Scientific Investigations Report 2010–5063
ABSTRACTThe principal source of streamflow during periods of low flow in the Susquehanna River basin of New York is the discharge of groundwater from sand-and-gravel deposits. Spatial variation in low flow is mostly a function of differences in three watershed properties: the amount of water that is introduced to the watershed and available for runoff, the extent of surficial sand and gravel relative to till-mantled bedrock, and the extent of wetlands. These three properties were consistently significant in regression equations that were developed to estimate several indices of low flow expressed in cubic feet per second or in cubic feet per second per square mile. The equations explain 90 to 99 percent of the spatial variation in low flow. A few equations indicate that underflow that bypasses streamflow-measurement sites through permeable sand and gravel can significantly decrease low flows. Analytical and numerical groundwater-flow models indicate that spatial extent, hydraulic conductivity and thickness, storage capacity, and topography of stratified sandand- gravel deposits affect low-flow yields from those deposits. Model-simulated discharge of groundwater to streams at low flow reaches a maximum where hydraulic-conductivity values are about 15 feet per day (in valleys 0.5 mile wide) to 60 feet per day (in valleys 1 mile wide). These hydraulic-conductivity values are much larger than those that are considered typical of till and bedrock, but smaller than values reported for productive sand-and-gravel aquifers in some valley reaches in New York. Differences in the properties of till and bedrock and in land-surface slope or relief within the Susquehanna River basin of New York apparently have little effect on low flow. Three regression equations were selected for practical application in estimating 7-day mean low flows in cubic feet per second with 10-year and 2-year recurrence intervals, and 90-percent flow duration, at ungaged sites draining more than 30 square miles; standard errors were 0.88, 1.40, and 1.95 cubic feet per second, respectively. Equations that express low flows in cubic feet per second per square mile were selected for estimating these three indices at ungaged sites draining less than 30 square miles; standard errors were 0.012, 0.018, and 0.022 cubic feet per second per square mile, respectively. |
First posted February 4, 2011 For additional information contact: Part or all of this report is presented in Portable Document Format (PDF); the latest version of Adobe Reader or similar software is required to view it. Download the latest version of Adobe Reader, free of charge. |
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.
Abstract
Introduction
Watershed Properties and Processes that Control Spatial Variation in Low Flow of Streams
Amount of Water Available for Runoff
Annual Precipitation and Runoff
Seasonal Precipitation and Runoff
Extent and Properties of Surficial Sand and Gravel
Modeling Properties of Surficial Sand and Gravel that Affect Groundwater
Discharge to Streams
Analytical Model
Numerical Model
Model Configuration and Operation
Model Evaluation
Evapotranspiration from Wetlands
Underflow beneath Sites of Streamflow Measurement
Less Important Factors that Affect Low Flows
Watershed Area
Extent and Properties of Till
Extent and Properties of Bedrock Units
Watershed Relief and Slope
Degree of Stream Regulation and Watershed Urbanization
Regression Analysis
Data Set
Low-Flow Statistics
Watershed Properties
Format of Regression Equations
Standard-Value Equations to Estimate 7-Day, 10-Year Low Flows
Effect of Sand-and-Gravel Area and Wetland Area
Effect of Amount of Water Available
Other Variables and Data Manipulations Tested
Per-Square-Mile Equations to Estimate 7-Day, 10-Year Low Flows
Comparison of Per-Square-Mile Equations with Standard-Value Equations
Evaluation of the Data Set and Equations
Effect of Underflow
Logarithmic Equations to Estimate 7-Day, 10-Year Low Flows
A Critique of the Rationale for Logarithmic Transformation
Comparison of Logarithmic Equations with Standard-Value Equations
Equations to Estimate 7-Day, 2-Year Low Flow and 90-Percent Flow Duration
Suggested Methods for Estimating Low-Flow Characteristics at Ungaged Sites
Watersheds Larger Than 30 Square Miles
Watersheds Smaller Than 30 Square Miles
Summary
Acknowledgments
References Cited