Figure 1. Central Everglades and adjoining areas, south Florida, showing locations of WCAs, ENR Project, and STAs. [larger version]

Recharge and discharge have increased since predrainage times, in part due to the increased gradients driving recharge of surface water near the WCA boundaries. At those boundaries, levees separate the WCA wetlands from areas outside where water levels have been lowered by canal drainage, subsidence, or ground water pumping for water supply (Harvey et al. 2002). For example, significant recharge now occurs along the northern and western border of the Everglades as a response to 80 years of subsidence in the neighboring Everglades Agricultural Area, an area of former wetlands now managed for agriculture located northwest of the WCAs (Figure 1). Significant recharge also occurs on the eastern side of the Everglades, replenishing water-supply wells and draining to the ocean via canals. Another factor that may have increased recharge and discharge is the possible increase in fluctuations of surface water levels under water management. Several times each year, water managers release surface water through control structures to quickly move large quantities of water from upstream to downstream basins. The sudden releases of water create gravity waves that are sometimes higher than 1.2 m and propagate southward through the basins. Hydrologic simulations by the South Florida Water Management District (SFWMD) using the South Florida Water Management model and natural system model suggest that the range of annual surface water fluctuations in the WCA-2A interior may have increased by ~50% since predrainage times, from ~60 cm to 90 cm (Tarboton et al. 1999) (also see www.sfwmd.gov/org/pld/restudy/hpm/index.html). It is difficult to determine precisely how recharge and discharge have been affected by an increase in surface water level fluctuations. Our research focused on quantifying recharge and discharge that is presently occurring in the central Everglades. We attempted to identify the specific roles of different aspects of water management, including the role of levees, in causing increases in recharge and discharge near WCA basin boundaries, as well as the possible role of surface water level fluctuations resulting from the operation of water-control structures in causing increases in recharge and discharge in the interior wetlands.

Although most scientists acknowledge a connection between ground water and surface water in the Everglades, the locations of recharge and discharge, and the volumes of water exchanged between wetland surface water and aquifer, remain uncertain. Past investigations of recharge and discharge were mainly conducted on wetland areas in the immediate vicinity of canals (Klein and Sherwood 1961; Miller 1978; Swayze 1988; Chin 1990; Genereux and Slater 1999; Rohrer 1999; Nemeth and Solo-Garbriele 2001; Sonenshein 2001). Due in part to logistical constraints, investigations of surface water and ground water interactions in the vast interior areas of the Everglades are almost nonexistent. Until recently, it was still common for recharge and discharge in the Everglades to be computed as net estimates averaged over large areas and long time periods as part of regional surface water budgets (Fennema et al. 1994). Recharge and discharge were estimated in a smaller Everglades basin (Everglades Nutrient Removal [ENR] Project) using surface water and chloride budgets (Choi and Harvey 2000), but that work required exceptionally dense instrumentation and frequent hydrologic and chemical measurements. There is an increasing need for information about recharge and discharge in the interior wetlands of the central Everglades, such as the WCAs. Estimates of recharge and discharge in the interior basins are important because the dominant percentage of the wetlands is far from levees, so that even relatively small recharge or discharge fluxes in the wetland interior could be significant to overall water or chemical budgets.

The goal of our research was to estimate recharge and discharge, and delineate spatial and temporal patterns of those fluxes in the central Everglades using data collected over a five-year period (1997 to 2002). The study documented some of the factors influencing recharge and discharge, including effects of compartmentalizing the wetlands with levees (i.e., due to the effect on hydraulic driving forces), and the effect of movement of pulses of surface water released through water-control structures at levees into the interior areas of the basins.

Figure 2. Schematic showing general relationships between topography, and surface water and ground water levels in (a) predrainage and (b and c) present-day hydrologic systems, central Everglades, south Florida. Approximate surface water levels (solid lines) and ground water levels (dashed lines) are shown for both wet and dry seasons. The vertical scale is substantially exaggerated to show detail. [click on images at right for larger versions] (below) The predrainage wetland was relatively wide and surface water fluctuations were muted (a). (below) Compartmentalization of the Everglades into enclosed basins narrowed the surface water flow-way, and increased surface water fluctuations. Subsidence, canal drainage, and ground water withdrawals on former wetlands to either side of the Everglades contributed to increased recharge in the wetlands, especially near levees (b). (below) Along a north-south axis, the retention of surface water in the enclosed basins created a stair step of water levels, causing recharge on the upgradient side and discharge on the downgradient side of levees (c). Under present-day management, when levee spillways are opened, a pulse of surface water is released that flows overland under the influence of gravity toward the basin interior. The pulsed behavior of surface water releases may play a role in driving cyclic reversals between recharge and discharge in the interior wetlands.