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Relations between precipitation, groundwater withdrawals, and changes in hydrologic conditions at selected sites in Volusia County, Florida

Project Chief: Louis Murray
Cooperator: St. Johns River Water Management District
Period of Project: October 2009 – September 2012

Problem Statement

Figure 1.  Location of study area including selected monitoring well and lake sites, municipal water-supply wells, and permitted agricultural sites in Volusia County, Florida.

Groundwater withdrawals have increased steadily during the latter half of the 20th century in east-central Florida. In Volusia County alone, withdrawals have more than doubled to about 97 million gallons per day between 1975 and 2000. Virtually all of the fresh groundwater required for municipal and agricultural supplies in central Florida is withdrawn from the Upper Floridan aquifer (UFA), a semiconfined sequence of highly transmissive carbonate rocks. Withdrawals from the UFA not only lower its potentiometric surface, but can potentially lower water levels in the overlying unconfined surficial aquifer system (SAS). Lowered SAS water levels, in turn, can adversely affect sensitive water resources such as wetlands, lakes, and associated ecosystems. The potential for groundwater withdrawals to impact wetland environments is of particular concern in north Volusia County, where large wetland areas occur near several municipal well fields (fig. 1). In addition to groundwater withdrawals, natural meteorological stressors such as precipitation and evapotranspiration influence wetland water levels and affect water resources. Consequently, trends and fluctuations observed in hydrologic conditions reflect the combined effects of both meteorological and anthropogenic influences. Difficulties in determining the relative influence of natural stressors and groundwater withdrawals on wetland hydrology pose a challenge for water managers tasked with assessing the impacts of new water-use permits in resource-sensitive areas, or of implementing water-use restrictions during periods of drought.

Objectives

The objective of the project was to examine the relative influences of precipitation, potential evapotranspiration (PET), and groundwater withdrawals on changes in hydrologic conditions in a wetland area of north Volusia County

Approach

Water-level data collected by the St. Johns River Water Management District (SJRWMD) at 18 groundwater and 3 lake sites were analyzed during varying periods of record between 1995 and 2010 to include a range of precipitation conditions. Streamflow measured between 1978 and 2001 at the USGS Tiger Bay gage (fig. 1) was analyzed to contrast flow conditions prior and subsequent to development of a nearby well field. Monthly precipitation data were acquired from the Daytona Beach International Airport National Oceanic and Atmospheric Administration (NOAA) rainfall station and from rainfall stations operated by SJRWMD at the Tomoka tower and SR40&11 sites. Monthly PET was estimated by the Priestley-Taylor method (this method is a modification of Penman’s more theoretical equation to estimate potential evapotranspiration). Data required for the model were obtained from land- and satellite-based measurements of net radiation and air temperature made within 2-km (1.24 mi) distances of the monitoring sites.

The relations between surface-water and groundwater levels, precipitation, and groundwater withdrawals were analyzed by multivariate linear regression. Regression models were limited to three explanatory variables and selected based on criteria developed to minimize serial correlation and multicollinearity of the data. Residuals were examined to assess adherence to assumptions of linearity, normality, and homoscedacticity. Streamflow at Tiger Bay was analyzed with flow-duration and double-mass curves, hypothesis testing, and trend testing. A significance criterion of 0.05 was applied to all statistical tests.

Information Product

A USGS Scientific Investigations Report (SIR) will describe the relations between precipitation, groundwater withdrawals, and changes in hydrologic conditions at selected sites in Volusia County, Florida.