Model simulations were consistent with field data in suggesting that recharge and discharge were highest near levees. Modeled vertical fluxes declined exponentially and were not significant beyond 600 m. In the wetland interior (farther than 1000 m), our measurements of recharge and discharge were significantly higher than simulated fluxes, and our observation in interior wetlands therefore could not be accounted for as a direct effect of ground water flow beneath levees. Field observations showed that most interior sites also demonstrated cyclic changes from recharge to discharge and vice versa, with reversals occurring on monthly or longer timescales.

Figure 8. Spatial trends in discharge and recharge in central Everglades, south Florida. Time-averaged values of precipitation, evapotranspiration, and surface flows in ENR Project (Choi and Harvey 2000) and WCA-2A (SFWMD 1999) are plotted for comparison. [larger version]

Estimates of recharge and discharge from a subset of our sites in the central Everglades are summarized in Figure 8 and contrasted with average precipitation, evapotranspiration, and basin-averaged surface flows in the central Everglades (SFWMD 1999). Recharge and discharge were greatest in the smaller basins (ENR and the Stormwater Treatment Areas [STAs]) of the north-central Everglades. At some sites in ENR, recharge was larger than precipitation and evapotranspiration, and was also significant relative to surface flows (Figure 8). Choi and Harvey (2000) showed that recharge in ENR accounted for a flux of water equal to 30% of surface flow. Farther south in the much larger WCAs, area-averaged recharge and discharge were smaller, although the fluxes were still significant relative to other water balance fluxes (Figure 8). The trend of decreasing vertical fluxes farther south in the central Everglades is probably the result of several factors, including larger basin size (and correspondingly less effect of ground water flow beneath levees) and also a smaller overall driving force for recharge due to surface water levels in the wetlands that are not as high relative to areas outside of the wetlands compared with wetlands farther north (Miller 1988).

Recharge and discharge are important to water budgets in the central Everglades. Over the long term, recharge generally exceeds discharge (Harvey et al. 2002). The present research suggests that in addition to net, long-term recharge, there are alternating periods of recharge and discharge in the interior wetlands. These fluxes cancel out in long-term hydrologic budgets, but they are of great potential importance to seasonal water balances and water quality. For example, seasonal recharge during rising surface water levels transports surface water (and surface water contaminants) into peat porewater and the aquifer (Harvey et al. 2002). The stored water is later released by ground water discharge during falling surface water levels, supplementing low flows and perhaps exporting subsurface constituents back to surface water. In this way, alternating periods of recharge and discharge can be viewed as a natural process of aquifer storage and recovery that may be important to ecosystem processes as well as storage and return of dissolved contaminants to surface water.

Comparison with Predrainage Conditions

Knowing exactly how recharge and discharge have changed since predrainage times in the central Everglades is impossible, which means that present-day spatial trends are the best source of information to extrapolate backward to estimate predrainage conditions. The best modern analog for recharge and discharge relationships in the central Everglades to represent predrainage conditions is probably WCA-3A, where driving forces for recharge and discharge by natural topographic gradients are small, and great distances from levees and water-control structures reduce the effects of increased surface water level fluctuations. Recharge and discharge in WCA-3A were the smallest measured in our study. We conclude that in most areas of the central Everglades, particularly the much smaller basins (thousands of hectares) near the northern and northwestern boundaries (i.e., ENR and the STAs), recharge and discharge have vastly increased due to the effects of water resources management. Our work confirms that isolation of surface water at different levels in basins surrounded by levees creates significant driving forces for recharge and discharge. Our work also identifies that, perhaps for the first time, greater surface water level fluctuations associated with water resources management have also played a role in increasing recharge and discharge in the interior areas of the central Everglades.