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

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.