Link to USGS home
.

The StreamStats Program

Obtaining Streamflow Statistics for Ungaged Sites

A basin delineation is needed before flow estimates can be obtained for an ungaged site. Refer to the Version 3 User Instructions or the Beta Version 4 User Instructions to determine how to obtain a basin delineation and get estimates of flow statistics for an ungaged site. The Estimate Flows Using Regression Equations tool, which provided estimates in StreamStats version 2 by measuring needed basin characteristics and solving USGS-developed regression equations is under development in beta version 4. The Estimate Flows Based on Similar Streamgaging Stations tool estimated 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.

The below instructions and description of the output for an ungaged site pertain to StreamStats version 3. Outputs for ungaged sites from beta version 4 appear differently than those in the captured screen images from version 3, but the content is mostly the same, with the exception that outputs from beta version 4 include boxes at the top in which users can type a title and a message about the output, and a map of the delineated basin is included.

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:

Click to View Larger Image

Return to Top

Streamflow Statistics Tables:

Click to View Larger Image

Return to Top

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.

Click to View Larger Image

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

Flow estimates obtained from regression equations for watersheds that span state boundaries may give different results depending on which state’s equations are used. Each state’s regression equations typically are applicable only within the state for which the equations were developed. Ries (2006, p. 8) indicates that in cases where a delineated watershed has area in multiple states, flow estimates should be determined using the regression equations for each state, and then final estimates should be determined by weighting the separate sets of flow estimates according to the proportion of the drainage area that is in each state. However, because of programing and data limitations, StreamStats typically will only provide estimates using the regression equations for the state in which the selected site is located. In cases where StreamStats is available for each state, it may be possible to determine weighted estimates by use of the following process:

  1. Determine the state in which the site of interest is located, and using that state’s application, (a) select the site of interest, (b) delineate the drainage basin using the Watershed Delineation from a Point tool, (c) obtain flow estimates using the Estimate Flows Using Regression Equations tool, and (d) then save the output.
  2. Open the application for the upstream state, and then follow the same process as in step 1, except select the point for delineation just upstream from where the stream of interest crosses the state border.
  3. It will be necessary to adjust the estimates that were determined using the upstream state’s application to represent the full drainage area at the initial point of interest. Such adjustments may be possible by one of the following approaches:
    1. If the output from the downstream state’s application has provided all of the basin characteristics needed to solve the upstream state’s regression equations, then use the Edit Parameters and Recompute Flow tool for the upstream state’s application, changing the computed basin characteristics to be those from the output for the downstream state.
    2. If the output from the downstream state’s application has not provided all of the basin characteristics needed to solve the upstream state’s regression equations, then determine if the downstream state’s Basin Characteristics tool will provide the additional basin characteristics needed to solve the upstream states’ equations. If so, compute the additional basin characteristics using the downstream state’s application, and then use the Edit Parameters and Recompute Flow tool for the upstream state’s application, as described above.
    3. If not all of the basin characteristics needed to solve the regression equations for the upstream state are available from the downstream state’s application, and if the proportion of the drainage area that is in the downstream state is small, then it may be reasonable to use the values of some basin characteristics that were obtained from the upstream state’s application to determine the final weighted estimates. For example, if the upstream state’s application requires the percent forest, but the downstream state’s application does not provide it, then if the area in the downstream state is small and the percent forest within the downstream state appears similar to that for the upstream state, then when using the Edit Parameters and Recompute Flow tool for the upstream state, edit the drainage area to be that from the output for downstream state, but do not edit the percent forest before recomputing the flow estimates.
    4. If none of the above three conditions exists, then it will not be possible to weight the flow estimates from the two state applications.
  4. If it was possible to adjust the estimates that were determined using the upstream state’s application to represent the full drainage area at the initial point of interest, then manually weight the adjusted flow estimate from the upstream state’s application with those from the downstream state’s application according to the proportion of the total drainage area that is in each state. An example manual computation is provided by Ries (2006, p. 8).

Return to Top

NOTICE: Reports with regression equations for some states recommend the use of different weighting methods than those that are used in StreamStats, although the results usually are very similar. StreamStats users should refer to the individual reports to determine if different weighting methods should be used. In some cases, report authors have provided spreadsheets or programs in which the basin characteristics from StreamStats can be inserted and estimates can be obtained according to the methods that are described in those reports. Links to the applicable reports are provided on the StreamStats introductory page for each individual state.

Return to Top



Best viewed in Internet Explorer 10 or higher with pop-up blocker disabled