Methods for Estimating Selected Low-Flow Frequency and Mean Annual Flow Statistics at Gaged and Ungaged Locations on Streams in Georgia, North Carolina, and South Carolina
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- Document: Report (9.25 MB pdf) , HTML , XML
- Plate: Plate 1 (2.92 MB pdf) - Map showing study area, Level III and selected Level IV ecoregions, and locations of 843 U.S. Geological Survey streamgages considered for inclusion in regional regression analysis in Georgia, North Carolina, South Carolina, and adjacent portions of Alabama, Florida, Tennessee, and Virginia
- Data Release: USGS Data Release - Methods for estimating selected low-flow frequency and mean annual flow statistics at gaged and ungaged locations on streams in Georgia, North Carolina, and South Carolina, 2022—Data
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Abstract
The U.S. Geological Survey, in cooperation with the Georgia Department of Natural Resources (Environmental Protection Division), North Carolina Department of Environmental Quality (Division of Water Resources), North Carolina Department of Public Safety (Office of Recovery and Resiliency), and South Carolina Department of Environmental Services, updated low-flow frequency, mean annual flow, and flow-duration statistics at 843 streamgages in and near Georgia, North Carolina, and South Carolina. The low-flow frequency statistics are annual minimum 1-day average flow with a 10-year recurrence interval (1Q10), annual minimum 7-day average flow for 2- and 10-year recurrence intervals (7Q2 and 7Q10, respectively), and annual minimum 30-day average flow with 2- and 3-year recurrence intervals (30Q2 and 30Q3, respectively). Monthly 1Q10 and 7Q10, and W7Q10 flow statistics for the winter period (November–March) also are presented. By using data from 604 of the streamgages on streams with streamflows that are not substantially affected by regulation or diversion and are not tidally influenced, regional regression equations were developed to predict flow statistics with prediction intervals at ungaged locations on streams with those same criteria. The regional regression analysis included data from 132 streamgages from adjacent States Alabama, Florida, Tennessee, and Virginia. The final regional regression equations include variables such as drainage area, streamflow variability, precipitation, percentage of impervious area, and percentage of the basin in various ecoregions. The low-flow statistics for the streamgages analyzed and the regional regression equations will be integrated into the U.S. Geological Survey StreamStats application (https://www.usgs.gov/streamstats) for Georgia, North Carolina, and South Carolina. StreamStats generates basin characteristics needed to compute low-flow frequency statistics for ungaged locations.
A trend analysis of annual minimum 7-day average flows was done for 78 streamgages with at least 30 years of continuous record. Trends were evaluated for 30-, 50‑, 70-, and 90-year periods, ending in climate year 2021, and independence and short- and long-term persistence assumptions were considered. For all trend analysis assumptions, most streamgages did not exhibit significant trends in annual minimum 7-day average flows. Trends in annual precipitation and air temperature were similarly evaluated for the period 1895–2021 to assess the variability of climate for Georgia, North Carolina, and South Carolina.
Plain Language Summary
Low flow is the sustained flow in a stream, supplied by groundwater. Low flows are important to Federal, State, regional, and local agencies and municipalities making decisions related to water quality, supply planning, infrastructure design, and more.
In this study, the U.S. Geological Survey, in cooperation with several State agencies, calculated low-flow statistics for select streamgages in and near Georgia, North Carolina, and South Carolina. A subset of those streamgages where streamflows were not substantially affected by water regulation (for example, by reservoirs and dams), diversion, or tides were used to develop regional regression equations to estimate mean annual flow and 30 low-flow statistics.
Benefits of using the three-State region include having (1) a larger dataset for modeling regional regression equations, which typically improves statistical analyses, and (2) regional regression equations that are consistent across the three-State region and particularly beneficial for basins that cross State boundaries.
As a result of this study, low-flow statistics for the streamgages analyzed and regional regression equations will be integrated into the U.S. Geological Survey StreamStats online application for computing low-flow statistics, including at ungaged locations on streams not substantially affected by regulation or diversion and not affected by tides. The low-flow frequency regression equations and resulting statistics are important for processes such as State water plans, permitting, evaluating drought impacts, and more.
Suggested Citation
Feaster, T.D., Harken, B.J., Aulenbach, B.T., Kolb, K.R., Mitchell, C.E., and Weaver, J.C., 2026, Methods for estimating selected low-flow frequency and mean annual flow statistics at gaged and ungaged locations on streams in Georgia, North Carolina, and South Carolina: U.S. Geological Survey Scientific Investigations Report 2026–5021, 48 p., https://doi.org/10.3133/sir20265021.
ISSN: 2328-0328 (online)
Study Area
Table of Contents
- Acknowledgments
- Abstract
- Plain Language Summary
- Introduction
- Purpose and Scope
- Previous Studies
- Description of Study Area
- Low-Flow Frequency and Flow-Duration Statistics at Gaged Locations
- Long-Term Trend Analyses for Annual Minimum 7-Day Average Flows, Precipitation, and Air Temperature in Georgia, North Carolina, and South Carolina
- Streamflow Record Selection for Regional Regression Analysis
- Estimating Low-Flow Frequency and Mean Annual Flow Statistics at Ungaged Locations
- Application of Low-Flow Frequency Methods
- Summary
- References Cited
| Publication type | Report |
|---|---|
| Publication Subtype | USGS Numbered Series |
| Title | Methods for estimating selected low-flow frequency and mean annual flow statistics at gaged and ungaged locations on streams in Georgia, North Carolina, and South Carolina |
| Series title | Scientific Investigations Report |
| Series number | 2026-5021 |
| DOI | 10.3133/sir20265021 |
| Publication Date | July 08, 2026 |
| Year Published | 2026 |
| Language | English |
| Publisher | U.S. Geological Survey |
| Publisher location | Reston, VA |
| Contributing office(s) | South Atlantic Water Science Center |
| Description | Report: x, 48 p.; 1 Plate: 36.00 x 36.00 inches; Data Release |
| Country | United States |
| State | Georgia, North Carolina, South Carolina |
| Online Only (Y/N) | Y |
| Additional Online Files (Y/N) | N |