USGS Scientific Investigations Report 2007-5111

By Gerard J. Gonthier

**U.S. Geological Survey Scientific Investigations Report 2007-5111; 29 pages** (Published July 2007)

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A graphical method that uses continuous water-level and barometric-pressure data was developed to estimate barometric efficiency. A plot of nearly continuous water level (on the y-axis), as a function of nearly continuous barometric pressure (on the x-axis), will plot as a line curved into a series of connected elliptical loops. Each loop represents a barometric-pressure fluctuation. The negative of the slope of the major axis of an elliptical loop will be the ratio of water-level change to barometric-pressure change, which is the sum of the barometric efficiency plus the error.

The negative of the slope of the preferred orientation of many elliptical loops is an estimate of the barometric efficiency. The slope of the preferred orientation of many elliptical loops is approximately the median of the slopes of the major axes of the elliptical loops. If water-level change that is not caused by barometric-pressure change does not correlate with barometric-pressure change, the probability that the error will be greater than zero will be the same as the probability that it will be less than zero. As a result, the negative of the median of the slopes for many loops will be close to the barometric efficiency.

The graphical method provided a rapid assessment of whether a well was affected by barometric-pressure change and also provided a rapid estimate of barometric efficiency. The graphical method was used to assess which wells at Air Force Plant 6, Marietta, Georgia, had water levels affected by barometric-pressure changes during a 2003 constant-discharge aquifer test. The graphical method was also used to estimate barometric efficiency. Barometric-efficiency estimates from the graphical method were compared to those of four other methods: average of ratios, median of ratios, Clark, and slope. The two methods (the graphical and median-of-ratios methods) that used the median values of water-level change divided by barometric-pressure change appeared to be most resistant to error caused by barometric-pressure-independent water-level change. The graphical method was particularly resistant to large amounts of barometric-pressure-independent water-level change, having an average and standard deviation of error for control wells that was less than one-quarter that of the other four methods.

When using the graphical method, it is advisable that more than one person select the slope or that the same person fits the same data several times to minimize the effect of subjectivity. Also, a long study period should be used (at least 60 days) to ensure that loops affected by large amounts of barometric-pressure-independent water-level change do not significantly contribute to error in the barometric-efficiency estimate.

Abstract

Introduction

Purpose and Scope

Acknowledgments

Barometric-Efficiency Concepts

Barometric-Pressure Effects on Water Levels

Barometric Efficiency

Factors Influencing Water-Level Change

Identifying Water-Level Change that Causes Error in Barometric-Efficiency Estimation

Systematic Error

Constant Barometric-Pressure-Independent Water-Level Change

Previous Methods of Estimating the Barometric Efficiency

Average-of-Ratios Method

Median-of-Ratios Method

Clark Method

Slope Method on Water-Level and Barometric-Pressure Change

Using Continuous Data to Estimate Barometric Efficiency

Filtering Water-Level Change that Causes Error in the Estimation of Barometric Efficiency

Long-Term Barometric-Pressure-Independent Water-Level Change

Daily Fluctuations

Other Barometric-Pressure-Independent Water-Level Changes

Time-Interval Selection

A Graphical Method for Estimating Barometric Efficiency from Continuous Water-Level Data

Theory Behind the Graphical Method

Examples of Elliptical Loops in Graphs

Determining the Slope of the Preferred Orientation of Loops

Application to a Site in the Piedmont Province, Air Force Plant 6, Marietta, Georgia

Comparison of Methods

Barometric-Efficiency Estimates Compared to the Domain of the Barometric Efficiency

Barometric-Efficiency Estimates of Control Wells

Summary

References Cited

Suggested citation:

Gonthier, G.J., 2007, A Graphical Method for Estimation of Barometric Efficiency from Continuous Data—Concepts and Application to a Site in the Piedmont, Air Force Plant 6, Marietta, Georgia: U.S. Geological Survey Scientific Investigations Report 2007-5111, 29 p., Web-only publication available at http://pubs.usgs.gov/sir/2007/5111/.

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