Certain areas, which typically have only internal drainage, are considered noncontributing relative to the rest of the basin. To view these noncontributing areas on the map, first click on the down arrow on the right side of the Map Contents tab to the left of the map, next click on the plus sign to the left of the initial line in panel that appears, and then put a checkmark in the Noncontributing Drainage Area box.
Obtaining Flow Estimates in Noncontributing Areas
The StreamStats output will show the contributing drainage area as zero and peak flows will not be calculated when using the Estimate Flows using Regression Equations tool to estimate flows at any point within a noncontributing area. However, flow estimates from regression equations still can be obtained for points within noncontributing areas by the following process: (1) use the Watershed Delineation from a Point tool to delineate the drainage basin, (2) use the Estimate Flows using Regression Equations tool to obtain an output with the flow estimates missing, (3) use the Basin Characteristics tool to obtain a list of the available basin characteristics for the site, and (4) use the Edit Parameters and Recompute Flows tool and enter the total drainage area obtained from the output from the Basin Characteristics tool in place of the contributing drainage area to obtain correct peak flow estimates. Because waterbodies along a stream can store water and attenuate flows, it is recommended that this technique not be used to force peak flows to be computed for streams that connect waterbodies within a noncontributing area.
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