Low Flow of Streams in the Susquehanna River Basin of New York Scientific Investigations Report 2010–5063 U.S. Department of the Interior U.S. Geological Survey Cover. Cuthrie Run, 3 miles north of Big Flats, Chemung County, New York, within an upland valley that drains 5.3 square miles. Photograph by Matthew B. Jennings, U.S. Geological Survey, June 29, 2007, when streamflow in this region was moderately low (exceeded about 80 percent of the time). Low Flow of Streams in the Susquehanna River Basin of New York By Allan D. Randall Scientific Investigations Report 2010–5063 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior KEN SALAZAR, Secretary U.S. Geological Survey Marcia K. McNutt, Director U.S. Geological Survey, Reston, Virginia: 2010 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment, visit http://www.usgs.gov or call 1-888-ASK-USGS For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/pubprod To order this and other USGS information products, visit http://store.usgs.gov Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report. Suggested citation: Randall, A.D., 2010, Low flow of streams in the Susquehanna River basin of New York: U.S. Geological Survey Scientific Investigations Report 2010–5063, 57 p., at http://pubs.usgs.gov/sir/2010/5063. iii Contents Abstract ...........................................................................................................................................................1 Introduction.....................................................................................................................................................1 Watershed Properties and Processes that Control Spatial Variation in Low Flow of Streams ........3 Amount of Water Available for Runoff .............................................................................................3 Annual Precipitation and Runoff ...............................................................................................3 Seasonal Precipitation and Runoff ...........................................................................................5 Extent and Properties of Surficial Sand and Gravel .......................................................................8 Modeling Properties of Surficial Sand and Gravel that Affect Groundwater Discharge to Streams .................................................................................................10 Analytical Model ..............................................................................................................10 Numerical Model ..............................................................................................................11 Model Configuration and Operation .....................................................................14 Model Evaluation .....................................................................................................15 Evapotranspiration from Wetlands ..................................................................................................18 Underflow beneath Sites of Streamflow Measurement ..............................................................19 Less Important Factors that Affect Low Flows ..............................................................................22 Watershed Area .........................................................................................................................22 Extent and Properties of Till .....................................................................................................22 Extent and Properties of Bedrock Units .................................................................................22 Watershed Relief and Slope ....................................................................................................23 Degree of Stream Regulation and Watershed Urbanization ..............................................24 Regression Analysis ....................................................................................................................................24 Data Set ...............................................................................................................................................24 Low-Flow Statistics ...................................................................................................................24 Watershed Properties ...............................................................................................................28 Format of Regression Equations .....................................................................................................32 Standard-Value Equations to Estimate 7-Day, 10-Year Low Flows ...................................33 Effect of Sand-and-Gravel Area and Wetland Area ...................................................33 Effect of Amount of Water Available .............................................................................33 Other Variables and Data Manipulations Tested .........................................................35 Per-Square-Mile Equations to Estimate 7-Day, 10-Year Low Flows ..................................38 Comparison of Per-Square-Mile Equations with Standard-Value Equations .........38 Evaluation of the Data Set and Equations ....................................................................38 Effect of Underflow ..........................................................................................................42 Logarithmic Equations to Estimate 7-Day, 10-Year Low Flows ..........................................42 A Critique of the Rationale for Logarithmic Transformation ......................................42 Comparison of Logarithmic Equations with Standard-Value Equations ..................43 Equations to Estimate 7-Day, 2-Year Low Flow and 90-Percent Flow Duration ..............44 Suggested Methods for Estimating Low-Flow Characteristics at Ungaged Sites ............................46 Watersheds Larger Than 30 Square Miles .....................................................................................47 Watersheds Smaller Than 30 Square Miles ...................................................................................47 Summary........................................................................................................................................................48 Acknowledgments .......................................................................................................................................49 References Cited .........................................................................................................................................49 iv Figures 1. Map showing location of the Susquehanna River basin and physiographic regions in New York and adjacent States ...............................................................................................2 2. Diagram showing principal environmental factors that control the low flow of streams in the glaciated northeastern United States .............................................................4 3. Maps showing spatial variation in A, mean annual precipitation and B, mean annual runoff in the Susquehanna River basin of New York, 1951–80 .................................6 4. Map showing ratios of mean August–October runoff to mean annual runoff for the period April 1937 to March 1963 in the Susquehanna River basin and adjacent areas ...9 5. Diagram showing idealized aquifer represented by analytical equations of Rorabaugh (1963) ........................................................................................................................11 6. Graphs showing effect of A, valley width and B, aquifer transmissivity on groundwater discharge to a stream during a period of low flow, as calculated from equations of Rorabaugh (1963) .................................................................................................12 7. Diagram showing dimensions and properties of a two-dimensional groundwater-flow model that represents an idealized vertical section across an upland hillside and the adjacent valley ...................................................................................13 8. Graph showing simulated groundwater discharge, to a stream and as evapotranspiration, from an idealized upland hillside and valley fill after 4 months without recharge, as a function of hydraulic conductivity of the valley fill ......................16 9. Graph showing simulated groundwater discharge to a stream from an idealized valley fill after 4 months without recharge, as a function of valley half-width ................18 10. Map showing locations of streamflow-measurement stations in the Susquehanna River basin of New York .............................................................................................................20 11. Graph showing correlation of 7Q10 calculated from streamflow measurements with 7Q10 estimated using equation 3 .....................................................................................34 12. Graph showing distribution of residuals from equation 5 ....................................................36