Home Introduction Hydrogeology -Hydrostatigraphy -Aquifer Prop. -Water-Budget -Fresh/Saltwater Simulation of GW Discharge Conclusions References Appendix 1 Appendix 2 Appendix 3 Appendix 4 Plates PDF Version

Hydrogeology of Southeastern Florida

Water-Budget Components

To properly simulate ground-water flow, it is necessary to understand and mathematically represent water-budget components that affect flow. This section identifies these components and summarizes their importance. Shallow ground-water flow is gravitationally driven by hydraulic gradients, which are caused by spatial variations in the height of the water table. Using this general principle, maps of the water table can be used in conjunction with an understanding of hydrogeologic characteristics to provide information regarding the processes that affect ground-water flow. A map report by Sonenshein and Koszalka (1996) displays the elevation of the water table in the Biscayne aquifer in Miami-Dade County for May 1993 (end of the dry season) and November 1993 (end of the wet season). Both maps (figure 4 and figure 5) are discussed in this report because they represent the general state and seasonal variability of the water table in the study area.

Rainfall, Evapotranspiration, and Runoff

The water-table maps (figure 4 and figure 5) suggest that the general flow of ground water is toward the coast. This implies that the Biscayne aquifer receives some form of recharge in order to maintain a water-table elevation above sea level. A comparison of figure 4 and figure 5, also shows that this recharge quantity is seasonally variable because the elevation of the water table changes during the year. Recharge to the Biscayne aquifer is primarily driven by rainfall, with large quantities of the total rainfall lost to evapotranspiration and surface runoff. Rainfall, evapotranspiration, and runoff combine to form recharge and require characterization for appropriate treatment in a numerical flow model.

Figure 4. Water-table elevation for May 1993, Miami-Dade County, Florida (from Sonenshein and Koszalka, 1996). [larger image]	Figure 5. Water-table elevation for November 1993, Miami-Dade County, Florida (from Sonenshein and Koszalka, 1996). [larger image]

One of the most comprehensive collections of rainfall data for southern Florida is maintained by the South Florida Water Management District. The District database includes rainfall records for stations operated by Federal, State, and local agencies. Rainfall stations within the study area that had at least 1 month of record for the period from 1989 to 1998 were used to calculate average values of monthly rainfall (fig. 6) for the domain of the regional-scale model (fig. 1). From January 1989 through September 1998, the annual rainfall average was about 141 cm/yr (centimeters per year); 75 percent of the rainfall occurred during the 5 wet-season months from June to October (fig. 6).

Figure 6. Totals of (A) annual and (B) monthly rainfall, 1989-98. Rainfall totals calculated for domain of regional-scale model using Thiessen polygons of rainfall monitoring stations. [click on images above for larger versions]

Evapotranspiration is defined as the rate of water loss to the atmosphere from evaporation and transpiration from plants. Direct measurements of evapotranspiration in the Everglades and other parts of southern Florida indicate that the rates are between 122 and 130 cm/yr (E.R. German, U.S. Geological Survey, oral commun., 2000). These rates were measured in areas where the water table is relatively high and may not be indicative of the entire study area. These rates, however, are in agreement with those simulated by Merritt (1996a) and suggest that the average evapotranspiration in some areas may be 90 percent of annual rainfall.

Runoff is the percentage of rainfall that drains directly into a flowing surface-water body, such as a canal or river. Runoff values are complicated to estimate because they depend on the moisture content of soils, soil properties, land use, and so forth. When soils are fully saturated, more runoff occurs than when the soils are dry. Land-use types further complicate quantification of runoff. For example, many urban areas in southern Florida are developed with french drains that route stormwater directly into the aquifer. This type of process is considered aquifer recharge. In other areas of southern Florida, however, stormwater flows directly into canals. This quantity is runoff and does not directly recharge the aquifer. Detailed estimates of runoff are not available for southern Florida.

Surface-Water/Ground-Water Interaction

Based on the shapes of contours in the water-table maps (figure 4 and figure 5), it is evident that the water-management canals have a substantial effect on ground-water flow within the study area. This is expected because the canals are in direct hydraulic connection with the highly permeable Biscayne aquifer. The canals recharge, or flow into, the Biscayne aquifer during the dry season (fig. 4) and drain the Biscayne aquifer during the wet season (fig. 5). From a hydrological perspective, the canals generally tend to serve the purpose for which they were designed. During the wet season, control structures within the canals are opened to allow discharge to Biscayne Bay. This drainage process lowers the water table and reduces the potential for flooding in urban and agricultural areas. During the dry season, surface water from farther inland is routed through the canal system to the coastal areas. This redistribution of surface water maintains relatively high water-table elevations near the coast and prevents saltwater intrusion. Based on their hydrologic influence, representation of canals in the numerical model of ground-water flow is essential to adequately simulate ground-water discharge to Biscayne Bay.

Control structures located within most of the canals are used to manage the water resources of southeastern Florida (figure 4 and figure 5). During the wet season, coastal control structures periodically open and allow discharge of surface water to Biscayne Bay. During the dry season, the coastal control structures generally remain closed to maintain relatively high water levels along the coast. Large differences in stage are commonly observed across the control structures. These stage differences likely cause ground-water flow around the structures, but there has been little research to quantify actual flow rates.

Ground-Water Withdrawals