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 U.S. Geological Survey Scientific Investigations Report 2014-5038 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. Overview of Data Long-term groundwater data from monitoring wells were spatially interpolated to generate a monthly time series of potentiometric surfaces in the Upper Floridan aquifer in the Northern Tampa Bay area of Florida. The resulting maps describe the potentiometric surface over a 573-square-mile area of west-central Florida between January 2000 and December 2009. Kriging was used to interpolate the monthly average potentiometric-surface elevation between wells in the Upper Floridan aquifer over a decade. Vertical elevations for the potentiometric surface are in feet and reference the National Geodetic Vertical Datum, NGVD 1929. Kriging was also used to describe the spatial distribution of the kriging standard error (feet) associated with the interpolated elevations. The complete monthly time series describes the potentiometric surface elevations, and the associated uncertainty in the elevations, beneath parts of six regionally important stream watersheds and 11 municipal well fields. Preview You can preview the entire 10-year time series of monthly potentiometric surfaces in the Animation File. This 2-minute long animation displays all 120 potentiometric surfaces. Color shades used on each map are classified equivalently and surfaces are displayed using a 5-foot elevation interval. Downloads - Filename Convention Results can be downloaded in zip files for each year: 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, and 2009. When extracted, each yearly zip file contains 24 rasters: two apiece for January through December. There are 12 rasters that contain gridded surfaces with the monthly average elevation of the Upper Floridan aquifer potentiometric surface in feet, NGVD 1929. The monthly-average potentiometric surfaces have the following filename convention: avgwlYYMM_c An additional 12 rasters contain gridded surfaces with the standard error of estimate in the monthly potentiometric surfaces, in feet. Standard error surfaces have the following filename convention: avgwlYYMM_e_c The YYMM in the filename conventions designates the year and month. For instance, 0001 represents January 2000 and 0701 represents January 2007. To Download 1. Download desired zip file(s) to local hard drive 2. Open with Winzip or other zip utility and click the "Extract" button (highlighting the individual files is not necessary) 3. Extract contents to a destination folder on the local hard drive Metadata Each raster has associated metadata viewable and exportable through ArcCatalog. To view the metadata, simply click on the raster in ArcCatalog, and then click the “Description” tab. Projections All rasters are projected in the Universal Transverse Mercator (UTM) projection referencing the High Accuracy Reference Network (HARN). Credits 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., http://dx.doi.org/10.3133/sir20145038. Use limitations This digital publication was prepared by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed in this report, or represents that its use would not infringe privately owned rights. Reference therein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. Any views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Although all data published in this report have been used by the U.S. Geological Survey (USGS), no warranty, expressed or implied, is made by the USGS as to the accuracy of the data and related materials and/or the functioning of the software. The act of distribution shall not constitute any such warranty, and no responsibility is assumed by the USGS in the use of this data, software, or related materials. Graphical map depictions are intended to be used within the map scale limits applicable to the source data. Although software enables the user to view data at various scales, the user is cautioned to refer to the source documentation for the appropriate map scale limitations. Corresponding Author Terrie M. Lee U.S. Geological Survey, Florida Water Science Center 4446 Pet Lane, Suite 108 Lutz, FL 33559 Telephone: 813-498-5030 Email: tmlee@usgs.gov