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Development of Regression Equations to Estimate Flow Durations, Low-Flow Frequencies, and Mean Flows at Ungaged Stream Sites in Connecticut Using Data Through Water Year 2022

Scientific Investigations Report 2025-5027
Prepared in cooperation with the Connecticut Department of Energy and Environmental Protection and the Connecticut Department of Transportation
By:  and 
https://doi.org/10.3133/sir20255027

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Abstract

To aid Federal and State regulatory agencies in the effective management of water resources, the U.S. Geological Survey, in cooperation with the Connecticut Department of Energy and Environmental Protection and the Connecticut Department of Transportation, updated flow statistics for 118 streamgages and developed 47 regression equations to estimate selected flow duration, low flow, and mean flow statistics for the entire State of Connecticut, for the following: 1-, 5-, 10-, 25-, 50-, 75-, 90-, 99-percent flow durations; 7-day, 10-year low-flow frequency and 30-day, 2-year low-flow frequency; and mean flow, spring mean flow, and harmonic mean flow. In addition, regression equations were developed for monthly and seasonal flow durations, ranging from 25 to 99 percent for aquatic biological processes of salmonid spawning (November), overwinter (December–February), clupeid spawning (May), resident spawning (June), and rearing and growth (July–October) periods, and for flow durations ranging from 1 to 99 percent for the habitat forming (March–April) period. Statistics were derived from daily mean streamflow data collected from streamgages with at least 10 years of data through water year 2022 in southern New England and eastern New York.

Forty streamgages in Connecticut and adjacent areas of neighboring States were used in the regression analysis. Regression methods of weighted least squares and generalized least squares were used to derive the final coefficients and measures of uncertainty for the regression equations. The equations used to estimate selected streamflow statistics were developed by relating the flow statistics to different basin characteristics (physical, land cover, and climatic) at the 40 streamgages. Nine basin characteristics served as the explanatory variables in the statewide regression equations: drainage area, percentage of area with coarse-grained stratified deposits, stream density, mean basin slope, mean basin elevation, percentage of area with hydrologic soil group A, mean monthly precipitation for November, mean seasonal precipitation in the winter (December, January, and February), and mean annual temperature. The root mean square error of the 47 equations ranged from 7.9 to 121.9 percent, with an average of 27.9 percent. The equations estimate flows most accurately near the mean (50-percent flow duration), become less accurate for low flows, and are the least accurate for extreme low flows. The root mean square error for the 50-percent flow duration is 15.1 percent, with an average of 17.6 percent across the six periods. The extreme low flow statistics of 7-day, 10-year low-flow frequency, 99-percent flow duration, and 99-percent rearing and growth period flow durations have root mean square errors of 121.9, 105.1, and 121.9 percent, respectively. The adjusted coefficient of determination of the 47 equations ranged from 73.4 to 99.5 percent, with an average of 95.1 percent.

Plain Language Summary

The U.S. Geological Survey, the Connecticut Department of Energy and Environmental Protection, and the Connecticut Department of Transportation collaboratively updated flow statistics for 118 streamgages and developed 47 regression equations to estimate key flow statistics in Connecticut. These included various flow durations and low-flow frequencies, as well as mean flow statistics for specific aquatic biological processes. The analysis used daily mean streamflow data from 40 streamgages with at least 10 years of data and incorporated basin characteristics such as drainage area and precipitation. The equations were most accurate near the mean flow (50-percent flow duration), with an average root mean square error of 27.9 percent, while accuracy decreased for low and extreme low flows. The adjusted coefficient of determination ranged from 73.4 to 99.5 percent, averaging 95.1 percent.

Suggested Citation

Ahearn, E.A., and Bent, G.C., 2025, Development of regression equations to estimate flow durations, low-flow frequencies, and mean flows at ungaged stream sites in Connecticut using data through water year 2022: U.S. Geological Survey Scientific Investigations Report 2025–5027, 54 p., https://doi.org/10.3133/sir20255027.

ISSN: 2328-0328 (online)

Study Area

Table of Contents

  • Abstract
  • Introduction
  • Purpose and Scope
  • Previous Studies
  • Physical Setting
  • Computation of Streamflow Statistics at Streamgages
  • Statistical Analysis of Trends in the Annual 7-Day Low Flows
  • Basin and Climatic Characteristics of Streamgages
  • Development of Regression Equations for Estimating Selected Flow Statistics
  • Summary
  • Acknowledgments
  • Selected References
  • Appendix 1. Streamgages Used To Estimate Flow-Durations, Low-Flow Frequencies, and Mean Flows at Ungaged Stream Sites in Connecticut
Publication type Report
Publication Subtype USGS Numbered Series
Title Development of regression equations to estimate flow durations, low-flow frequencies, and mean flows at ungaged stream sites in Connecticut using data through water year 2022
Series title Scientific Investigations Report
Series number 2025-5027
DOI 10.3133/sir20255027
Publication Date July 23, 2025
Year Published 2025
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) New England Water Science Center
Description Report: vi, 54 p.; Data Release
Country United States
State Connecticut
Online Only (Y/N) Y
Additional Online Files (Y/N) N
Additional publication details