|Scientific Investigations Report 2006–5024|
By Keith J. Halford
Drawdowns during aquifer tests can be obscured by barometric pressure changes, earth tides, regional pumping, and recharge events in the water-level record. These stresses can create water-level fluctuations that should be removed from observed water levels prior to estimating drawdowns. Simple models have been developed for estimating unpumped water levels during aquifer tests that are referred to as synthetic water levels. These models sum multiple time series such as barometric pressure, tidal potential, and background water levels to simulate non-pumping water levels. The amplitude and phase of each time series are adjusted so that synthetic water levels match measured water levels during periods unaffected by an aquifer test. Differences between synthetic and measured water levels are minimized with a sum-of-squares objective function. Root-mean-square errors during fitting and prediction periods were compared multiple times at four geographically diverse sites. Prediction error equaled fitting error when fitting periods were greater than or equal to four times prediction periods.
The proposed drawdown estimation approach has been implemented in a spreadsheet application. Measured time series are independent so that collection frequencies can differ and sampling times can be asynchronous. Time series can be viewed selectively and magnified easily. Fitting and prediction periods can be defined graphically or entered directly. Synthetic water levels for each observation well are created with earth tides, measured time series, moving averages of time series, and differences between measured and moving averages of time series. Selected series and fitting parameters for synthetic water levels are stored and drawdowns are estimated for prediction periods. Drawdowns can be viewed independently and adjusted visually if an anomaly skews initial drawdowns away from 0. The number of observations in a drawdown time series can be reduced by averaging across user-defined periods. Raw or reduced drawdown estimates can be copied from the spreadsheet application or written to tab-delimited ASCII files.
This report is contained in the following files:
For viewing and printing upon download. (This version of the report is accessible as defined in Section 508 of the Rehabilitation Act of 1973, Amendments of 1998.)
Template and spreadsheets for download.
Purpose and Scope
Background Water Levels
Moving Averages and Differences
Drawdown Estimation with Synthetic Water Levels
Drawdown Detection Limits
Instructions for Time-Series Analysis Workbook
Cell Formatting in the Time-Series Analysis Workbook
Pasting Data into the Time-Series Analysis Workbook
Initializing and Filtering Time Series
Viewing Time Series
Magnifying Selected Periods of Time Series
Graphically Defining Fitting, Estimation, and Feel-Good Periods
Major Features of the DETREND Page
Loading and Fitting Synthetic Series
Viewing Components and Estimating Drawdowns
Viewing, Filtering, and Exporting Drawdowns
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|U.S. Department of the Interior, U.S. Geological Survey
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Last modified: Thursday, March 07 2013, 07:34:56 PM