Ungaged Site Reports
The output from StreamStats for ungaged sites 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, "Streamflow Statistics Report" 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, and total drainage are provided for future reference. Below this information is a series of two or more tables.
Most states have been subdivided into hydrologic regions based on similarity of climate and physical characteristics, and separate regression equations have been developed for each region and each flow statistic. The ungaged site reports list only the basin characteristics that are used in the regression equations for the hydrologic region in which the site is located.
The reports will always include at least one table for peak-flow basin characteristics and one table for peak-flow statistics. If equations are available in the region for other types of streamflow statistics, additional tables will appear to show the basin characteristics needed to estimate the statistics and to show the statistics. In addition, if the drainage basin for the ungaged site is located in more than one region, basin characteristics and streamflow statistics will appear for each region along with tables that give area-averaged estimates of the streamflow statistics for the site.
Basin Characteristics Tables:
- The top line in the table identifies the type of streamflow statistics for which the basin characteristics were measured.
- If the ungaged site has a basin boundary in two or more regions, header lines appear above the basin characteristics listed for each region. This line contains the percentage of the basin area that is within the region, the name of the region and the drainage area, in square miles, in the region.
- The names of the basin characteristics needed to compute the streamflow statistics for the region are listed in the first column, named Parameters, along with the units of measure. These are brief names. More detailed definitions of the basin characteristics can be found on the Basin Characteristic page. Names shown in the table above correspond to the Statistic Name field in the linked table.
- The second column, named Value, contains the values of the basin characteristics. These values are for the entire area of the drainage basin, not just the values within the region.
- The third and fourth columns, named Min and Max, contain the minimum and maximum values of the basin characteristics that can be entered to obtain estimates of streamflow statistics with known errors.
- In the example above, the mean basin elevation computed for the site is lower than the minimum value shown for region 2. Because of this, the message “below min value 2966.3” is shown along with the value in the Value column in the table. In addition, the message “Warning: some parameters are outside the suggested range. Estimates will be extrapolations with unknown errors” appears below the table.
Streamflow Statistics Tables:
- The top line in the table identifies the type of streamflow statistics and the region.
- The names of the statistics are shown in the first column, named Statistic. These are brief names. For peak-flow statistics, the names shown, Q1.5 to Q500, represent the 1.5- to 500-year recurrence interval floods. More detailed definitions of other streamflow statistics can be found on the Statistics page. Names in the table above correspond to the StatLabel field in the linked table.
- The estimated streamflows are shown in the second column, named Flow, in cubic feet per second.
- The average standard errors of prediction, in percent, are shown in the third column, named Prediction Error. About two thirds of the estimated statistics for ungaged sites will have errors within the percentages shown. Because percentage errors are not normally distributed, negative percentage errors tend to be smaller than positive percentage errors.
- The equivalent years of record are shown in the fourth column. These values indicate the length of time that a streamgaging station would need to be operated at the ungaged site to obtain an estimate of the streamflow statistic that is equal in accuracy to the estimate provided by the regression equation.
- The fifth and sixth columns contain the minimum and maximum values of the 90-percent prediction interval. There is a 90-percent probability that the actual flow for the ungaged site is between the given values.
- Values in the fourth through sixth columns are not available for some equations in some regions. They also will not be shown for estimates in a region when the values for any of the basin characteristics are outside of the Min and Max values shown in the Basin Characteristics tables.
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 information shown in the table is very similar to a normal streamflow statistics table.
- The 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. 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.
