The U.S. Geological Survey (USGS), in cooperation with Connecticut Department of Transportation, completed a study to improve flood-frequency estimates in Connecticut. This companion data release is a Microsoft Excel workbook for: (1) computing flood discharges for the 50- to 0.2-percent annual exceedance probabilities from peak-flow regression equations, and (2) computing additional prediction intervals, not available through the USGS StreamStats web application. The current StreamStats application (version 4) only computes the 90-percent prediction interval for stream sites in Connecticut. The Excel workbook can be used to compute the 70-, 80-, 90-, 95-, and 99-percent prediction intervals. The prediction interval provides upper and lower limits of the estimated flood discharge with a certain probability, or level of confidence in the accuracy of the estimate. The standard error of prediction for the Connecticut peak-flow regression equations ranged from 26.3 to 45.0 percent (Ahearn and Hodgkins, 2020). The Excel workbook consists of four worksheets. The worksheets provide an overview of how the application works; input and output tables of the explanatory variables and flood discharges, and graphical display of the results; and the computational formulas used to estimate the flood discharges and prediction intervals.

These data include annual instantaneous maximum discharge record for 156 streams and rivers in Vermont and adjacent areas of New Hampshire, Massachusetts, and New York through the 2023 water year. The data also include flood-frequency estimates for the 50-, 20-, 10-, 4-, 2-, 1-, 0.5-, and 0.2-percent annual exceedance probabilities that were analyzed using the discharge data. These flood-frequency data were used to develop regression equations for estimating the magnitude of floods at the selected annual exceedance probabilities on ungaged, unregulated, rural streams in Vermont. The regression equations use the basin characteristics of drainage area, percentage of wetland area in the basin, and the basinwide mean of the annual precipitation as explanatory variables. These basin characteristics for the streamgages and the Geographic Information System datasets for computing the basin characteristics are also in this data release. This data release is structured with the streamgage data, the input and output files for the version 7.5.1 PeakFQ software (U.S. Geological Survey, 2024) used to conduct flood-frequency analysis, the flood-frequency results, the streamgage basin characteristics, and the geographic information system datasets used to determine the basin characteristics on the main page. There is one sub-page which is used as a model archive for the regression analysis. U.S. Geological Survey, 2024, Water resources application software, PeakFQ, accessed March 4, 2024, at http://water.usgs.gov/software/peakfq.html.

To improve flood-frequency estimates at rural streams in Mississippi, annual exceedance probability (AEP) flows at gaged streams in Mississippi and regional-regression equations, used to estimate annual exceedance probability flows for ungaged streams in Mississippi, were developed by using current geospatial data, additional statistical methods, and annual peak-flow data through the 2013 water year. The regional-regression equations were derived from statistical analyses of peak-flow data, basin characteristics associated with 281 streamgages, the generalized skew from Bulletin 17B (Interagency Advisory Committee on Water Data, 1982), and a newly developed study-specific skew for select four-digit hydrologic unit code (HUC4) watersheds in Mississippi. Four flood regions were identified based on residuals from the regional-regression analyses. No analysis was conducted for streams in the Mississippi Alluvial Plain flood region because of a lack of long-term streamflow data and poorly defined basin characteristics. Flood regions containing sites with similar basin and climatic characteristics yielded better regional-regression equations with lower error percentages. The generalized least squares method was used to develop the final regression models for each flood region for annual exceedance probability flows. The peak-flow statistics were estimated by fitting a log-Pearson type III distribution to records of annual peak flows and then applying two additional statistical methods: (1) the expected moments algorithm to help describe uncertainty in annual peak flows and to better represent missing and historical record; and (2) the generalized multiple Grubbs-Beck test to screen out potentially influential low outliers and to better fit the upper end of the peak-flow distribution. Standard errors of prediction of the generalized least-squares models ranged from 28 to 46 percent. Pseudo coefficients of determination of the models ranged from 91 to 96 percent. Flood Region C, located in the southwest corner of Mississippi, contained 120 streamgages with drainage areas that ranged from 0.05 to 1,010 square miles. The 1% annual exceedance probability had a standard error of prediction of 41 percent.

To improve flood-frequency estimates at rural streams in Mississippi, annual exceedance probability (AEP) flows at gaged streams in Mississippi and regional-regression equations, used to estimate annual exceedance probability flows for ungaged streams in Mississippi, were developed by using current geospatial data, additional statistical methods, and annual peak-flow data through the 2013 water year. The regional-regression equations were derived from statistical analyses of peak-flow data, basin characteristics associated with 281 streamgages, the generalized skew from Bulletin 17B (Interagency Advisory Committee on Water Data, 1982), and a newly developed study-specific skew for select four-digit hydrologic unit code (HUC4) watersheds in Mississippi. Four flood regions were identified based on residuals from the regional-regression analyses. No analysis was conducted for streams in the Mississippi Alluvial Plain flood region because of a lack of long-term streamflow data and poorly defined basin characteristics. Flood regions containing sites with similar basin and climatic characteristics yielded better regional-regression equations with lower error percentages. The generalized least squares method was used to develop the final regression models for each flood region for annual exceedance probability flows. The peak-flow statistics were estimated by fitting a log-Pearson type III distribution to records of annual peak flows and then applying two additional statistical methods: (1) the expected moments algorithm to help describe uncertainty in annual peak flows and to better represent missing and historical record; and (2) the generalized multiple Grubbs-Beck test to screen out potentially influential low outliers and to better fit the upper end of the peak-flow distribution. Standard errors of prediction of the generalized least-squares models ranged from 28 to 46 percent. Pseudo coefficients of determination of the models ranged from 91 to 96 percent. Flood Region C, located in the southwest corner of Mississippi, contained 120 streamgages with drainage areas that ranged from 0.05 to 1,010 square miles. The 1% annual exceedance probability had a standard error of prediction of 41 percent.

Current estimates of the magnitude and frequency of floods at gaged and ungaged stream sites are critical for assessing flood risk, delineating flood zones, designing hydraulic structures, and managing flood plains. The Connecticut Department of Transportation collaborated with U.S. Geological Survey (USGS) in a study to improve the flood-frequency estimates in Connecticut and develop regional regression equations for estimating annual exceedance probability discharges at ungaged sites in Connecticut. The results of the study are found in Scientific Investigations Report (add link). This companion data release consists of data compiled and used for the flood-frequency analysis of annual peak flows and development of regional regression equations for Connecticut. The data release is composed of four tables that include: 1) descriptions and locations of streamgages for flood frequency analysis of annual peak flows; 2) updated flood discharges for 152 streamgages for the 50-, 20-, 10-, 4-, 2-, 1-, 0.5-, and 0.2-percent annual exceedance probabilities (AEP) using annual peak data through water year 2015; 3) representation of peak flow data using perception thresholds and interval data in the flood frequency analysis to describe uncertainty and represent missing and historical records; and 4) basin and climatic characteristics that were included as explanatory variables in the regression equations.

Current estimates of the magnitude and frequency of floods at gaged and ungaged stream sites are critical for assessing flood risk, delineating flood zones, designing hydraulic structures, and managing flood plains. The Connecticut Department of Transportation collaborated with U.S. Geological Survey (USGS) in a study to improve the flood-frequency estimates in Connecticut and develop regional regression equations for estimating annual exceedance probability discharges at ungaged sites in Connecticut. The results of the study are found in Scientific Investigations Report (add link). This companion data release consists of data compiled and used for the flood-frequency analysis of annual peak flows and development of regional regression equations for Connecticut. The data release is composed of four tables that include: 1) descriptions and locations of streamgages for flood frequency analysis of annual peak flows; 2) updated flood discharges for 152 streamgages for the 50-, 20-, 10-, 4-, 2-, 1-, 0.5-, and 0.2-percent annual exceedance probabilities (AEP) using annual peak data through water year 2015; 3) representation of peak flow data using perception thresholds and interval data in the flood frequency analysis to describe uncertainty and represent missing and historical records; and 4) basin and climatic characteristics that were included as explanatory variables in the regression equations.

One-percent annual exceedance probability (AEP) flood-flow estimates were computed at flood insurance study (FIS) locations across Pennsylvania using methods identified in Scientific Investigation Report (SIR) 2019-5094. Following guidance outlined in SIR 2016-5149, valid statistical reaches (VSRs) were identified for streamgages, which were used to assist with the determination of the applicable method used to compute a USGS-derived 1-percent AEP flood-flow estimate at an FIS location. Methods included: weighting, weighting and transferring, and regression equations. The USGS-derived 1-percent AEP flood-flow estimates were then compared to 1-percent AEP flood-flow estimates published in FIS's and furnished by the Federal Emergency Management Agency (FEMA).

One-percent annual exceedance probability (AEP) flood-flow estimates were computed at flood insurance study (FIS) locations across Pennsylvania using methods identified in Scientific Investigation Report (SIR) 2019-5094. Following guidance outlined in SIR 2016-5149, valid statistical reaches (VSRs) were identified for streamgages, which were used to assist with the determination of the applicable method used to compute a USGS-derived 1-percent AEP flood-flow estimate at an FIS location. Methods included: weighting, weighting and transferring, and regression equations. The USGS-derived 1-percent AEP flood-flow estimates were then compared to 1-percent AEP flood-flow estimates published in FIS's and furnished by the Federal Emergency Management Agency (FEMA).

These data include flood-frequency data for the 50-, 20-, 10-, 4-, 2-, 1-, 0.5-, and 0.2-percent annual exceedance probabilities (pfqprocessed.zip) and the basin characteristics (siteinfoVT.txt) for 156 streamgages in Vermont and adjacent areas in surrounding states. These data were used as input to the regression model developed for estimating the magnitude of floods at the selected annual exceedance probabilities on ungaged, unregulated, rural streams in Vermont. The model code is provided in R script language (R Core Team, 2024) and utilized the generalized-least-squares regression tools in the WREG package (Farmer, 2021) to develop the regression equations to estimated flood discharge in Vermont. The model output (VT_Regression_Equations.pdf) provides code and the regression equations. Farmer, W.H., 2021, WREG—Weighted least squares regression for streamflow frequency statistics (ver. 3.0): U.S. Geological Survey software release, accessed January 31, 2025, at https://doi.org/10.5066/P9ZCGLI1. R Core Team, 2024, R: A language and environment for statistical computing version 4.4.0: R Foundation for Statistical Computing, Vienna, Austria, accessed July 18, 2024 at, https://www.R-project.org.

The U.S. Geological Survey Central Midwest Water Science Center completed a report (Over and others, 2023) documenting the methods, results, and applications of an updated flood-frequency study for the State of Illinois. This data release contains data related to the analysis completed to determine peak-flow quantiles (flood frequency estimates) at streamgages in Illinois for 50-, 20-, 10-, 4-, 2-, 1-, 0.5-, and 0.2-percent annual exceedance probabilities (AEPs), as well as data used to develop regional regression equations that relate the peak-flow quantiles and the basin characteristics of selected streamgages in Illinois, Indiana, and Wisconsin, based on data through water year 2017 (a water year is the period from October 1 to September 30 and is designated by the year in which it ends; for example water year 2017 was from October 1, 2016 to September 30, 2017). There is one set of equations for each of Illinois’ seven hydrologic regions, and, for each region, one equation for each AEP. These regional equations allow the estimation of peak-flow quantiles at ungaged locations throughout the state and their uncertainties. This data release contains a figure of the study area in addition to seven tables that provide both input data and results in tabular format. See metadata for detailed information.