Water Balance

Recharge was the dominant interaction between the wetland and ground water at ENR when averaged over the entire basin and over the 4-year study period. Total recharge averaged 0.9 cm/d and discharge was 0.1 cm/d (Choi and Harvey, 2000). Expressed as a percentage of water inflows, those fluxes are 31 and 3 percent, respectively, of surface water pumped into ENR for treatment. Discharge at ENR is ignored in further discussion because recharge dominated vertical fluxes. Ground-water flow beneath levees accounted for approximately 94 percent of the total recharge flux in ENR. Choi and Harvey (2000) showed that approximately 73 percent of the total recharge was discharged to the seepage canal. The seepage canal collected a mixture of shallow ground-water flow beneath levees and deep ground water flowing vertically to the surface. At sites greater than 0.5 km from the western and northern levees, recharge also occurred but at much lower rates. The average recharge at interior sites was approximately 0.05 cm/d, which accounted for only about 6 percent of the total recharge in ENR (table 16). Having accounted for a total of 79 percent of all recharge in ENR, the remaining 21 percent was inferred to have bypassed the seepage canal, probably discharging instead in nearby agricultural land. Eventually, that water would have discharged to a canal at another location in the EAA, to be evapotranspired or pumped away through a different system of canals.

Recharge varied temporally in ENR. Whereas flow in the seepage canal was almost constant, total recharge varied over approximately a factor of 2, correlating positively with surface-water levels in ENR (Choi and Harvey, 2000). For that reason, Choi and Harvey (2000) concluded that recharge was controlled mainly by water management rather than other factors (such as seasonal or interannual variation in precipitation).

In WCA-2A, recharge also was the dominant interaction between the wetland and ground water. Averaged over 2 years throughout the entire basin, a recharge of 0.2 cm/d and discharge of 0.1 cm/d was estimated for the WCA-2A interior, which amount to 31 percent and 15 percent of average surface-water fluxes through WCA-2A, respectively (table 16). Using the SFWMM, researchers at SFWMD estimated recharge in WCA-2A to be 0.04 cm/d and discharge to be 0.02 cm/d, equal to 6 and 3 percent of surface-water inflow to WCA-2A, respectively. Lower values of recharge and discharge from SFWMD model simulations may be the result of the larger spatial scale and longer temporal scale of averaging inherent in their methods. The present research suggests that SFWMD estimates are minimum estimates, and that recharge and discharge in WCA-2A each are approximately a factor of 5 larger. It is important to keep in mind that the difference among various estimates of recharge and discharge is not a reflection on the relative value of different approaches–each has its merits. The detailed estimates of recharge and discharge developed by the present study are important to shorter term and smaller scale water-balance studies, and studies concerned with water quality. The estimates of recharge and discharge reported here are based on relatively short data sets and must be considered uncertain until a longer term data set is available. The larger scale water-balance work at SFWMD is well established and has primary importance to regional flow estimation. More detailed comparisons between the SFWMM and the measurements of the present report should be encouraged.

The main factor that caused recharge and discharge fluxes to increase at ENR relative to pre-drainage conditions was land subsidence, which was near its maximum in terms of the effects on the land slope in that vicinity. Another factor affecting recharge in ENR is the relatively large ratio of levee perimeter to wetland surface area (4 x 10^-4 ft/ft²) compared with WCAs. That ratio was a factor of 6 smaller in WCA-2A (6 x 10^-5 ft/ft²), suggesting a higher contribution of ground-water flow beneath levees per unit area of wetland. As discussed earlier, vertical fluxes within 0.5 km of levees usually were higher than 0.3 cm/d, whereas fluxes in the wetland interior typically were less. As a result, ground-water flow beneath levees accounted for 94 percent of recharge at ENR (whereas vertical fluxes in the wetland interior were thought to dominate in WCA-2A), and basin-averaged recharge was almost 5 times larger in ENR compared to WCA-2A (table 16).

Figure 21. Spatial trends in discharge and recharge in north-central Everglades, south Florida. Time-averaged precipitation, evapotranspiration, and surface flows in Everglades Nutrient Removal (ENR) project and Water Conservation Area 2A (WCA-2A) are plotted for comparison. [larger version]

The overall result of land subsidence and water-resources management in the north-central Everglades appears to be a general pattern of decreasing interactions between surface water and ground water, from areas where they are highest in the relatively small basins at the northern boundary (ENR and STAs), to the center of the Everglades’ largest enclosed basin (WCA-3A). The greatest recharge and discharge fluxes were observed in ENR, and fluxes decreased toward the interior areas of WCA-3A (fig. 21). The increasing size of WCAs to the south is accompanied by a decreasing ratio of levee length to wetland surface area, which is an important reason why recharge and discharge decrease toward WCA-3A.

In contrast to basin-averaged recharge and discharge, fluxes in the wetland interior increased toward the south. Greater vertical fluxes in the wetland interior of WCA-2A (compared with ENR) are at least partly the result of greater driving forces associated with surface-water fluctuations in WCA-2A. Higher hydraulic conductivity of the peat in WCA-2A, compared with ENR, also contributed to higher vertical fluxes in the interior of WCA-2A.