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Scientific Investigations Report 2014–5038

Prepared in cooperation with the Southwest Florida Water Management District

Creating a Monthly Time Series of the Potentiometric Surface in the Upper Floridan Aquifer, Northern Tampa Bay Area, Florida, January 2000–December 2009

By Terrie M. Lee and Geoffrey G. Fouad

Thumbnail of and link to report PDF (3.51 MB)Abstract

In Florida’s karst terrain, where groundwater and surface waters interact, a mapping time series of the potentiometric surface in the Upper Floridan aquifer offers a versatile metric for assessing the hydrologic condition of both the aquifer and overlying streams and wetlands. Long-term groundwater monitoring data were used to generate a monthly time series of potentiometric surfaces in the Upper Floridan aquifer over a 573-square-mile area of west-central Florida between January 2000 and December 2009. Recorded groundwater elevations were collated for 260 groundwater monitoring wells in the Northern Tampa Bay area, and a continuous time series of daily observations was created for 197 of the wells by estimating missing daily values through regression relations with other monitoring wells. Kriging was used to interpolate the monthly average potentiometric-surface elevation in the Upper Floridan aquifer over a decade. The mapping time series gives spatial and temporal coherence to groundwater monitoring data collected continuously over the decade by three different organizations, but at various frequencies. Further, the mapping time series describes the potentiometric surface beneath parts of six regionally important stream watersheds and 11 municipal well fields that collectively withdraw about 90 million gallons per day from the Upper Floridan aquifer.

Monthly semivariogram models were developed using monthly average groundwater levels at wells. Kriging was used to interpolate the monthly average potentiometric-surface elevations and to quantify the uncertainty in the interpolated elevations. Drawdown of the potentiometric surface within well fields was likely the cause of a characteristic decrease and then increase in the observed semivariance with increasing lag distance. This characteristic made use of the hole effect model appropriate for describing the monthly semivariograms and the interpolated surfaces. Spatial variance reflected in the monthly semivariograms decreased markedly between 2002 and 2003, timing that coincided with decreases in well-field pumping. Cross-validation results suggest that the kriging interpolation may smooth over the drawdown of the potentiometric surface near production wells.

The groundwater monitoring network of 197 wells yielded an average kriging error in the potentiometric-surface elevations of 2 feet or less over approximately 70 percent of the map area. Additional data collection within the existing monitoring network of 260 wells and near selected well fields could reduce the error in individual months. Reducing the kriging error in other areas would require adding new monitoring wells. Potentiometric-surface elevations fluctuated by as much as 30 feet over the study period, and the spatially averaged elevation for the entire surface rose by about 2 feet over the decade. Monthly potentiometric-surface elevations describe the lateral groundwater flow patterns in the aquifer and are usable at a variety of spatial scales to describe vertical groundwater recharge and discharge conditions for overlying surface-water features.

First posted May 19, 2014

  • Downloads
    Downloads directory contains a Readme.txt, 10 zip files (gridded-surface data layers), and a table (XLS 261 KB) that provides the well identification number, xy-coordinates, and monthy-average potentiometric elevation at each well from December 2000-January 2009. These data were used to interpolate the monthly time series of potentiometric surfaces over 10 years.

For additional information, contact:
Florida Water Science Center
U.S. Geological Survey
4446 Pet Lane, Suite 108
Lutz, Florida 33559

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Suggested citation:

Lee, T.M., and Fouad, G.G., 2014, Creating a monthly time series of the potentiometric surface in the Upper Floridan aquifer, Northern Tampa Bay area, Florida, January 2000–December 2009: U.S. Geological Survey Scientific Investigations Report 2014–5038, 26 p.,

ISSN 2328-0328 (online)




Methods Used to Create Monthly Potentiometric Surfaces

Interpolation of Potentiometric Surfaces in the Upper Floridan Aquifer

Summary and Conclusions

References Cited

Appendix 1. Characteristics of the Upper Floridan aquifer monitoring wells in the Northern Tampa Bay study area.

Appendix 2. Linear equation data for estimating missing daily values of potentiometric elevation in the 197 monitoring wells.

Appendix 3. Parameters used to curve-fit the monthly hole-effect semivariograms.

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