The reports for user-selected sites contain text entry boxes near the top that allow users to enter a report title and comments that will display on the printed reports. Below these, the reports identify the state or regional application name, the workspace ID, the latitude and longitude of the user-selected point, and the time the report was created. If there are any problems with the report, then the workspace ID should be provided to the StreamStats development team, along with a description of the problem, by filling out the provided form after clicking on the Help button above the map on the user interface.
The reports also contain maps showing the user-selected points and their drainage areas. Users can pan and zoom within the map before printing it. Buttons at the bottom of the reports allow users to (1) download the basin as either a shapefile or an ESRI geodatabase file; (2) download the output report as a .csv file, which can be input into various spreadsheet, GIS, and database programs; (3) print the report; and (4) close it.
StreamStats will display the warning below if the watershed boundary was edited before computing basin characteristics or flow statistics. This warning will appear just below the Basin Characteristics section in the report.
If it was chosen to compute basin characteristics, then a table of the computed basin characteristics will be displayed in the report just below the map. The first column in the table contains the label used for the basin characteristic internally within StreamStats. The following three columns contain a description of the basin characteristic, the computed value, and its units of measure, respectively (see the example below). Methods and geospatial data used to compute the basin characteristics are identified on the General Information page for each state or region. A link to this page is provided on the State/Regional Info tab of the About link above the map frame.
Below the basin characteristics tables, the reports contain separate sections for each selected scenario. Within each of these sections are a table of parameters (the basin characteristics used as explanatory variables in the regression equations) and a table of the estimated streamflow statistics. Headings for each of the tables begin with names of the scenarios that were selected. As an example, the screen capture below shows the report section that corresponds to the Flow-Duration Statistics scenario that was shown above in the Select Scenarios section of this users’ manual, thus the table names are Flow-Duration Statistics Parameters and Flow-Duration Statistics Flow Report. To the right of the headings, in smaller lettering, is the percentage of the basin that is within the noted hydrologic region and the hydrologic region name. In the example for a site in New Hampshire, the basin is 100 percent within the Low Flow Statewide region.
The columns in the parameters tables list the names of the parameters, their computed values, and then the minimum (Min) and maximum (Max) limit values, respectively. Definitions for the parameters can be found on the Basin Characteristic Definitions page.
The Min and Max limit values provide the ranges of the values of the basin characteristics that were available for the streamgages used to develop the regression equations. If any of the computed values of the parameters for the user-selected site are outside of the Min/Max range the value is highlighted in orange in the table and a message will appear below the table indicating that “One or more of the parameters is outside the suggested range. Estimates were extrapolated with unknown errors.” This is further discussed below.
If any of the basin characteristics used as parameters in the regression equations were edited before creating a report, the report will contain a warning message above the scenario sections, such as the one below. Messages also will be provided if the basin boundary has been edited, or the selected point was in an exclusion area. For Colorado, when applicable, a disclaimer also will be provided to warn that the flow at the selected site is affected by dam regulations. In addition, if any of the basin characteristics used as explanatory variables in the regression equations are outside of the given min limit or max limit, then the value of the basin characteristic will be highlighted in orange and an orange disclaimer will be provided.
The columns in the streamflow statistics tables list the names of the statistics, their values, and their units of measure, respectively, followed by any indicators that are available of the errors that are associated with the estimates. No error indicators will be displayed when any of the parameters are outside their indicated Min and Max ranges. Definitions for the statistics can be found on the Streamflow Statistics Definitions page.
Regression equations for estimating streamflow statistics usually are developed as part of studies for an entire state. As part of the studies, the states very often are divided geographically or on some other basis into regions of similar hydrology. In most cases, delineated drainage areas for user-selected sites will fall within a single hydrologic region within a state. However, in some cases the drainage areas will span multiple regions. When this occurs, StreamStats reports will present flow estimates based on the regression equations for each hydrologic region and also area-averaged estimates that are determined by weighting the flow estimates for each region by the percentage of the drainage area that is within the region.
The StreamStats Web application provides access to automated procedures and very large, complex data sets. These data sets are known to contain occasional errors. Users are advised to carefully check all results for accuracy and to exercise their own professional judgment in evaluating the appropriateness of the results for their application. Basin delineations, in particular, frequently have been found to be erroneous. StreamStats provides tools and base maps useful for verifying the accuracy of the basin delineations and for correcting them, if necessary.
Estimates of streamflow statistics provided by StreamStats for user-selected ungaged sites are determined by solving USGS-developed regression equations that have been compiled for the nation in the National Streamflow Statistics (NSS) program, which StreamStats uses 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 also are listed on the State/Regional Info tab 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.
Indicators of the errors associated with estimates of streamflow statistics provided by StreamStats may include the equivalent years of record, the average standard error of estimate, the average standard error of prediction, the minimum and maximum values of the 90-percent prediction interval, and the percent correct. Typically, only some of these indicators will be available for an estimated statistic. Definitions for the error indicators are:
- Equivalent years of record indicates the number of years that a streamgage would have operated at the user-selected site to obtain an estimate of the streamflow statistic that is equal in accuracy to the estimate provided by the regression equation.
- Average standard error of estimate (labeled as estimate error in the output) measures the average variation between the regression estimates and estimates derived from the data collected for the streamgages used to develop the regression equations. About two-thirds of the regression estimates for the streamgages used in the regression analyses have errors less than the average standard error of estimate. About one-third of the estimates have errors larger than the average standard error of estimate.
- Average standard error of prediction (labeled as prediction error in the output) measures the average accuracy of the regression equations when predicting values for user-selected sites, which is the condition under which regression equations are most often applied for StreamStats. The average standard error of prediction is usually a few percent larger than the average standard error of estimate. About two-thirds of the regression estimates for ungaged sites will have errors less than the given average standard errors of prediction, and about one-third of estimates will have errors larger than the given standard errors of prediction.
- Minimum (Min in report) and maximum (Max in report) 90-percent prediction interval indicates the range of flows for which estimates at user-selected sites will have actual flows that are within the given values 90 percent of the time.
- Percent correct applies only to probability estimates, such as the probability of perennial flow. It indicates the percentage of the stations that were used to develop the equation that had correct estimates determined using the equation.
StreamStats will provide estimates of flow statistics but error indicators will not be provided when any of the basin characteristics used as explanatory variables in the regression equations are not within the Min and Max limit values provided in the parameters tables. In these circumstances, the flow statistics are estimated by extrapolation. Although extrapolated estimates are often the best estimates that can be obtained for the site, the errors associated with them are unknown and could be substantial.
When the delineated basin for a user-selected site is in more than one hydrologic region, StreamStats normally provides tables of the basin characteristics for each region, tables of the streamflow statistics estimated using the regression equations for each region, and a table of area-averaged estimates, as seen in the example below for a site in New Jersey, which has 67.35 percent of its area in the Central region and 32.65 percent in the Piedmont region. The area-averaged estimates are obtained by multiplying the estimated flow for each region by the drainage area for each region, summing these values, and then dividing by the total drainage area. If available, prediction errors and equivalent years of record are computed by the same weighting method. Prediction errors and equivalent years of record will be provided for area-averaged estimates only if all regions have this information available.
Estimates of streamflow statistics for user-selected sites with basin characteristics that are not within the given Min and Max values have errors that are of unknown magnitude. When this occurs, the output will have the computed value highlighted in red, and a warning message will be displayed at the bottom of the section.
- Reports for some states recommend that only the regression equations for the hydrologic region in which the selected site is located should be used to determine estimates of streamflow statistics for the site. Virginia is an example. In these cases, StreamStats operates as suggested in the reports and does not provide area-weighted estimates.
- 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 State/Regional Info tab of the About link above the map frame.
Flow estimates obtained from regression equations for watersheds that span state boundaries may give different results depending on which state’s equations are used. Unless otherwise stated, each state’s regression equations 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. This is the same method that StreamStats uses to determine area-averaged estimates for sites with drainage area in multiple hydrologic regions within a state.
Programing and data limitations usually do not allow StreamStats to provide estimates using the regression equations for each state with area in a delineated basin. In cases where StreamStats is available for each state, it may be possible to determine area-averaged estimates by use of the following process:
- Zoom into the vicinity of site of interest, select the state in which the site is located, delineate the drainage basin, obtain all possible basin characteristics and the desired flow estimates, and then save the output.
- Refresh the StreamStats user interface and then follow the same process as in step 1, except select the upstream state and then select the point for delineation just upstream from where the stream of interest crosses the state border.
- 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:
- 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 before building the report, click on the Show Basin Characteristics button and change the computed basin characteristics to be those from the output for the downstream state. Build the report after making the changes and the adjusted flows will appear in the output.
- 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 Show Basin Characteristics 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.
- If neither of the above two conditions exists, then it will not be possible to weight the flow estimates from the two state applications.
- 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 estimates 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).