Flooded area and plant zonation in isolated wetlands on well fields in the northern Tampa Bay region following reductions in well-field groundwater withdrawal rates
Project Chief: Kim H. Haag
Cooperator: Tampa Bay Water
Period of Project: October 2008- September 2010
Problem Statement
Figure 1. Location of study wetlands in the northern Tampa Bay region of west-central Florida.
Groundwater withdrawals for municipal water supply in the northern Tampa Bay region have contributed to the lowering of aquifer water levels and water levels in isolated wetlands that are located in well field areas. Efforts have been made to develop alternative water-supply sources so that reductions in groundwater withdrawals can be maintained. Since alternative water sources came on-line in late 2002, groundwater withdrawal rates have been reduced. The subsequent recovery of aquifer water levels is expected to reduce leakage from the surficial aquifer system and allow expansion of flooded areas within the wetlands, thereby increasing the amount of time that various zones within the wetlands are flooded and available as habitat for wetland plant communities. Increases in functional wetland areas can be assessed by comparing the sizes of routinely- flooded areas prior and subsequent to reductions in groundwater withdrawal rates. Analysis of plant distribution information collected by Tampa Bay Water and the Southwest Florida Water Management District (SWFWMD), using their Wetland Assessment Procedure (WAP), can help determine if increased flooded area results in changes in plant distribution.
Objectives
The objectives of the study were to (1) describe and contrast the extent and duration of the flooded areas in isolated well-field wetlands before and after reductions in groundwater withdrawal rates; and (2) describe changes in the zonation of vegetation in the wetlands after reductions in groundwater withdrawal rates.
Figure 2. A) Bathymetric map of W-33 cypress wetland in west-central Florida, and graphs of B) wetland water level and groundwater levels in the wetland monitor well, C) the duration of inundation, in percent of time, and D) flooded area in percent of total wetland area for W-33.
Approach
The project included nine study wetlands and two reference wetlands unaffected by groundwater pumpage (fig. 1). Wetland bathymetric data were used to describe relationships between wetland stage, area, and volume. Long-term stage data were used to reconstruct historical flooded-area frequency distributions for the wetlands during a period before reductions in groundwater withdrawal rates. Historical flooded-area frequencies were compared to flooded-area frequencies observed after groundwater withdrawal reductions (fig. 2). Patterns of flooded area were interpreted with long-term records of rainfall, well-field groundwater withdrawal rates, and the elevation of the potentiometric surface in the Upper Floridan aquifer.
Vegetation data were collected by Tampa Bay Water and SWFWMD. The mean percent cover of the five dominant plant species in each wetland zone in each wetland was compiled. A wetland prevalence index for each sample in the wetland zones was determined. The wetland prevalence index was computed by weighting the percent cover of the dominant species with an index value for each Wetland Indicator Category [Obligate wetland species (OBL) = 5, Facultative wetland species (FACW) = 4, Facultative species (FAC) = 3, Facultative upland species (FACU) = 2, Obligate upland species (UPL) = 1]. A higher prevalence index indicates more hydrophytic vegetation.
Results
Five well-field wetlands had an increase in the extent and duration of the flooded area after well-field groundwater withdrawal rates were reduced. The median elevation of the potentiometric surface of the Upper Floridan aquifer increased 4 to 8 feet beneath these five well-field wetlands. The other four well-field wetlands did not have an increase in flooded area extent or duration. Changes in wetland vegetation were generally consistent with the results of the flooded area analysis.
Information Product
Haag, K.H. and Pfeiffer, W.C., 2012, Flooded area and plant zonation in isolated wetlands in well fields in the northern Tampa Bay region, Florida, following reductions in groundwater withdrawal rates: U.S. Geological Survey Scientific Investigations Report 2012-5039, 49 p.
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U.S. Department of the Interior |
U.S. Geological Survey
