# Illinois Streamstats Version History

Questions regarding the issues described below should be directed to support@streamstats.freshdesk.com.

__July 2021__: Urban peak-flow equations from Over and others (2016) updated with the weighted skew coefficient values for the calculation of at-site flood frequencies. Previous equations were using the regional skew only.

__July 2016__: Urban peak-flow equations from Over and others (2016) are implemented.

__July 2015__: Illinois StreamStats migrated to version 3. Error notice of February 2015 (below) is not relevant to current StreamStats results, but may be of concern to some users of StreamStats Version 2 between May 2010 and July 2015.

__February 2015__ (with clarification on the Basin Characteristics bug added in March 2015):

## Potential errors in peak-flow estimates for user-selected sites in Region 2

On Feb. 24, 2015, it was found that StreamStats has been providing erroneous peak-flow estimates in some cases for user-selected sites in hydrologic region 2, in northeastern Illinois. This area appears in pink when the "Peak-flow regions" layer is displayed on the version 2 StreamStats map. The errors in the flow estimates are occurring because the version 2 StreamStats *Estimate Flows Using Regression Equations* tool has been incorrectly computing the percent area of open water and herbaceous wetlands (%Water in the report, WATWET in StreamStats) in cases where both this basin characteristic and main channel slope (MCS in the report, and CSL10_85 in StreamStats) are used as explanatory variables in the regression equations. **The errors have been found only occasionally when all or part of the delineated drainage basin for a user-selected site is outside of Illinois.** When errors occur, the values of open water and herbaceous wetlands are larger than they should be, sometimes by an order of magnitude, leading to peak-flow estimates that are lower than they should be. The erroneous values may also appear in the Basin Characteristics output for any region in which part of the delineated basin is outside of Illinois, unless the percent area of open water and herbaceous wetlands is computed alone, without the other basin characteristics.

The StreamStats team is working to determine why these errors are occurring and to correct the problem. It is not known precisely when the errors began occurring, but it is possible that they may have been occurring since 2010. Users can determine if they have received erroneous flow estimates by (1) using the *Basin Characteristics* tool, with all other basin characteristics turned off, to get the percentage of open water and herbaceous wetlands for your selected site, and then (2) comparing the value from the Basin Characteristics tool with the value in the output from the *Estimate Flows Using Regression Equations* tool. If the values agree closely, then the flow estimates are correct. If they disagree, then the value from the *Basin Characteristics* tool output is correct. The *Edit Parameters and Recompute Flows* tool can be used to correct the value of open water and herbaceous wetlands and recompute the flow estimates.

__March 2014 and November 2014__: This notice appeared in March 2014, but database updates may have caused the issue to recur up to November 2014.

## Peak-flow statistics for all gaging stations are now consistent with SIR_2004_5103

Previous to March 2014, an earlier version of the peak-flow statistics for 23 gaging stations had been inadvertently entered to the StreamStats streamflow statistics database. The peak-flow statistics published in SIR_2004_5103 should have been entered, because these are the preferred values. The differences were caused by the inclusion or exclusion of historical (non-systematic) peaks in the gaging station record used in the flood-frequency analysis. These values have now (March 2014) been corrected. The 23 stations were:

03337500, 03338000, 03378900, 05437000, 05439500, 05440000, 05441000, 05441500, 05442000, 05448000, 05466000, 05467500,05468500,05469000, 05469500, 05502040, 05512500, 05513000, 05527500, 05559000, 05580500, 05592050, 05592500

__November 2013__:

## Peak-flow statistics for 23 gaging stations are not consistent with SIR_2004_5103

An earlier version of the peak-flow statistics for 23 gaging stations was inadvertently entered to the StreamStats streamflow statistics database. The peak-flow statistics published in SIR_2004_5103 should have been entered, because these are the preferred values. The differences are caused by the inclusion or exclusion of historical (non-systematic) peaks in the gaging station record used in the flood-frequency analysis. Until the database is updated with the published values, the peak-flow statistics published in SIR_2004_5103 should be used for gage-weighted computations for these 23 stations. This does not affect the Estimate Flows Based on Similar Gages tool computations that do not use any of these 23 stations, nor any ungaged computations using the Estimate Flow Using Regression Equation tool. The 23 stations are:

03337500, 03338000, 03378900, 05437000, 05439500, 05440000, 05441000, 05441500, 05442000, 05448000, 05466000, 05467500, 05468500, 05469000, 05469500, 05502040, 05512500, 05513000, 05527500, 05559000, 05580500, 05592050, 05592500

__May 2012__:

## Errors in the computation of prediction intervals for Region 7 were corrected on May 10, 2012

Typographical errors were found in the data needed for computing prediction intervals for some peak-flow equations for Region 7 from Illinois report number SIR_2004_5103. These errors, which now have been corrected, created inaccurate prediction intervals for the PK2, PK10, PK25, PK50, and PK 500 flood estimates. While the peak-flow estimates themselves have not changed as a result of the modifications, the prediction intervals provided previously by StreamStats may have been in error by more than 100% in some cases.

__October 2007 – Initial release of Illinois StreamStats__:

The data and methods used by StreamStats to compute basin characteristics are somewhat different than those that were described by Soong and others (2004). Drainage areas and water percentages from StreamStats were not significantly different than those published in the Soong and others (2004) report. StreamStats main-channel slopes and basin lengths required adjustments to remove bias when compared to the published values. Even after application of the corrections for bias, the main-channel slopes were slightly different from the published values on a statewide basis, but the differences were not statistically significant. (The definitions for main channel slope are found on page 73 of Soong and others (2004), and for StreamStats, in the Basin Characteristics link, where it is listed as Stream_Slope_10-85_Method.) Where the differences exceeded 60 percent, the StreamStats slopes were higher, resulting in rural flood-quantile estimates that are larger than the published regional equation estimates.

A very small mean difference in average permeability (about 0.25%) was found on a statewide basis, but the sensitivity of the equations to this basin characteristic is also very small, so no correction was applied. Both the adjusted and unadjusted values for main channel slope and basin length can be displayed by selecting the *Basin Characteristics* tool after delineating a watershed; however, only the adjusted value is displayed on the Streamflow Statistics report obtained by using the *Estimate Flows Using Regression Equations* tool.

The overall differences in rural flood-peak discharges resulting from the differences in basin characteristics were statistically insignificant on a sampled region-by-region basis, except for Region 1, which had mean differences ranging from 3.8% for the 2-year peak discharge estimate to 4.3% for the 500-year peak discharge estimate, where differences are computed as 100*(2004 estimate-StreamStats estimate)/2004 estimate. These mean differences are less than one-tenth of the average prediction error for the regression-equation estimates. However, the percent differences can be large for small watersheds of less than 0.5 square miles. The automatic delineation of watersheds should be reviewed and, if necessary, corrected using the *Edit a Delineated Basin* tool.