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The StreamStats Program

Obtaining Streamflow Statistics for Ungages Sites

Estimates of streamflow statistics for ungaged sites can be obtained by two methods in StreamStats. The Watershed Delineation from a Point tool must be used first to obtain the drainage-basin boundary for the point of interest before either of the flow-estimation tools may be used. The Estimate Flows Using Regression Equations tool provides estimates by measuring needed basin characteristics and solving USGS-developed regression equations. The Estimate Flows Based on Similar Streamgaging Stations tool estimates streamflow statistics by applying the flows per unit area for streamflow statistics at a nearby gaging station to the drainage area for the ungaged site. Directions for use of both of these tools are provided on the User Instructions page.

Estimate Flows Using Regression Equations Tool Output

When the Estimate Flows Using Regression Equations tool is used, StreamStats first measures whatever basin or climatic characteristics are used as explanatory variables in the regression equations that are available for the selected location. StreamStats then uses the National Streamflow Statistics (NSS) program to solve the equations. The NSS report by Ries (2006) provides a general description of the development and application of regression equations. The NSS Web site contains links to all reports that contain regression equations included in the software. The reports are also listed on the StreamStats introductory Web page for each State. These reports describe how the equations were developed and their limitations. Users should read and understand the limitations described in these reports before attempting to use the Estimate Flows Using Regression Equations tool to obtain flow estimates for ungaged sites.

The output from the Estimate Flows Using Regression Equations tool appears in a pop-up Web browser window. At the top is a banner identifying the output as a product of the USGS StreamStats program. The title, "StreamStats Ungaged Site Report" is below the banner. Following the title are several lines of text that give the processing date, the name of the state in which the ungaged site is located, the total drainage area, and the latitude and longitude for the site. Below this information is a series of two or more tables, described below.

Most states have been subdivided into hydrologic regions based on similarity of climate and physical characteristics, and regression equations have been developed separately for each region. The ungaged site reports list only the basin characteristics that are used in the regression equations for any hydrologic regions in which the site has drainage area.

The reports will always include at least one pair of tables; one for basin characteristics and one for streamflow statistics. One pair of tables will be provided for peak-flow statistics and the basin characteristics needed to solve the equations for peak-flow statistics. Another pair of tables will be provided for all other types of statistics and the basin characteristics needed to solve the equations for those statistics. Tables of basin characteristics are always presented before the tables of streamflow statistics.

Basin Characteristics Tables:

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Streamflow Statistics Tables:

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Area-Averaged Streamflow Statistics Tables:

StreamStats provides area-averaged estimates of streamflow statistics when the drainage basin for an ungaged site is in more than one region. The area-averaged estimates will appear below the basin characteristics tables and above the tables of estimates for individual regions.

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Obtaining Estimates for User-Selected Sites With Drainage Area in More Than One State

The regression equations that are used by StreamStats to estimate flows for user-selected site typically are applicable only within the state for which the equations were developed. If the delineated basin for a user-selected site has drainage area in more than one state, then StreamStats will only provide estimates using the regression equations for the state in which the selected site is located. Improved estimates can be obtained.

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Estimate Flows Based on Similar Streamgaging Stations Tool Output:

The Estimate Flows Based on Similar Streamgaging Stations tool uses stream-network navigation to search upstream and downstream from a user-selected ungaged site to identify streamgaging stations along the same stream or its upstream tributaries. The drainage-area ratio is computed for all streamgaging stations identified in the search by dividing the drainage area for the streamgaging station by the drainage area for the ungaged site. Normally, the method is applied only if the drainage area ratio is between 0.5 and 1.5, but the ratios can be set differently for each state if information is available to support changing them. The equation used to determine the drainage-area ratio estimates, modified from Ries (2006), is:

Qu = (Au/Ag)bQg

where Qu is the estimated flow statistic for the ungaged site, Au is the drainage area for the ungaged site, Ag is the drainage area for the streamgaging station, Qg is the flow statistic for the streamgaging station, and b, depending on the state, may be the exponent of drainage area from the appropriate regression equation, a value determined by the author of the state report, or 1 where not defined in the state report.

The flow statistic for the streamgaging station, Qg, may be computed from the systematic record for the station or it may be a weighted estimate that combines the estimate from the systematic record with an estimate obtained from a regression equation. The NSS report by Ries (2006) explains how weighted estimates for streamgaging stations can be computed. StreamStats does not compute these weighted estimates, but if weighted estimates were computed previously and stored in the StreamStats database, then StreamStats can use them to compute the drainage-area ratio estimates.

StreamStats will generate drainage-area ratio estimates based on both the closest upstream and downstream streamgaging station if both of the streamgaging stations have drainage-area ratios within the set limits, usually 0.5 to 1.5. For any flow statistics that were estimated based on drainage-area ratios from both streamgaging stations, StreamStats will then obtain final weighted estimates for the ungaged site based on linear interpolation between the streamgaging stations.

The output from the Estimate Flows Based on Similar Gages tool appears in a pop-up Web browser window. Across the top is a banner identifying the output as a product of the USGS StreamStats program. The title, "Flow estimates based on flows at nearby streamgaging stations" is below the banner. Following the title is the processing date, the name of the state in which the ungaged site in located, and its latitude, longitude, the NHD reach code and measure, the total drainage area, and an indication of whether or not regulated streamgaging stations were allowed to be used in the estimation process. Below this information is a series of two or more tables.

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