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Climate Change and Hydrologic Extreme Events

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Investigating the Effects of Climate Extremes on Lake/Groundwater Interactions in Central Florida Lakes

Project Chief: Terrie M. Lee
Cooperator: Southwest Florida Water Management District
Period of Project: October 2006 Ė September 2009

Problem Statement

Figure 1. Lake Starr basin showing topographic-bathymetric elevations used in the model, the plan view of the model grid, the maximum extent of the lake in the model, and the location of monitoring wells used for model calibration.

Figure 1. Lake Starr basin showing topographic-bathymetric elevations used in the model, the plan view of the model grid, the maximum extent of the lake in the model, and the location of monitoring wells used for model calibration.


With the increase in global warming, climate researchers predict that there will be greater climate variability in sub-tropical regions, with both droughts and wet periods more frequent than in the past. The effect of increased climate variability over a range of time scales on lake-water budgets is unknown. Lake-basin models typically cover 1 to 2 year periods, and results of these models are somewhat limited because they reflect short-term hydrologic conditions. Lack of longer-term data sets for calibration has made it difficult to fully test the ability of models to simulate climate extremes.

Objectives

1) Document the annual and seasonal variability in the water-budget components of a representative seepage lake in central Florida for a 15-year period (1996-2010) that included historic wet and dry climate conditions

2) Describe the relation between the lake/groundwater exchanges and environmental conditions in the basin including rainfall, recharge, and groundwater levels for variable climate conditions

3) Use variably-saturated and saturated groundwater flow modeling to improve our understanding of the effect of rainfall variability and Upper Floridan aquifer levels on the groundwater recharge to the surficial aquifer and annual groundwater inflow to the lake and leakage from the lake

Approach

1. Modify the previous model of Lake Starr to include the MODFLOW Lake Package and the variably-saturated zone flow package (UZF), which uses a kinematic-wave approximation to Richardís equation to represent variably-saturated flow. The Richardís equation represents the movement of water in unsaturated soils.

2. Use daily rainfall and evapotranspiration as driving variables.

3. Simulate daily groundwater exchanges with the lake and the regions of inflow and outflow along the lakebed.

4. Calibrate the model simulation to the 15-year dataset.

The model uses an original conceptual approach to allow lake size and position in the basin to change in response to wet and dry climate conditions while accurately representing the daily lake volume and three dimensional variably-saturated groundwater flow responses in the basin (fig. 1).

Results

The divergent climate extremes over the decade subjected nearly 70 percent of the maximum lakebed area and 75 percent of the maximum shoreline perimeter to both groundwater inflow and lake leakage. About half of the lakebed area subject to flow reversals also went dry. A flow-through pattern present for 73 percent of the decade caused net leakage from the lake 80 percent of the time. Runoff from the saturated lake margin offset the groundwater deficit only about half of that time. A wet-extreme flow pattern present for 6 percent of the decade accounted for most of the surplus groundwater inflow and the recovery of lake stage. Pumping effects superimposed on dry climate extremes induced the least frequent but most cautionary flow pattern of leakage from over 90 percent of the actual lakebed area.

Information Products

Virdi, M.L., Lee, T.M., and Swancar, Amy, Simulating lake-groundwater interactions during decadal climate cycles: accounting for variable lake area in the watershed. Abstract - American Geophysical Union Fall Meeting, December 2009, San Francisco.

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